Visual Basic Hoja de referencia

' Option Explicit On  -- requires declaration (recommended)
Module Variables
    Sub Main()
        Dim name As String = "Alice"          ' text
        Dim age As Integer = 30                ' 32-bit integer
        Dim salary As Decimal = 50000.50D      ' precise money
        Dim pi As Double = 3.14159             ' 64-bit float
        Dim isDev As Boolean = True            ' True/False
        Dim letter As Char = "A"c              ' single character
        Dim today As Date = #2024-06-18#       ' date literal

        ' type inference (Option Infer On)
        Dim count = 10          ' inferred as Integer
        Dim message = "Hello"   ' inferred as String

        ' nullable types (Value types that can be Nothing)
        Dim score As Integer? = Nothing
        If score.HasValue Then Console.WriteLine(score.Value)

        ' varType() returns the Type object
        Console.WriteLine(name.GetType().Name)  ' String
    End Sub
End Module

' constants (compile-time, implicitly Shared)
Public Const Pi As Double = 3.14159265358979
Public Const MaxRetries As Integer = 3

' enum: named integer constants
Public Enum LogLevel
    Debug = 0
    Info = 1
    Warning = 2
    [Error] = 3     ' brackets for reserved words
End Enum

Module EnumsDemo
    Sub Main()
        Dim level As LogLevel = LogLevel.Warning
        Console.WriteLine(level)         ' Warning
        Console.WriteLine(CInt(level))   ' 2

        ' parse string to enum
        Dim parsed = [Enum].Parse(GetType(LogLevel), "Info")
        Console.WriteLine(parsed)        ' Info

        ' Flags attribute for bit-field enums
        Dim perms As FilePermission = FilePermission.Read Or FilePermission.Write
        If (perms And FilePermission.Read) <> 0 Then
            Console.WriteLine("Has read")
        End If
    End Sub
End Module

<Flags>
Public Enum FilePermission As Integer
    None = 0
    Read = 1
    Write = 2
    Execute = 4
End Enum

' widening conversions (implicit, safe)
Dim i As Integer = 42
Dim d As Double = i        ' Integer -> Double (no data loss)

' narrowing conversions (explicit, may lose data)
Dim d2 As Double = 3.99
Dim i2 As Integer = CInt(d2)       ' 4 (rounds, not truncates)
Dim i3 As Integer = Int(d2)        ' 3 (floor)
Dim s As String = CStr(42)         ' "42"
Dim n As Integer = CInt("100")     ' 100 (throws if invalid)

' DirectCast vs CType vs TryCast
Dim obj As Object = "Hello"
Dim s2 As String = DirectCast(obj, String)  ' strict, same type only
Dim s3 As String = CType(obj, String)       ' flexible, converts

' TryCast: returns Nothing if cast fails (reference types only)
Dim s4 As String = TryCast(obj, String)
If s4 IsNot Nothing Then Console.WriteLine(s4)

' conversion functions
Dim b As Boolean = CBool(1)         ' True
Dim dt As Date = CDate("2024-06-18")
Dim dec As Decimal = CDec("99.99")

' IsNumeric, IsDate checks
If IsNumeric("42") Then Console.WriteLine("is number")
If IsDate("2024-06-18") Then Console.WriteLine("is date")

' Parse vs TryParse (safer)
Dim num As Integer
If Integer.TryParse("123", num) Then
    Console.WriteLine(num)   ' 123
End If

' arithmetic
Dim a As Integer = 10
Console.WriteLine(a + 3)    ' 13
Console.WriteLine(a - 4)    ' 6
Console.WriteLine(a * 2)    ' 20
Console.WriteLine(a / 3)    ' 3.333 (Double, real division)
Console.WriteLine(a \ 3)   ' 3 (integer division)
Console.WriteLine(a Mod 3)  ' 1 (remainder)
Console.WriteLine(a ^ 2)    ' 100 (exponent)

' comparison (return Boolean)
Console.WriteLine(5 > 3)        ' True
Console.WriteLine(5 = 5)        ' True (VB uses = for equality)
Console.WriteLine(5 <> 4)       ' True (not equal)
Console.WriteLine("a" < "b")    ' True (string comparison)

' logical operators
Dim x As Boolean = True
Dim y As Boolean = False
Console.WriteLine(x And y)      ' False (evaluates both)
Console.WriteLine(x Or y)       ' True
Console.WriteLine(Not x)        ' False
Console.WriteLine(x Xor y)      ' True (exclusive or)
Console.WriteLine(x AndAlso y)  ' False (short-circuit)
Console.WriteLine(x OrElse y)   ' True (short-circuit)

' bitwise (on integers)
Console.WriteLine(5 And 3)      ' 1 (0101 And 0011 = 0001)
Console.WriteLine(5 Or 3)       ' 7
Console.WriteLine(5 Xor 3)      ' 6

' string concatenation
Dim s As String = "Hello" & " " & "World"   ' & operator
Dim s2 As String = $"Hello {a}"             ' interpolation

Module IO
    Sub Main()
        ' console output
        Console.WriteLine("Hello, World!")    ' with newline
        Console.Write("No newline")           ' without newline
        Console.WriteLine()                   ' blank line

        ' string interpolation ($ before the string)
        Dim name As String = "Alice"
        Dim age As Integer = 30
        Console.WriteLine($"Hello, {name}. You are {age} years old.")
        Console.WriteLine($"Next year: {age + 1}")     ' expressions work
        Console.WriteLine($"Pi: {3.14159:F2}")         ' format specifier

        ' String.Format (positional)
        Dim msg As String = String.Format("{0} is {1} years old", name, age)

        ' composite formatting with alignment
        Console.WriteLine("{0,-10} {1,5}", "Alice", 30)   ' left/right align
        Console.WriteLine("{0:C}", 1234.56)   ' currency: $1,234.56
        Console.WriteLine("{0:P}", 0.25)      ' percent: 25.00 %
        Console.WriteLine("{0:X}", 255)       ' hex: FF

        ' console input
        Console.Write("Enter name: ")
        Dim input As String = Console.ReadLine()
        Console.Write("Enter age: ")
        Dim ageInput As Integer = Integer.Parse(Console.ReadLine())

        ' StringBuilder for efficient concatenation
        Dim sb As New System.Text.StringBuilder()
        For i As Integer = 1 To 100
            sb.Append($"Line {i}").AppendLine()
        Next
        Console.WriteLine(sb.ToString())
    End Sub
End Module

Dim score As Integer = 85

' multi-line If
If score >= 90 Then
    Console.WriteLine("A")
ElseIf score >= 80 Then
    Console.WriteLine("B")
ElseIf score >= 70 Then
    Console.WriteLine("C")
Else
    Console.WriteLine("F")
End If

' single-line If (no End If)
If score > 50 Then Console.WriteLine("Pass")

' nested If with AndAlso (short-circuit)
Dim age As Integer = 25
Dim hasLicense As Boolean = True
If age >= 18 AndAlso hasLicense Then
    Console.WriteLine("Can drive")
End If

' If as expression (VB 14+ ternary)
Dim grade As String = If(score >= 60, "Pass", "Fail")

' nullable coalescing
Dim name As String = Nothing
Dim displayName As String = If(name, "Unknown")  ' "Unknown"

' IIf function (old-style, always evaluates both sides)
Dim result As String = IIf(score > 50, "Pass", "Fail")

Dim grade As String = "B"

' basic Select Case
Select Case grade
    Case "A"
        Console.WriteLine("Excellent")
    Case "B", "C"          ' multiple values
        Console.WriteLine("Good")
    Case "D" To "F"        ' range
        Console.WriteLine("Needs improvement")
    Case Else
        Console.WriteLine("Invalid grade")
End Select

' numeric ranges
Dim score As Integer = 85
Select Case score
    Case Is >= 90          ' comparison operator
        Console.WriteLine("A")
    Case 80 To 89          ' range
        Console.WriteLine("B")
    Case 70 To 79
        Console.WriteLine("C")
    Case Is < 70
        Console.WriteLine("F")
End Select

' Case with expressions
Dim day As Integer = 3
Select Case day
    Case 1, 2, 3, 4, 5
        Console.WriteLine("Weekday")
    Case 6, 7
        Console.WriteLine("Weekend")
End Select

' For loop with step
For i As Integer = 0 To 4
    Console.WriteLine(i)      ' 0 1 2 3 4
Next

For i As Integer = 10 To 1 Step -2
    Console.WriteLine(i)      ' 10 8 6 4 2
Next

' nested loops
For row As Integer = 1 To 3
    For col As Integer = 1 To 3
        Console.Write($"{row * col} ")
    Next
    Console.WriteLine()
Next

' For Each (iterates collections)
Dim fruits() As String = {"apple", "banana", "cherry"}
For Each fruit As String In fruits
    Console.WriteLine(fruit)
Next

' For Each with List
Dim nums As New List(Of Integer) From {1, 2, 3}
For Each n As Integer In nums
    Console.WriteLine(n)
Next

' Exit For / Continue For
For i As Integer = 1 To 100
    If i > 5 Then Exit For         ' break
    If i Mod 2 = 0 Then Continue For ' skip even
    Console.WriteLine(i)            ' 1 3 5
Next

' Iterator function (yields one item at a time)
Iterator Function Evens(max As Integer) As IEnumerable(Of Integer)
    For i As Integer = 2 To max Step 2
        Yield i
    Next
End Function

For Each e In Evens(10)
    Console.WriteLine(e)   ' 2 4 6 8 10
Next

' Do While: test at top (may never run)
Dim count As Integer = 0
Do While count < 3
    Console.WriteLine(count)   ' 0 1 2
    count += 1
Loop

' Do Until: runs UNTIL condition is true
count = 0
Do Until count >= 3
    Console.WriteLine(count)   ' 0 1 2
    count += 1
Loop

' Do...Loop While: test at bottom (runs at least once)
count = 0
Do
    Console.WriteLine(count)
    count += 1
Loop While count < 3

' Do...Loop Until: test at bottom, runs until true
count = 0
Do
    Console.WriteLine(count)
    count += 1
Loop Until count >= 3

' While...End While (older syntax)
Dim i As Integer = 0
While i < 3
    Console.WriteLine(i)
    i += 1
End While

' Exit Do / Continue Do
Dim n As Integer = 0
Do While n < 100
    n += 1
    If n Mod 2 = 0 Then Continue Do   ' skip even
    If n > 10 Then Exit Do            ' break
    Console.WriteLine(n)              ' 1 3 5 7 9
Loop

' With...End With: access members of one object repeatedly
Dim sb As New System.Text.StringBuilder()
With sb
    .Append("Hello")
    .Append(", ")
    .Append("World")
    .AppendLine()
End With
Console.WriteLine(sb.ToString())

' With on anonymous types
Dim person = New With {.Name = "Alice", .Age = 30}
With person
    Console.WriteLine($"{.Name} is {.Age}")
End With

' nested With (use caution — confusing)
With New System.Drawing.Point(3, 4)
    Console.WriteLine($"X={.X}, Y={.Y}")
End With

' GoTo (use sparingly — only for error handling or deep breaks)
For i As Integer = 1 To 3
    For j As Integer = 1 To 3
        If i + j > 4 Then GoTo done   ' break out of both loops
        Console.WriteLine($"{i},{j}")
    Next
Next
done:
Console.WriteLine("Finished")

' On Error GoTo (legacy error handling — prefer Try/Catch)
On Error GoTo errorHandler
Dim result As Integer = 10 \ 0
Exit Sub
errorHandler:
Console.WriteLine("Error: " & Err.Description)
Resume Next

' Sub: performs an action, returns nothing
Sub Greet(name As String)
    Console.WriteLine($"Hello, {name}!")
End Sub

' Function: returns a value
Function Add(a As Integer, b As Integer) As Integer
    Return a + b          ' explicit return
End Function

' Function without Return: use function name
Function Multiply(a As Integer, b As Integer) As Integer
    Multiply = a * b      ' old-style: assign to function name
End Function

' calling
Greet("Alice")
Dim sum As Integer = Add(3, 4)
Console.WriteLine(sum)       ' 7

' parentheses are optional for Subs with no return value
Greet "Alice"                ' valid (no parens)

' passing arguments by position
Console.WriteLine(Add(1, 2))

' Sub with multiple statements
Sub PrintReport(title As String, lines As Integer)
    Console.WriteLine(New String("-"c, 40))
    Console.WriteLine(title)
    Console.WriteLine(New String("-"c, 40))
    For i As Integer = 1 To lines
        Console.WriteLine($"Line {i}")
    Next
End Sub

PrintReport("Monthly Report", 5)

' ByVal (default): pass by value (copy)
Sub ResetValue(ByVal x As Integer)
    x = 0   ' only changes the copy
End Sub

' ByRef: pass by reference (can modify caller's variable)
Sub Increment(ByRef x As Integer)
    x += 1   ' modifies the original
End Sub

Dim n As Integer = 5
ResetValue(n)
Console.WriteLine(n)   ' 5 (unchanged)
Increment(n)
Console.WriteLine(n)   ' 6 (modified)

' Optional parameters (must have default values, must be last)
Function Power(base As Double, Optional exp As Double = 2) As Double
    Return Math.Pow(base, exp)
End Function

Console.WriteLine(Power(3))      ' 9 (exp defaults to 2)
Console.WriteLine(Power(2, 10))  ' 1024

' ParamArray: variable number of arguments (must be last)
Function Sum(ParamArray nums() As Integer) As Integer
    Dim total As Integer = 0
    For Each n In nums
        total += n
    Next
    Return total
End Function

Console.WriteLine(Sum(1, 2, 3))         ' 6
Console.WriteLine(Sum(1, 2, 3, 4, 5))   ' 15
Console.WriteLine(Sum())                 ' 0

' named arguments (improve readability)
Sub CreateAccount(name As String, age As Integer, active As Boolean)
    ' ...
End Sub
CreateAccount(name:="Bob", active:=True, age:=25)

' single-line lambda (Function returns a value)
Dim square As Func(Of Integer, Integer) = Function(x) x * x
Console.WriteLine(square(5))   ' 25

' multi-line lambda
Dim factorial As Func(Of Integer, Integer) = Function(n)
    Dim result As Integer = 1
    For i As Integer = 2 To n
        result *= i
    Next
    Return result
End Function
Console.WriteLine(factorial(5))   ' 120

' Sub lambda (no return value)
Dim log As Action(Of String) = Sub(msg) Console.WriteLine($"[LOG] {msg}")
log("Hello")

' lambda with multiple parameters
Dim add As Func(Of Integer, Integer, Integer) = Function(a, b) a + b
Console.WriteLine(add(3, 4))   ' 7

' use lambdas with LINQ
Dim nums = {1, 2, 3, 4, 5}
Dim evens = nums.Where(Function(n) n Mod 2 = 0).ToList()
Dim doubled = nums.Select(Function(n) n * 2).ToList()
Dim total = nums.Sum(Function(n) n)

' delegate definition
Delegate Function MathOp(a As Integer, b As Integer) As Integer

' assign methods to delegates
Dim op As MathOp
op = AddressOf AddNumbers    ' AddNumbers is a regular Function
Console.WriteLine(op(3, 4))  ' 7

' Action and Func (built-in delegate types)
Dim print As Action(Of String) = AddressOf Console.WriteLine
print("Hello")

' define an event
Public Class TemperatureSensor
    Public Event TemperatureChanged(temp As Double)

    Private _current As Double

    Public Property Current As Double
        Get
            Return _current
        End Get
        Set(value As Double)
            If value <> _current Then
                _current = value
                RaiseEvent TemperatureChanged(value)   ' fire the event
            End If
        End Set
    End Property
End Class

' subscribe to an event
Module SensorDemo
    WithEvents sensor As New TemperatureSensor()

    Sub Main()
        sensor.Current = 20.5   ' triggers handler
        sensor.Current = 21.0   ' triggers handler
    End Sub

    ' event handler (must match the event signature)
    Sub OnTempChanged(temp As Double) Handles sensor.TemperatureChanged
        Console.WriteLine($"Temperature changed to {temp:F1}°C")
    End Sub
End Module

' alternative: AddHandler (dynamic, runtime)
Module DynamicDemo
    Sub Main()
        Dim s As New TemperatureSensor()
        AddHandler s.TemperatureChanged, AddressOf HandleTemp
        s.Current = 25.0
        RemoveHandler s.TemperatureChanged, AddressOf HandleTemp
    End Sub

    Sub HandleTemp(temp As Double)
        Console.WriteLine($"Got: {temp}")
    End Sub
End Module

' overloading: same name, different parameters
Class Calculator
    Overloads Function Add(a As Integer, b As Integer) As Integer
        Return a + b
    End Function

    Overloads Function Add(a As Double, b As Double) As Double
        Return a + b
    End Function

    Overloads Function Add(values() As Integer) As Integer
        Dim total As Integer = 0
        For Each v In values
            total += v
        Next
        Return total
    End Function
End Class

' recursion: function calls itself
Function Factorial(n As Integer) As Long
    If n <= 1 Then Return 1
    Return n * Factorial(n - 1)
End Function
Console.WriteLine(Factorial(5))   ' 120

' Fibonacci (inefficient — for demo)
Function Fib(n As Integer) As Integer
    If n < 2 Then Return n
    Return Fib(n - 1) + Fib(n - 2)
End Function

' Shared (static) members: belong to the class, not instances
Class MathHelper
    Public Shared ReadOnly Pi As Double = 3.14159265358979

    Public Shared Function CircleArea(radius As Double) As Double
        Return Pi * radius * radius
    End Function
End Class

' call without creating an instance
Console.WriteLine(MathHelper.Pi)
Console.WriteLine(MathHelper.CircleArea(5))

' Shared constructor (runs once, before any access)
Class Config
    Shared Sub New()
        Console.WriteLine("Config initialized")
    End Sub
End Class

Dim s As String = "Hello, World"

' properties
Console.WriteLine(s.Length)              ' 12
Console.WriteLine(s(0))                  ' H (indexer, read-only)

' comparison
Console.WriteLine("abc" = "abc")         ' True (= is equality in VB)
Console.WriteLine("abc".Equals("abc"))   ' True
Console.WriteLine(String.Equals("a", "A", StringComparison.OrdinalIgnoreCase))  ' True

' case conversion (returns new string — strings are immutable)
Console.WriteLine(s.ToUpper())           ' HELLO, WORLD
Console.WriteLine(s.ToLower())           ' hello, world

' searching
Console.WriteLine(s.IndexOf("o"))        ' 4 (first occurrence)
Console.WriteLine(s.LastIndexOf("o"))    ' 8
Console.WriteLine(s.Contains("World"))   ' True
Console.WriteLine(s.StartsWith("Hello")) ' True
Console.WriteLine(s.EndsWith("World"))   ' True

' substring
Console.WriteLine(s.Substring(7))        ' World
Console.WriteLine(s.Substring(0, 5))     ' Hello

' split and join
Dim parts() As String = "a,b,c,d".Split(","c)
Console.WriteLine(String.Join("-", parts))   ' a-b-c-d

' replace and trim
Console.WriteLine(s.Replace("o", "0"))   ' Hell0, W0rld
Console.WriteLine("  hi  ".Trim())       ' hi
Console.WriteLine("  hi  ".TrimStart())  ' hi<spaces>
Console.WriteLine("xxhelloxx".Trim("x"c)) ' hello

Imports System.Text

' StringBuilder: mutable string for efficient concatenation
Dim sb As New StringBuilder()
For i As Integer = 1 To 1000
    sb.Append("Line ").Append(i).AppendLine()   ' chainable
Next
Dim result As String = sb.ToString()

' StringBuilder vs & concatenation
' BAD: creates 1000 intermediate strings
Dim bad As String = ""
For i As Integer = 1 To 1000
    bad &= $"Line {i}"   ' creates new string each time
Next

' GOOD: StringBuilder mutates in place
Dim good As New StringBuilder()
For i As Integer = 1 To 1000
    good.AppendLine($"Line {i}")
Next

' useful StringBuilder methods
Dim sb2 As New StringBuilder("Hello")
sb2.Append(", World")        ' append string
sb2.AppendLine()             ' append newline
sb2.AppendFormat("Count: {0}", 42)  ' formatted append
sb2.Insert(0, ">> ")         ' insert at position
sb2.Remove(0, 3)             ' remove 3 chars at position 0
sb2.Replace("World", "VB")   ' replace substring
Console.WriteLine(sb2.ToString())
Console.WriteLine(sb2.Length)

' pre-allocate capacity for known sizes
Dim sb3 As New StringBuilder(10000)   ' initial capacity

Dim name As String = "Alice"
Dim age As Integer = 30
Dim salary As Decimal = 50000.5D
Dim today As Date = Date.Now

' interpolated string ($ prefix)
Console.WriteLine($"Name: {name}, Age: {age}")
Console.WriteLine($"Next year: {age + 1}")          ' expressions
Console.WriteLine($"Upper: {name.ToUpper()}")        ' method calls

' format specifiers in interpolation
Console.WriteLine($"Salary: {salary:C}")             ' $50,000.50
Console.WriteLine($"Salary: {salary:N2}")            ' 50,000.50
Console.WriteLine($"Pi: {Math.PI:F4}")               ' 3.1416
Console.WriteLine($"Date: {today:yyyy-MM-dd}")       ' 2024-06-18
Console.WriteLine($"Time: {today:HH:mm:ss}")         ' 14:30:00
Console.WriteLine($"Hex: {255:X}")                   ' FF
Console.WriteLine($"Padded: {42:D5}")                ' 00042
Console.WriteLine($"Percent: {0.25:P0}")             ' 25%

' alignment in interpolation
Console.WriteLine($"{"Name",-10} | {"Age",5}")       ' left/right align
Console.WriteLine($"{name,-10} | {age,5}")

' String.Format (positional, older style)
Dim msg As String = String.Format("{0} is {1} years old", name, age)

' composite formatting with format string
Console.WriteLine(String.Format("Price: {0:C2}", 19.99))

' custom date formats
Console.WriteLine(today.ToString("dddd, MMMM d, yyyy"))
Console.WriteLine(today.ToString("yyyy/MM/dd HH:mm"))

' culture-specific formatting
Dim ci As Globalization.CultureInfo = New Globalization.CultureInfo("fr-FR")
Console.WriteLine(salary.ToString("C", ci))   ' 50 000,50 €

Imports System.Text.RegularExpressions

' basic matching
Dim pattern As String = "d+"   ' one or more digits
Dim input As String = "abc 123 def 456"

If Regex.IsMatch(input, pattern) Then
    Console.WriteLine("Contains digits")
End If

' extract matches
For Each m As Match In Regex.Matches(input, "d+")
    Console.WriteLine(m.Value)   ' 123, 456
Next

' single match with groups
Dim m2 As Match = Regex.Match("2024-06-18", "(d{4})-(d{2})-(d{2})")
If m2.Success Then
    Console.WriteLine(m2.Groups(1).Value)   ' 2024 (year)
    Console.WriteLine(m2.Groups(2).Value)   ' 06 (month)
    Console.WriteLine(m2.Groups(3).Value)   ' 18 (day)
End If

' replace
Dim cleaned As String = Regex.Replace("phone: 123-456-7890", "D", "")
Console.WriteLine(cleaned)   ' 1234567890 (digits only)

' replace with match reference ($1, $2)
Dim swapped As String = Regex.Replace("John Doe", "(w+) (w+)", "$2, $1")
Console.WriteLine(swapped)   ' Doe, John

' split
Dim parts() As String = Regex.Split("a,b;;c", "[,;]+")

' common patterns
Dim emailPattern As String = "^[w.-]+@[w.-]+.w+$"
Dim urlPattern As String = "^https?://[w./-]+$"
Dim phonePattern As String = "^d{3}-d{3}-d{4}$"

' compiled regex (faster for repeated use)
Dim rx As New Regex("d+", RegexOptions.Compiled)
Dim count As Integer = rx.Matches(input).Count

' Char basics
Dim c As Char = "A"c          ' char literal (c suffix)
Console.WriteLine(Char.IsDigit("5"c))      ' True
Console.WriteLine(Char.IsLetter("A"c))     ' True
Console.WriteLine(Char.IsWhiteSpace(" "c)) ' True
Console.WriteLine(Char.ToUpper("a"c))      ' A
Console.WriteLine(Char.ToLower("A"c))      ' a

' convert between char and ASCII code
Dim code As Integer = Asc("A"c)    ' 65
Dim ch As Char = Chr(66)           ' B

' iterate characters in a string
Dim s As String = "Hello"
For Each c In s
    Console.Write($"{Asc(c)} ")    ' 72 101 108 108 111
Next

' char array <-> string
Dim chars() As Char = s.ToCharArray()
Array.Reverse(chars)
Dim reversed As String = New String(chars)
Console.WriteLine(reversed)   ' olleH

' encoding: string <-> bytes
Dim text As String = "Hello"
Dim bytes() As Byte = System.Text.Encoding.UTF8.GetBytes(text)
Console.WriteLine(bytes.Length)   ' 5

Dim decoded As String = System.Text.Encoding.UTF8.GetString(bytes)

' ASCII encoding (1 byte per char, no Unicode)
Dim asciiBytes() As Byte = System.Text.Encoding.ASCII.GetBytes(text)

' Base64 encoding (for binary data in text)
Dim b64 As String = Convert.ToBase64String(bytes)
Dim original() As Byte = Convert.FromBase64String(b64)

' StringBuilder from char array
Dim sb As New System.Text.StringBuilder(New String(chars))

' fixed-size array
Dim nums(4) As Integer          ' 5 elements (0-4), default 0
nums(0) = 10
nums(1) = 20

' array literal
Dim fruits() As String = {"apple", "banana", "cherry"}
Dim nums2() As Integer = {1, 2, 3, 4, 5}

' type inference
Dim mixed = {1, 2, 3}           ' Integer()
Dim names = {"Alice", "Bob"}    ' String()

' indexing (0-based)
Console.WriteLine(fruits(0))         ' apple
Console.WriteLine(fruits.Length)     ' 3
Console.WriteLine(nums2.GetUpperBound(0))  ' 4 (last index)

' multi-dimensional arrays
Dim grid(2, 2) As Integer      ' 3x3
grid(0, 0) = 1
grid(1, 2) = 5

' jagged arrays (array of arrays)
Dim jagged()() As Integer = {
    New Integer() {1, 2},
    New Integer() {3, 4, 5},
    New Integer() {6}
}
Console.WriteLine(jagged(1)(2))   ' 5

' Array class methods
Array.Sort(nums2)                    ' sort in place
Array.Reverse(nums2)                 ' reverse
Array.Clear(nums2, 0, 2)             ' set first 2 to 0
Dim found As Integer = Array.IndexOf(nums2, 3)   ' search
Dim copy(4) As Integer
Array.Copy(nums2, copy, 5)           ' copy

' resize (creates new array)
ReDim Preserve nums2(9)              ' resize, keep existing values
' ReDim without Preserve clears values

' iterate
For Each n As Integer In nums2
    Console.WriteLine(n)
Next

Imports System.Collections.Generic

' create and initialize
Dim nums As New List(Of Integer) From {1, 2, 3}
Dim names As New List(Of String)

' add and insert
nums.Add(4)                    ' add to end
nums.AddRange({5, 6, 7})       ' add multiple
nums.Insert(0, 0)              ' insert at index
Console.WriteLine(nums.Count)  ' 8

' access and modify
Console.WriteLine(nums(0))     ' 0 (indexer)
nums(0) = 100                  ' modify

' remove
nums.Remove(3)                 ' remove by value (first match)
nums.RemoveAt(0)               ' remove by index
nums.RemoveRange(0, 2)         ' remove 2 starting at index 0
nums.Clear()                   ' remove all

' search
nums = New List(Of Integer) From {1, 2, 3, 4, 5}
Console.WriteLine(nums.Contains(3))           ' True
Console.WriteLine(nums.IndexOf(3))            ' 2
Dim found As Boolean = nums.Exists(Function(n) n > 3)
Dim first As Integer = nums.Find(Function(n) n > 3)        ' 4
Dim all As List(Of Integer) = nums.FindAll(Function(n) n > 2)  ' 3,4,5

' iterate
For Each n As Integer In nums
    Console.WriteLine(n)
Next

' convert to array
Dim arr() As Integer = nums.ToArray()

' sort and reverse
nums.Sort()
nums.Reverse()

' List with custom objects
Dim people As New List(Of (Name As String, Age As Integer))
people.Add(("Alice", 30))
people.Add(("Bob", 25))
people.Sort(Function(a, b) a.Age.CompareTo(b.Age))   ' sort by age

Imports System.Collections.Generic

' Dictionary: key-value pairs
Dim ages As New Dictionary(Of String, Integer) From {
    {"Alice", 30},
    {"Bob", 25}
}
ages("Eve") = 28               ' add or update
ages.Add("Carol", 22)          ' add (throws if key exists)

' access
Console.WriteLine(ages("Alice"))           ' 30
Console.WriteLine(ages.ContainsKey("Bob")) ' True
Console.WriteLine(ages.ContainsValue(28))  ' True

' safe access (TryGetValue avoids double lookup)
Dim age As Integer
If ages.TryGetValue("Alice", age) Then
    Console.WriteLine(age)   ' 30
End If

' iterate
For Each kv As KeyValuePair(Of String, Integer) In ages
    Console.WriteLine($"{kv.Key}: {kv.Value}")
Next

' remove
ages.Remove("Bob")

' HashSet: unique elements
Dim unique As New HashSet(Of Integer) From {1, 2, 3}
unique.Add(2)                 ' no effect (already exists)
unique.Add(4)
Console.WriteLine(unique.Count)  ' 4

' set operations
Dim setA As New HashSet(Of Integer) From {1, 2, 3}
Dim setB As New HashSet(Of Integer) From {3, 4, 5}
setA.UnionWith(setB)          ' {1,2,3,4,5}
setA.IntersectWith(setB)      ' {3}
setA.ExceptWith(setB)         ' remove setB from setA

' Queue: FIFO
Dim q As New Queue(Of String)
q.Enqueue("first")
q.Enqueue("second")
Console.WriteLine(q.Dequeue())  ' first
Console.WriteLine(q.Peek())     ' second (without removing)

' Stack: LIFO
Dim stk As New Stack(Of Integer)
stk.Push(1)
stk.Push(2)
Console.WriteLine(stk.Pop())    ' 2
Console.WriteLine(stk.Peek())   ' 1

Imports System.Linq

Dim nums = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
Dim people = {
    New With {.Name = "Alice", .Age = 30, .City = "NYC"},
    New With {.Name = "Bob", .Age = 25, .City = "LA"},
    New With {.Name = "Carol", .Age = 35, .City = "NYC"}
}

' query syntax (SQL-like)
Dim adults = From p In people
             Where p.Age >= 30
             Select p.Name
' Alice, Carol

Dim nycNames = From p In people
               Where p.City = "NYC"
               Order By p.Age
               Select p.Name, p.Age

' method syntax (fluent, with lambdas)
Dim evens = nums.Where(Function(n) n Mod 2 = 0).ToList()       ' 2,4,6,8,10
Dim doubled = nums.Select(Function(n) n * 2).ToList()
Dim sum = nums.Sum()                                            ' 55
Dim avg = nums.Average()                                        ' 5.5
Dim max = nums.Max()
Dim count = nums.Count(Function(n) n > 5)                       ' 5

' ordering
Dim sorted = nums.OrderBy(Function(n) n).ToList()
Dim descSorted = nums.OrderByDescending(Function(n) n).ToList()

' grouping
Dim byCity = people.GroupBy(Function(p) p.City)
For Each g In byCity
    Console.WriteLine($"{g.Key}: {g.Count()} people")
Next

' take / skip (pagination)
Dim first3 = nums.Take(3).ToList()        ' 1,2,3
Dim skip3 = nums.Skip(3).Take(3).ToList() ' 4,5,6

' first / single / elementAt
Dim first = nums.First()                          ' 1
Dim firstEven = nums.First(Function(n) n Mod 2 = 0)  ' 2
Dim maybeFirst = nums.FirstOrDefault(Function(n) n > 100)  ' 0 (default)

' any / all
Dim hasEven = nums.Any(Function(n) n Mod 2 = 0)   ' True
Dim allPositive = nums.All(Function(n) n > 0)     ' True

' aggregate
Dim product = nums.Aggregate(Function(acc, n) acc * n)  ' factorial-like

Imports System.Collections.Generic

' collection initializers (From clause)
Dim nums As New List(Of Integer) From {1, 2, 3, 4, 5}
Dim dict As New Dictionary(Of String, Integer) From {
    {"one", 1},
    {"two", 2},
    {"three", 3}
}
Dim set_ As New HashSet(Of String) From {"a", "b", "c"}

' tuples (VB 2017+)
Dim point As (X As Integer, Y As Integer) = (X := 3, Y := 4)
Console.WriteLine(point.X)   ' 3
Console.WriteLine(point.Y)   ' 4

' tuple without named fields (Item1, Item2, ...)
Dim t As (String, Integer) = ("Alice", 30)
Console.WriteLine(t.Item1)   ' Alice
Console.WriteLine(t.Item2)   ' 30

' tuple as function return (multiple values)
Function GetStats(nums As List(Of Integer)) As (Count As Integer, Sum As Integer, Avg As Double)
    Return (
        nums.Count,
        nums.Sum(),
        nums.Average()
    )
End Function

Dim stats = GetStats(nums)
Console.WriteLine($"Count={stats.Count}, Sum={stats.Sum}, Avg={stats.Avg:F2}")

' deconstruct tuples
Dim (name, age) = ("Bob", 25)
Console.WriteLine($"{name}, {age}")

' list of tuples
Dim people As New List(Of (Name As String, Age As Integer)) From {
    ("Alice", 30),
    ("Bob", 25)
}

' iterate with index
For i As Integer = 0 To nums.Count - 1
    Console.WriteLine($"{i}: {nums(i)}")
Next

' LINQ to dictionary
Dim nameToAge = people.ToDictionary(Function(p) p.Name, Function(p) p.Age)
Console.WriteLine(nameToAge("Alice"))   ' 30

Public Class Person
    ' private field (backing store)
    Private _name As String
    Private _age As Integer

    ' auto-implemented property (compiler creates the field)
    Public Property Email As String

    ' property with custom logic
    Public Property Name As String
        Get
            Return _name
        End Get
        Set(value As String)
            If String.IsNullOrEmpty(value) Then
                Throw New ArgumentException("Name required")
            End If
            _name = value
        End Set
    End Property

    ' property with validation
    Public Property Age As Integer
        Get
            Return _age
        End Get
        Set(value As Integer)
            If value < 0 OrElse value > 150 Then
                Throw New ArgumentOutOfRangeException("Age")
            End If
            _age = value
        End Set
    End Property

    ' read-only property (computed)
    Public ReadOnly Property IsAdult As Boolean
        Get
            Return _age >= 18
        End Get
    End Property

    ' constructor
    Public Sub New(name As String, age As Integer)
        Me.Name = name    ' Me refers to the current instance
        Me.Age = age
    End Sub
End Class

' usage
Dim p As New Person("Alice", 30)
Console.WriteLine(p.Name)      ' Alice
Console.WriteLine(p.IsAdult)   ' True
p.Age = 31                     ' uses setter
' p.Age = 200                  ' throws (validation)

' base class
Public Class Animal
    Public Property Name As String

    Public Sub New(name As String)
        Me.Name = name
    End Sub

    Public Overridable Function Speak() As String
        Return Name & " makes a sound"
    End Function

    Public Overridable Sub Eat()
        Console.WriteLine(Name & " is eating")
    End Sub
End Class

' derived class (Inherits)
Public Class Dog
    Inherits Animal

    Public Property Breed As String

    ' call base constructor with MyBase
    Public Sub New(name As String, breed As String)
        MyBase.New(name)      ' must be first line
        Me.Breed = breed
    End Sub

    ' override virtual method
    Public Overrides Function Speak() As String
        Return Name & " says Woof"
    End Function

    ' extend base method
    Public Overrides Sub Eat()
        MyBase.Eat()          ' call base implementation
        Console.WriteLine(Name & " wags tail")
    End Sub

    ' new method (not override — hides base)
    Public Shadows Sub Description()
        Console.WriteLine($"{Name} ({Breed})")
    End Sub
End Class

Dim d As New Dog("Rex", "Labrador")
Console.WriteLine(d.Speak())   ' Rex says Woof
d.Eat()                        ' Rex is eating / Rex wags tail

' polymorphism: base reference, derived object
Dim a As Animal = New Dog("Buddy", "Poodle")
Console.WriteLine(a.Speak())   ' Buddy says Woof (virtual dispatch)

' interface: contract (no implementation)
Public Interface IComparable(Of T)
    Function CompareTo(other As T) As Integer
End Interface

Public Interface IDrawable
    Sub Draw()
    Property X As Integer
    Property Y As Integer
End Interface

' implement an interface
Public Class Circle
    Implements IDrawable

    Public Property X As Integer Implements IDrawable.X
    Public Property Y As Integer Implements IDrawable.Y
    Public Property Radius As Integer

    Public Sub Draw() Implements IDrawable.Draw
        Console.WriteLine($"Drawing circle at ({X}, {Y}) r={Radius}")
    End Sub
End Class

Public Class Square
    Implements IDrawable

    Public Property X As Integer Implements IDrawable.X
    Public Property Y As Integer Implements IDrawable.Y
    Public Property Side As Integer

    Public Sub Draw() Implements IDrawable.Draw
        Console.WriteLine($"Drawing square at ({X}, {Y}) side={Side}")
    End Sub
End Class

' polymorphism via interface
Dim shapes As New List(Of IDrawable) From {
    New Circle With {.X = 0, .Y = 0, .Radius = 5},
    New Square With {.X = 10, .Y = 10, .Side = 4}
}
For Each s In shapes
    s.Draw()   ' calls the right implementation
Next

' check and cast
Dim shape As IDrawable = New Circle()
If TypeOf shape Is Circle Then
    Dim c As Circle = DirectCast(shape, Circle)
    c.Radius = 10
End If

' multiple interfaces
Public Class TextBox
    Implements IDrawable, IComparable(Of TextBox)
    ' ... implement both
End Class

' generic class
Public Class Stack(Of T)
    Private items As New List(Of T)

    Public Sub Push(item As T)
        items.Add(item)
    End Sub

    Public Function Pop() As T
        If items.Count = 0 Then Throw New InvalidOperationException("Empty")
        Dim last As T = items(items.Count - 1)
        items.RemoveAt(items.Count - 1)
        Return last
    End Function

    Public ReadOnly Property Count As Integer
        Get
            Return items.Count
        End Get
    End Property
End Class

' use with different types
Dim intStack As New Stack(Of Integer)
intStack.Push(1)
intStack.Push(2)
Console.WriteLine(intStack.Pop())   ' 2

Dim strStack As New Stack(Of String)
strStack.Push("Hello")
Console.WriteLine(strStack.Pop())   ' Hello

' generic method with constraint
Public Class Repository(Of T As {Class, New})
    ' T must be a reference type (Class) and have a parameterless constructor (New)
    Private items As New List(Of T)

    Public Sub Add(item As T)
        items.Add(item)
    End Sub

    Public Function Find(match As Predicate(Of T)) As T
        Return items.Find(match)
    End Function
End Class

' generic function
Function FirstOrDefault(Of T)(list As IEnumerable(Of T), predicate As Func(Of T, Boolean)) As T
    For Each item In list
        If predicate(item) Then Return item
    Next
    Return Nothing    ' default for reference types
End Function

' constraints:
'   Class      - reference type
'   Structure  - value type
'   New        - has parameterless constructor
'   BaseClass  - inherits from a specific class
'   Interface  - implements an interface

' File1.vb
Namespace MyApp.Models
    Partial Public Class User
        Public Property Id As Integer
        Public Property Name As String
        Public Property Email As String
    End Class
End Namespace

' File2.vb (same class, different file)
Namespace MyApp.Models
    Partial Public Class User
        Public Sub SendWelcomeEmail()
            Console.WriteLine($"Welcome {Name}!")
        End Sub

        Public Function Validate() As Boolean
            Return Not String.IsNullOrEmpty(Email)
        End Function
    End Class
End Namespace

' usage (combines both files)
Dim u As New MyApp.Models.User With {
    .Id = 1,
    .Name = "Alice",
    .Email = "[email protected]"
}
u.SendWelcomeEmail()
Console.WriteLine(u.Validate())   ' True

' namespace nesting
Namespace MyApp.Services
    Public Class UserService
        Public Sub Create(name As String)
            ' ...
        End Sub
    End Class
End Namespace

' import to use short names
Imports MyApp.Models
Imports MyApp.Services

Module Program
    Sub Main()
        Dim user As New User()        ' no full namespace needed
        Dim svc As New UserService()
    End Sub
End Module

Try
    Dim x As Integer = 10
    Dim y As Integer = 0
    Dim z As Integer = x \ y          ' integer division by zero
Catch ex As DivideByZeroException
    Console.WriteLine("Cannot divide by zero: " & ex.Message)
Catch ex As OverflowException
    Console.WriteLine("Number too large: " & ex.Message)
Catch ex As Exception                  ' catch-all (must be last)
    Console.WriteLine("Unexpected error: " & ex.Message)
Finally
    ' always runs (even with Return or exception)
    Console.WriteLine("Cleanup complete")
End Try

' nested Try
Try
    Try
        Dim arr() As Integer = {1, 2, 3}
        Console.WriteLine(arr(10))     ' IndexOutOfRangeException
    Catch ex As IndexOutOfRangeException
        Console.WriteLine("Index error: " & ex.Message)
        Throw                          ' re-throw to outer handler
    End Try
Catch ex As Exception
    Console.WriteLine("Outer caught: " & ex.Message)
End Try

' When filter (conditional catch)
Try
    ' risky code
Catch ex As Exception When ex.Message.Contains("network")
    Console.WriteLine("Network error, retrying...")
End Try

' exception properties
Try
    Dim n As Integer = Integer.Parse("abc")
Catch ex As FormatException
    Console.WriteLine(ex.Message)        ' human-readable
    Console.WriteLine(ex.GetType().Name) ' FormatException
    Console.WriteLine(ex.StackTrace)     ' where it happened
    Console.WriteLine(ex.Source)         ' which assembly
End Try

' throw a built-in exception
Sub CheckAge(age As Integer)
    If age < 0 Then
        Throw New ArgumentOutOfRangeException(NameOf(age), "Age cannot be negative")
    ElseIf age > 150 Then
        Throw New ArgumentOutOfRangeException(NameOf(age), "Age unrealistic")
    End If
End Sub

' throw with inner exception (wrapping)
Function LoadConfig(path As String) As String
    Try
        Return File.ReadAllText(path)
    Catch ex As FileNotFoundException
        Throw New ConfigurationException("Config not found", ex)   ' wrap
    End Try
End Function

' custom exception (inherit from Exception)
<Serializable>
Public Class InvalidTransactionException
    Inherits Exception

    Public Property TransactionId As String

    Public Sub New(message As String, transactionId As String)
        MyBase.New(message)
        Me.TransactionId = transactionId
    End Sub

    Public Sub New(message As String, transactionId As String, inner As Exception)
        MyBase.New(message, inner)
        Me.TransactionId = transactionId
    End Sub

    Public Overrides Function ToString() As String
        Return $"Transaction {TransactionId} failed: {Message}"
    End Function
End Class

' using the custom exception
Sub ProcessPayment(amount As Decimal, txnId As String)
    If amount <= 0 Then
        Throw New InvalidTransactionException("Amount must be positive", txnId)
    End If
    ' ... process
End Sub

' catch custom exception
Try
    ProcessPayment(-100, "TXN-001")
Catch ex As InvalidTransactionException
    Console.WriteLine($"Failed: {ex.TransactionId} - {ex.Message}")
End Try

Imports System.IO

' Using ensures Dispose is called (even on exception)
Using writer As New StreamWriter("output.txt")
    writer.WriteLine("Hello")
    writer.WriteLine("World")
End Using   ' writer.Dispose() called automatically

' equivalent to:
Dim writer2 As StreamWriter = Nothing
Try
    writer2 = New StreamWriter("output.txt")
    writer2.WriteLine("Hello")
Finally
    writer2?.Dispose()
End Using

' multiple resources in one Using
Using conn As New SqlClient.SqlConnection(connStr),
      cmd As New SqlClient.SqlCommand("SELECT 1", conn)
    conn.Open()
    Dim result = cmd.ExecuteScalar()
End Using

' implement IDisposable for your class
Public Class FileManager
    Implements IDisposable

    Private stream As FileStream
    Private disposed As Boolean = False

    Public Sub New(path As String)
        stream = New FileStream(path, FileMode.Open)
    End Sub

    Public Function ReadLine() As String
        ' ... read from stream
    End Function

    Protected Overridable Sub Dispose(disposing As Boolean)
        If Not disposed Then
            If disposing Then
                ' free managed resources
                stream?.Dispose()
            End If
            ' free unmanaged resources (if any)
            disposed = True
        End If
    End Sub

    Public Sub Dispose() Implements IDisposable.Dispose
        Dispose(True)
        GC.SuppressFinalize(Me)
    End Sub
End Class

' usage
Using fm As New FileManager("data.txt")
    Dim line = fm.ReadLine()
End Using

' 1. Don't catch exceptions you can't handle
' BAD: swallows all errors
Try
    File.Delete(path)
Catch ex As Exception
    ' silent failure — bug hidden!
End Try

' GOOD: only catch what you expect
Try
    File.Delete(path)
Catch ex As FileNotFoundException
    ' file already gone — that's fine
Catch ex As UnauthorizedAccessException
    Console.WriteLine("Permission denied: " & ex.Message)
    ' re-throw or handle meaningfully
End Try

' 2. Log exceptions (don't swallow)
Try
    ProcessData()
Catch ex As Exception
    Logger.Error(ex, "ProcessData failed")
    Throw   ' re-throw after logging
End Try

' 3. Validate input (avoid exceptions for expected conditions)
' BAD: exception for control flow
Try
    Dim n = Integer.Parse(input)
Catch ex As FormatException
    n = 0
End Try

' GOOD: TryParse
Dim n As Integer
If Not Integer.TryParse(input, n) Then
    n = 0
    Console.WriteLine("Invalid number, using 0")
End If

' 4. Use specific exception types
If amount < 0 Then
    Throw New ArgumentOutOfRangeException(NameOf(amount))
End If

' 5. Exception filters (When) for logging without catching
Try
    RiskyOperation()
Catch ex As Exception When LogError(ex)
    ' only enters if LogError returns True
End Try

Function LogError(ex As Exception) As Boolean
    Logger.Error(ex)
    Return False   ' don't catch, just log
End Function

Imports System.Diagnostics

' Debug.Assert: only in Debug builds
Debug.Assert(age >= 0, "Age should be non-negative")

' Debug.WriteLine: only in Debug builds
Debug.WriteLine($"Processing {name}, age {age}")

' Trace: works in Release builds too
Trace.WriteLine("App started")
Trace.WriteLineIf(age > 100, "Unusual age")

' conditional compilation
#If DEBUG Then
    Console.WriteLine("Debug build")
#Else
    Console.WriteLine("Release build")
#End If

' Debugger.Break: pause in debugger (like a breakpoint)
If count > 1000 Then
    Debugger.Break()
End If

' Stopwatch for performance measurement
Dim sw As Stopwatch = Stopwatch.StartNew()
For i As Integer = 1 To 1000000
    ' some work
Next
sw.Stop()
Console.WriteLine($"Elapsed: {sw.ElapsedMilliseconds} ms")

' StackTrace for debugging
Dim st As New StackTrace(True)
Console.WriteLine(st.ToString())   ' current call stack

' EventLog for Windows event logging (requires admin)
If Not EventLog.SourceExists("MyApp") Then
    EventLog.CreateEventSource("MyApp", "Application")
End If
EventLog.WriteEntry("MyApp", "Started", EventLogEntryType.Information)

' process info
Dim p As Process = Process.GetCurrentProcess()
Console.WriteLine($"Memory: {p.WorkingSet64 \ 1024} KB")
Console.WriteLine($"PID: {p.Id}")

Imports System.IO

' write all text at once (simple)
File.WriteAllText("output.txt", "Hello, World!")

' append text
File.AppendAllText("log.txt", $"[{Date.Now}] Started{Environment.NewLine}")

' read all text
Dim content As String = File.ReadAllText("output.txt")

' read all lines into an array
Dim lines() As String = File.ReadAllLines("data.csv")
For Each line In lines
    Console.WriteLine(line)
Next

' write all lines
File.WriteAllLines("nums.txt", {"one", "two", "three"})

' append lines
File.AppendAllLines("log.txt", {"line1", "line2"})

' stream-based writing (for large files)
Using writer As New StreamWriter("big.txt")
    For i As Integer = 1 To 1000000
        writer.WriteLine($"Line {i}")
    Next
End Using   ' flushes and closes automatically

' stream-based reading (line by line, memory-efficient)
Using reader As New StreamReader("big.txt")
    Dim line As String
    Do
        line = reader.ReadLine()
        If line Is Nothing Then Exit Do
        Console.WriteLine(line)
    Loop
End Using

' check if file exists
If File.Exists("data.txt") Then
    Console.WriteLine("Found")
End If

' File.ReadAllLinesAsync (VB 15+ with async)
Async Function ReadAsync(path As String) As Task(Of String())
    Return Await File.ReadAllLinesAsync(path)
End Function

Imports System.IO
Imports System.Runtime.Serialization.Formatters.Binary

' write binary data
Using fs As New FileStream("data.bin", FileMode.Create)
    Using writer As New BinaryWriter(fs)
        writer.Write(42)              ' Integer
        writer.Write(3.14)            ' Double
        writer.Write("Hello")         ' String (length-prefixed)
        writer.Write(True)            ' Boolean
        writer.Write(New Byte() {1, 2, 3})  ' byte array
    End Using
End Using

' read binary data (must read in same order!)
Using fs As New FileStream("data.bin", FileMode.Open)
    Using reader As New BinaryReader(fs)
        Dim n As Integer = reader.ReadInt32()
        Dim d As Double = reader.ReadDouble()
        Dim s As String = reader.ReadString()
        Dim b As Boolean = reader.ReadBoolean()
        Dim bytes() As Byte = reader.ReadBytes(3)
        Console.WriteLine($"{n}, {d}, {s}, {b}")
    End Using
End Using

' JSON serialization (Newtonsoft.Json or System.Text.Json)
Imports System.Text.Json

Public Class User
    Public Property Name As String
    Public Property Age As Integer
End Class

Dim u As New User With {.Name = "Alice", .Age = 30}
Dim json As String = JsonSerializer.Serialize(u)
' {"Name":"Alice","Age":30}
File.WriteAllText("user.json", json)

Dim restored As User = JsonSerializer.Deserialize(Of User)(json)

' JSON with options
Dim opts As New JsonSerializerOptions With {
    .WriteIndented = True,
    .PropertyNamingPolicy = JsonNamingPolicy.CamelCase
}
Dim prettyJson = JsonSerializer.Serialize(u, opts)

Imports System.IO

' directory operations
Directory.CreateDirectory("backup/2024/june")

' list files
Dim files() As String = Directory.GetFiles("C:\temp")
Dim csvFiles() As String = Directory.GetFiles("C:\temp", "*.csv")
Dim allFiles() As String = Directory.GetFiles("C:\temp", "*.*", SearchOption.AllDirectories)

' list directories
Dim dirs() As String = Directory.GetDirectories("C:\temp")

' check and delete
If Directory.Exists("old") Then
    Directory.Delete("old", recursive := True)   ' delete with contents
End If

' Path class (cross-platform path handling)
Dim fullPath As String = Path.Combine("folder", "sub", "file.txt")
' folder\sub\file.txt (on Windows)

Console.WriteLine(Path.GetFileName("C:\temp\data.txt"))    ' data.txt
Console.WriteLine(Path.GetExtension("photo.JPG"))            ' .JPG
Console.WriteLine(Path.GetFileNameWithoutExtension("data.txt"))  ' data
Console.WriteLine(Path.GetDirectoryName("C:\temp\data.txt"))   ' C:\temp
Console.WriteLine(Path.GetFullPath("data.txt"))              ' absolute path

' temp files
Dim tempFile As String = Path.GetTempFileName()   ' creates a temp file
Dim tempDir As String = Path.GetTempPath()
Console.WriteLine(tempFile)

' file info
Dim fi As New FileInfo("data.txt")
If fi.Exists Then
    Console.WriteLine($"Size: {fi.Length} bytes")
    Console.WriteLine($"Created: {fi.CreationTime}")
    Console.WriteLine($"Modified: {fi.LastWriteTime}")
    Console.WriteLine($"Extension: {fi.Extension}")
    fi.CopyTo("data_backup.txt", overwrite := True)
    fi.MoveTo("renamed.txt")
    fi.Delete()
End If

' directory info
Dim di As New DirectoryInfo("C:\temp")
For Each f As FileInfo In di.GetFiles("*.txt")
    Console.WriteLine(f.Name)
Next

Imports System.IO
Imports Microsoft.VisualBasic.FileIO

' simple CSV split (doesn't handle quoted commas)
Dim lines() As String = File.ReadAllLines("data.csv")
For Each line In lines
    Dim fields() As String = line.Split(","c)
    Console.WriteLine($"{fields(0)}, {fields(1)}")
Next

' robust CSV with TextFieldParser (handles quotes, commas)
Using parser As New TextFieldParser("data.csv")
    parser.TextFieldType = FieldType.Delimited
    parser.Delimiters = New String() {","}
    parser.HasFieldsEnclosedInQuotes = True

    ' skip header
    If Not parser.EndOfData Then parser.ReadLine()

    While Not parser.EndOfData
        Dim fields() As String = parser.ReadFields()
        Console.WriteLine($"Name: {fields(0)}, Age: {fields(1)}")
    End While
End Using

' write CSV
Using writer As New StreamWriter("output.csv")
    writer.WriteLine("Name,Age,City")
    writer.WriteLine("Alice,30,NYC")
    writer.WriteLine("Bob,25,LA")
    ' quote fields with commas
    writer.WriteLine($"""Smith, John"",40,Chicago")
End Using

' parse fixed-width files
Using parser As New TextFieldParser("fixed.txt")
    parser.TextFieldType = FieldType.FixedWidth
    parser.FieldWidths = New Integer() {10, 5, 20}
End Using

' read with My.Computer.FileSystem (VB-specific shortcut)
Dim contents As String = My.Computer.FileSystem.ReadAllText("data.txt")
Dim allLines As Object = My.Computer.FileSystem.ReadAllBytes("data.bin")

' write bytes
File.WriteAllBytes("data.bin", New Byte() {1, 2, 3, 4, 5})

Imports System.IO
Imports System.Threading.Tasks

' async read (non-blocking)
Async Function ReadFileAsync(path As String) As Task(Of String)
    Using reader As New StreamReader(path)
        Return Await reader.ReadToEndAsync()
    End Using
End Function

' async write
Async Function WriteFileAsync(path As String, content As String) As Task
    Using writer As New StreamWriter(path)
        Await writer.WriteAsync(content)
    End Using
End Function

' async line-by-line (memory-efficient for large files)
Async Function ProcessLargeFileAsync(path As String) As Task
    Using reader As New StreamReader(path)
        Dim line As String
        Do
            line = Await reader.ReadLineAsync()
            If line Is Nothing Then Exit Do
            ' process line
            Console.WriteLine(line)
        Loop
    End Using
End Function

' call async methods (must be in an Async Sub/Function)
Async Sub Main()
    Dim content As String = Await ReadFileAsync("data.txt")
    Console.WriteLine(content)

    Await WriteFileAsync("output.txt", "Async write")
End Sub

' parallel file processing
Async Function ProcessFilesAsync(paths As String()) As Task
    Dim tasks As New List(Of Task)
    For Each path In paths
        tasks.Add(ProcessFileAsync(path))
    Next
    Await Task.WhenAll(tasks)   ' wait for all
End Function

Async Function ProcessFileAsync(path As String) As Task
    Dim content = Await File.ReadAllTextAsync(path)
    ' process content
End Function

' File.ReadAllLinesAsync (.NET Core+)
Async Function CountLinesAsync(path As String) As Task(Of Integer)
    Dim lines = Await File.ReadAllLinesAsync(path)
    Return lines.Length
End Function

Imports System.Windows.Forms
Imports System.Drawing

Public Class MainForm
    Inherits Form

    Public Sub New()
        Me.Text = "My App"             ' form title
        Me.Size = New Size(400, 300)
        Me.StartPosition = FormStartPosition.CenterScreen

        ' create a button
        Dim btn As New Button With {
            .Text = "Click Me",
            .Location = New Point(150, 100),
            .Size = New Size(100, 30)
        }
        AddHandler btn.Click, AddressOf OnButtonClick
        Me.Controls.Add(btn)

        ' create a label
        Dim lbl As New Label With {
            .Text = "Hello",
            .Location = New Point(150, 50),
            .AutoSize = True
        }
        Me.Controls.Add(lbl)
    End Sub

    Private Sub OnButtonClick(sender As Object, e As EventArgs)
        MessageBox.Show("Button clicked!", "Info",
                        MessageBoxButtons.OK, MessageBoxIcon.Information)
    End Sub

    ' form events
    Protected Overrides Sub OnLoad(e As EventArgs)
        MyBase.OnLoad(e)
        Console.WriteLine("Form loaded")
    End Sub

    Protected Overrides Sub OnFormClosing(e As FormClosingEventArgs)
        Dim result = MessageBox.Show("Close?", "Confirm",
                                     MessageBoxButtons.YesNo)
        If result = DialogResult.No Then
            e.Cancel = True
        End If
        MyBase.OnFormClosing(e)
    End Sub
End Class

' run the app
Module Program
    Sub Main()
        Application.EnableVisualStyles()
        Application.Run(New MainForm())
    End Sub
End Module

Imports System.Windows.Forms
Imports System.Drawing

Public Class ControlsForm
    Inherits Form

    Public Sub New()
        ' TextBox (single-line input)
        Dim txt As New TextBox With {
            .Location = New Point(20, 20),
            .Width = 200
        }

        ' multiline TextBox
        Dim txtMulti As New TextBox With {
            .Multiline = True,
            .Location = New Point(20, 60),
            .Size = New Size(200, 80),
            .ScrollBars = ScrollBars.Vertical
        }

        ' CheckBox
        Dim chk As New CheckBox With {
            .Text = "Enable feature",
            .Location = New Point(20, 160),
            .Checked = True
        }

        ' RadioButton (group in GroupBox for mutual exclusion)
        Dim grp As New GroupBox With {.Text = "Gender", .Location = New Point(20, 190), .Size = New Size(120, 80)}
        Dim rb1 As New RadioButton With {.Text = "Male", .Location = New Point(10, 20), .Checked = True}
        Dim rb2 As New RadioButton With {.Text = "Female", .Location = New Point(10, 40)}
        grp.Controls.AddRange({rb1, rb2})

        ' ComboBox (dropdown)
        Dim cmb As New ComboBox With {.Location = New Point(150, 190), .DropDownStyle = ComboBoxStyle.DropDownList}
        cmb.Items.AddRange({"Red", "Green", "Blue"})
        cmb.SelectedIndex = 0

        ' NumericUpDown
        Dim num As New NumericUpDown With {
            .Location = New Point(150, 220),
            .Minimum = 0,
            .Maximum = 100,
            .Value = 50
        }

        ' ListBox
        Dim lst As New ListBox With {.Location = New Point(20, 280), .Size = New Size(150, 80)}
        lst.Items.AddRange({"Apple", "Banana", "Cherry"})

        Me.Controls.AddRange({txt, txtMulti, chk, grp, cmb, num, lst})
    End Sub
End Class

Imports System.Windows.Forms
Imports System.Drawing

Public Class LayoutForm
    Inherits Form

    Public Sub New()
        Me.Size = New Size(600, 400)

        ' Panel: container for grouping controls
        Dim topPanel As New Panel With {
            .Dock = DockStyle.Top,        ' fill the top
            .Height = 50,
            .BackColor = Color.LightBlue
        }

        Dim bottomPanel As New Panel With {
            .Dock = DockStyle.Bottom,
            .Height = 40,
            .BackColor = Color.LightGray
        }

        ' Fill remaining space
        Dim centerPanel As New Panel With {
            .Dock = DockStyle.Fill,
            .BackColor = Color.White
        }

        ' Add in reverse Z-order (Fill last to actually fill)
        Me.Controls.Add(centerPanel)
        Me.Controls.Add(bottomPanel)
        Me.Controls.Add(topPanel)

        ' Anchoring: control resizes with parent edges
        Dim btn As New Button With {
            .Text = "Anchored",
            .Location = New Point(10, 10),
            .Anchor = AnchorStyles.Top Or AnchorStyles.Left Or AnchorStyles.Right
        }
        centerPanel.Controls.Add(btn)

        ' TableLayoutPanel: grid layout
        Dim tlp As New TableLayoutPanel With {
            .Dock = DockStyle.Fill,
            .ColumnCount = 2,
            .RowCount = 2
        }
        tlp.ColumnStyles.Add(New ColumnStyle(SizeType.Percent, 50))
        tlp.ColumnStyles.Add(New ColumnStyle(SizeType.Percent, 50))
        tlp.Controls.Add(New Label With {.Text = "Name:"}, 0, 0)
        tlp.Controls.Add(New TextBox(), 1, 0)
        tlp.Controls.Add(New Label With {.Text = "Age:"}, 0, 1)
        tlp.Controls.Add(New NumericUpDown(), 1, 1)

        ' FlowLayoutPanel: stacks controls left-to-right or top-to-bottom
        Dim flp As New FlowLayoutPanel With {
            .FlowDirection = FlowDirection.LeftToRight,
            .Dock = DockStyle.Top,
            .Height = 40
        }
        flp.Controls.AddRange({New Button With {.Text = "New"}, New Button With {.Text = "Open"}})

        Me.Controls.Add(flp)
        Me.Controls.Add(tlp)
    End Sub
End Class

Imports System.Windows.Forms

Public Class DialogForm
    Inherits Form

    Private Sub OpenFile()
        Using ofd As New OpenFileDialog
            ofd.Title = "Select a file"
            ofd.Filter = "Text files (*.txt)|*.txt|CSV files (*.csv)|*.csv|All files (*.*)|*.*"
            ofd.FilterIndex = 1
            ofd.Multiselect = False
            ofd.InitialDirectory = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments)

            If ofd.ShowDialog() = DialogResult.OK Then
                Dim path As String = ofd.FileName
                MessageBox.Show($"Selected: {path}")
            End If
        End Using
    End Sub

    Private Sub SaveFile()
        Using sfd As New SaveFileDialog
            sfd.Filter = "Text files (*.txt)|*.txt|All files (*.*)|*.*"
            sfd.DefaultExt = "txt"
            sfd.AddExtension = True
            sfd.OverwritePrompt = True    ' ask before overwriting

            If sfd.ShowDialog() = DialogResult.OK Then
                System.IO.File.WriteAllText(sfd.FileName, "Saved content")
            End If
        End Using
    End Sub

    Private Sub PickColor()
        Using cd As New ColorDialog
            cd.AllowFullOpen = True
            cd.FullOpen = True
            cd.Color = Color.Red

            If cd.ShowDialog() = DialogResult.OK Then
                Me.BackColor = cd.Color
            End If
        End Using
    End Sub

    Private Sub PickFont()
        Using fd As New FontDialog
            fd.ShowColor = True
            fd.Font = Me.Font

            If fd.ShowDialog() = DialogResult.OK Then
                Me.Font = fd.Font
            End If
        End Using
    End Sub

    Private Sub PickFolder()
        Using fbd As New FolderBrowserDialog
            fbd.Description = "Select a folder"
            fbd.SelectedPath = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments)

            If fbd.ShowDialog() = DialogResult.OK Then
                MessageBox.Show($"Folder: {fbd.SelectedPath}")
            End If
        End Using
    End Sub
End Class

Imports System.Windows.Forms
Imports System.ComponentModel

Public Class User
    Implements INotifyPropertyChanged

    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged

    Private _name As String
    Public Property Name As String
        Get
            Return _name
        End Get
        Set(value As String)
            If _name <> value Then
                _name = value
                RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(NameOf(Name)))
            End If
        End Set
    End Property

    Public Property Age As Integer
End Class

Public Class DataForm
    Inherits Form

    Public Sub New()
        ' simple binding: TextBox <-> object property
        Dim user As New User With {.Name = "Alice", .Age = 30}
        Dim txt As New TextBox With {.Location = New Point(20, 20)}
        txt.DataBindings.Add("Text", user, "Name")   ' two-way binding

        ' DataGridView with a list
        Dim people As New BindingList(Of User) From {
            New User With {.Name = "Alice", .Age = 30},
            New User With {.Name = "Bob", .Age = 25}
        }

        Dim dgv As New DataGridView With {
            .Location = New Point(20, 60),
            .Size = New Size(350, 200),
            .AutoGenerateColumns = True,
            .AllowUserToAddRows = True,
            .AllowUserToDeleteRows = True,
            .AutoSizeColumnsMode = DataGridViewAutoSizeColumnsMode.Fill
        }
        dgv.DataSource = people   ' bind to list

        ' BindingSource for navigation
        Dim bs As New BindingSource With {.DataSource = people}
        dgv.DataSource = bs

        ' DataTable binding
        Dim dt As New DataTable()
        dt.Columns.Add("Name", GetType(String))
        dt.Columns.Add("Age", GetType(Integer))
        dt.Rows.Add("Carol", 28)
        dt.Rows.Add("Dave", 35)
        dgv.DataSource = dt

        Me.Controls.Add(txt)
        Me.Controls.Add(dgv)
    End Sub
End Class

Imports System.IO

' write text file
Using writer As New StreamWriter("output.txt")
    writer.WriteLine("First line")
    writer.WriteLine("Second line")
    writer.Write("No newline ")
    writer.WriteLine("appended")
End Using  ' automatically disposes/closes

' append to file
Using writer As New StreamWriter("log.txt", append:=True)
    writer.WriteLine($"{Date.Now}: Application started")
End Using

' read all lines
Dim lines() As String = File.ReadAllLines("input.txt")
For Each line In lines
    Console.WriteLine(line)
Next

' read line by line (memory efficient for big files)
Using reader As New StreamReader("big.txt")
    Dim line As String
    Do While reader.Peek() >= 0
        line = reader.ReadLine()
        ' process line
    Loop
End Using

' read all text at once
Dim content As String = File.ReadAllText("input.txt")

' async I/O (non-blocking)
Dim text As String = Await File.ReadAllTextAsync("input.txt")
Await File.WriteAllTextAsync("output.txt", "Hello")

Imports System.IO

' binary file with BinaryWriter/Reader
Using fs As New FileStream("data.bin", FileMode.Create),
      bw As New BinaryWriter(fs)
    bw.Write(42)              ' Integer
    bw.Write(3.14)            ' Double
    bw.Write("Hello")         ' String (length-prefixed)
    bw.Write(True)            ' Boolean
End Using

Using fs As New FileStream("data.bin", FileMode.Open),
      br As New BinaryReader(fs)
    Dim num As Integer = br.ReadInt32()
    Dim pi As Double = br.ReadDouble()
    Dim s As String = br.ReadString()
    Dim flag As Boolean = br.ReadBoolean()
End Using

' CSV parsing (manual)
Dim csvLines = File.ReadAllLines("data.csv")
For Each line In csvLines.Skip(1)  ' skip header
    Dim fields = line.Split(","c)
    Dim name = fields(0)
    Dim age = Integer.Parse(fields(1))
Next

' CSV with TextFieldParser (handles quoted fields)
Using parser As New Microsoft.VisualBasic.FileIO.TextFieldParser("data.csv")
    parser.TextFieldType = FileIO.FieldType.Delimited
    parser.SetDelimiters(",")
    While Not parser.EndOfData
        Dim fields() As String = parser.ReadFields()
    End While
End Using

Imports System.IO

' file operations
File.Copy("source.txt", "dest.txt", overwrite:=True)
File.Move("old.txt", "new.txt")
File.Delete("unwanted.txt")
File.Exists("check.txt")          ' Boolean

' file info
Dim fi As New FileInfo("data.txt")
Console.WriteLine($"Size: {fi.Length} bytes")
Console.WriteLine($"Created: {fi.CreationTime}")
Console.WriteLine($"Modified: {fi.LastWriteTime}")
Console.WriteLine($"Extension: {fi.Extension}")

' directory operations
Directory.CreateDirectory("path	o
ewdir")
Directory.Exists("somedir")
Directory.Delete("dir", recursive:=True)

' enumerate files (lazy, memory-efficient)
For Each path In Directory.EnumerateFiles("C:data", "*.txt", SearchOption.AllDirectories)
    Console.WriteLine(path)
Next

' directory info
Dim di As New DirectoryInfo("C:data")
For Each file In di.GetFiles("*.csv")
    Console.WriteLine($"{file.Name} - {file.Length}")
Next

' path operations
Path.Combine("C:", "data", "file.txt")  ' "C:dataile.txt"
Path.GetExtension("file.txt")           ' ".txt"
Path.GetFileNameWithoutExtension("file.txt")  ' "file"
Path.GetTempFileName()                  ' temp file path

Imports System.Text.Json

' define a class
Public Class Person
    Public Property Name As String
    Public Property Age As Integer
    Public Property Email As String
End Class

' JSON serialize
Dim p As New Person With {.Name = "Alice", .Age = 30, .Email = "[email protected]"}
Dim json As String = JsonSerializer.Serialize(p)
File.WriteAllText("person.json", json)

' JSON with options
Dim opts As New JsonSerializerOptions With {
    .WriteIndented = True,
    .PropertyNamingPolicy = JsonNamingPolicy.CamelCase
}
json = JsonSerializer.Serialize(p, opts)

' JSON deserialize
Dim text = File.ReadAllText("person.json")
Dim p2 = JsonSerializer.Deserialize(Of Person)(text)

' serialize a list
Dim people = New List(Of Person) From {p, p2}
json = JsonSerializer.Serialize(people)

' XML serialization
Imports System.Xml.Serialization
Dim xs As New XmlSerializer(GetType(Person))
Using fs As New FileStream("person.xml", FileMode.Create)
    xs.Serialize(fs, p)
End Using
Using fs As New FileStream("person.xml", FileMode.Open)
    Dim p3 = CType(xs.Deserialize(fs), Person)
End Using

Imports System.IO

' async copy with progress
Async Function CopyWithProgress(src As String, dest As String, progress As IProgress(Of Long)) As Task
    Const bufferSize As Integer = 81920
    Dim buffer(bufferSize - 1) As Byte
    Using srcStream As FileStream = File.OpenRead(src),
          destStream As FileStream = File.Create(dest)
        Dim read As Integer
        Dim total As Long = 0
        Do
            read = Await srcStream.ReadAsync(buffer, 0, bufferSize)
            If read = 0 Then Exit Do
            Await destStream.WriteAsync(buffer, 0, read)
            total += read
            progress?.Report(total)
        Loop
    End Using
End Function

' usage with progress bar
Dim prog As New Progress(Of Long)(Sub(bytes)
    ProgressBar1.Value = CInt(bytes * 100  totalSize)
End Sub)
Await CopyWithProgress("big.zip", "copy.zip", prog)

' memory stream (in-memory)
Using ms As New MemoryStream()
    Dim bw As New BinaryWriter(ms)
    bw.Write("data")
    ms.Position = 0
    ' read back
End Using

' GZip compression
Imports System.IO.Compression
Using src = File.OpenRead("data.txt"),
      dest = File.Create("data.txt.gz"),
      gz As New GZipStream(dest, CompressionMode.Compress)
    src.CopyTo(gz)
End Using

Imports System.Data.SqlClient  ' or Microsoft.Data.SqlClient

' connection string (SQL Server)
Dim connStr = "Server=localhost;Database=MyDb;Integrated Security=True;"

' basic query
Using conn As New SqlConnection(connStr)
    conn.Open()
    Using cmd As New SqlCommand("SELECT Id, Name FROM Users WHERE Age > @age", conn)
        cmd.Parameters.AddWithValue("@age", 25)
        Using reader As SqlDataReader = cmd.ExecuteReader()
            While reader.Read()
                Dim id = reader.GetInt32(0)
                Dim name = reader.GetString(1)
                Console.WriteLine($"{id}: {name}")
            End While
        End Using
    End Using
End Using  ' connection auto-closed

' insert with parameters (prevent SQL injection!)
Using conn As New SqlConnection(connStr), _
      cmd As New SqlCommand("INSERT INTO Users (Name, Age) VALUES (@name, @age)", conn)
    cmd.Parameters.AddWithValue("@name", "Bob")
    cmd.Parameters.AddWithValue("@age", 30)
    conn.Open()
    Dim rowsAffected = cmd.ExecuteNonQuery()
End Using

' scalar query (single value)
Using conn As New SqlConnection(connStr), _
      cmd As New SqlCommand("SELECT COUNT(*) FROM Users", conn)
    conn.Open()
    Dim count = CInt(cmd.ExecuteScalar())
End Using

Imports System.Data.SqlClient

' fill DataTable
Dim dt As New DataTable()
Using adapter As New SqlDataAdapter("SELECT * FROM Products", connStr)
    adapter.Fill(dt)
End Using

' access rows
For Each row As DataRow In dt.Rows
    Console.WriteLine(row("Name"))
    Console.WriteLine(row.Field(Of Integer)("Price"))
Next

' modify and update
Dim newRow = dt.NewRow()
newRow("Name") = "Widget"
newRow("Price") = 9.99
dt.Rows.Add(newRow)

Using adapter As New SqlDataAdapter("SELECT * FROM Products", connStr),
      builder As New SqlCommandBuilder(adapter)
    adapter.Update(dt)  ' generates INSERT/UPDATE/DELETE
End Using

' typed DataSet (strongly typed)
' add .xsd file to project, drag tables from Server Explorer
' Dim ta As New ProductsTableAdapter()
' Dim products = ta.GetData()
' For Each p In products
'     Console.WriteLine(p.ProductName)
' Next

' filter and sort
Dim view As New DataView(dt)
view.RowFilter = "Price > 10"
view.Sort = "Price DESC"
For Each row As DataRowView In view
    Console.WriteLine(row("Name"))
Next

Imports System.Data.SqlClient

' explicit transaction
Using conn As New SqlConnection(connStr)
    conn.Open()
    Using tran As SqlTransaction = conn.BeginTransaction()
        Try
            Using cmd1 As New SqlCommand(
                "UPDATE Accounts SET Balance = Balance - 100 WHERE Id = 1",
                conn, tran)
                cmd1.ExecuteNonQuery()
            End Using
            Using cmd2 As New SqlCommand(
                "UPDATE Accounts SET Balance = Balance + 100 WHERE Id = 2",
                conn, tran)
                cmd2.ExecuteNonQuery()
            End Using
            tran.Commit()
        Catch ex As Exception
            tran.Rollback()
            Throw
        End Try
    End Using
End Using

' transaction scope (distributed, multi-resource)
Imports System.Transactions
Using scope As New TransactionScope()
    ' multiple connections, even different databases
    Using conn1 As New SqlConnection(connStr1)
        ' operations on DB1
    End Using
    Using conn2 As New SqlConnection(connStr2)
        ' operations on DB2
    End Using
    scope.Complete()  ' commit all
End Using  ' rollback if Complete not called

' connection pooling (automatic, configure in connection string)
' "Server=...;Pooling=True;Max Pool Size=100;Connection Lifetime=300"
' connections are reused — always Close/Dispose to return to pool

Imports System.Data.SqlClient

' async query
Async Function GetUsersAsync(minAge As Integer) As Task(Of List(Of User))
    Dim users As New List(Of User)()
    Using conn As New SqlConnection(connStr)
        Await conn.OpenAsync()
        Using cmd As New SqlCommand("SELECT Id, Name, Age FROM Users WHERE Age > @age", conn)
            cmd.Parameters.AddWithValue("@age", minAge)
            Using reader = Await cmd.ExecuteReaderAsync()
                While Await reader.ReadAsync()
                    users.Add(New User With {
                        .Id = reader.GetInt32(0),
                        .Name = reader.GetString(1),
                        .Age = reader.GetInt32(2)
                    })
                End While
            End Using
        End Using
    End Using
    Return users
End Function

' async execute (INSERT/UPDATE)
Async Function UpdateUserAsync(user As User) As Task(Of Integer)
    Using conn As New SqlConnection(connStr)
        Await conn.OpenAsync()
        Using cmd As New SqlCommand(
            "UPDATE Users SET Name=@name, Age=@age WHERE Id=@id", conn)
            cmd.Parameters.AddWithValue("@name", user.Name)
            cmd.Parameters.AddWithValue("@age", user.Age)
            cmd.Parameters.AddWithValue("@id", user.Id)
            Return Await cmd.ExecuteNonQueryAsync()
        End Using
    End Using
End Function

' usage in event handler
Private Async Sub btnLoad_Click(sender As Object, e As EventArgs) Handles btnLoad.Click
    btnLoad.Enabled = False
    Try
        Dim users = Await GetUsersAsync(25)
        DataGridView1.DataSource = users
    Catch ex As Exception
        MessageBox.Show(ex.Message)
    Finally
        btnLoad.Enabled = True
    End Try
End Sub

Imports Dapper  ' NuGet: Install-Package Dapper
Imports System.Data.SqlClient

' simple query
Using conn As New SqlConnection(connStr)
    Dim users = conn.Query(Of User)("SELECT * FROM Users WHERE Age > @age",
                                     New With { .age = 25 })
    For Each u In users
        Console.WriteLine(u.Name)
    Next
End Using

' single row
Using conn As New SqlConnection(connStr)
    Dim user = conn.QuerySingleOrDefault(Of User)(
        "SELECT * FROM Users WHERE Id = @id", New With { .id = 42 })
End Using

' insert
Using conn As New SqlConnection(connStr)
    Dim sql = "INSERT INTO Users (Name, Age, Email) VALUES (@Name, @Age, @Email)"
    conn.Execute(sql, New With { .Name = "Alice", .Age = 30, .Email = "[email protected]" })
End Using

' bulk insert
Using conn As New SqlConnection(connStr)
    Dim users = New List(Of User) From {
        New User With {.Name = "A", .Age = 1},
        New User With {.Name = "B", .Age = 2}
    }
    conn.Execute("INSERT INTO Users (Name, Age) VALUES (@Name, @Age)", users)
End Using

' multi-mapping (join)
Dim sql = "SELECT * FROM Orders o INNER JOIN Users u ON o.UserId = u.Id"
Dim orders = conn.Query(Of Order, User, Order)(sql,
    Function(o, u)
        o.User = u
        Return o
    End Function, splitOn:="Id")

' stored procedure
Dim result = conn.Query(Of User)("sp_GetActiveUsers",
    commandType:=Data.CommandType.StoredProcedure)

Dim numbers = New List(Of Integer) From {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}

' query syntax (SQL-like)
Dim evens = From n In numbers
            Where n Mod 2 = 0
            Select n
            Order By n Descending

' method syntax (fluent)
Dim evens2 = numbers.Where(Function(n) n Mod 2 = 0).
                     OrderByDescending(Function(n) n)

' both produce the same result
For Each n In evens
    Console.WriteLine(n)  ' 10, 8, 6, 4, 2
Next

' projection (transform)
Dim squares = numbers.Select(Function(n) n * n)
Dim names = numbers.Select(Function(n) $"Number {n}")

' filtering with multiple conditions
Dim filtered = From n In numbers
               Where n > 3 AndAlso n < 8
               Select n

' type inference (Dim is required)
Dim result = From p In people
             Where p.Age >= 18
             Select p.Name, p.Age
             Order By Age

' deferred execution (query runs when iterated)
Dim query = numbers.Where(Function(n) n > 5)  ' not executed yet
numbers.Add(11)  ' query will include 11!
For Each n In query  ' executes now
Next

Dim people = New List(Of Person) From {
    New Person With {.Name = "Al", .Dept = "Eng", .Salary = 90000},
    New Person With {.Name = "Bo", .Dept = "Eng", .Salary = 70000},
    New Person With {.Name = "Cy", .Dept = "Sales", .Salary = 60000}
}

' aggregation
Dim total = people.Sum(Function(p) p.Salary)
Dim avg = people.Average(Function(p) p.Salary)
Dim max = people.Max(Function(p) p.Salary)
Dim count = people.Count(Function(p) p.Dept = "Eng")
Dim any = people.Any(Function(p) p.Salary > 80000)
Dim all = people.All(Function(p) p.Salary > 0)

' grouping
Dim byDept = From p In people
             Group p By p.Dept Into Group
             Select Dept, Count = Group.Count(), Avg = Group.Average(Function(x) x.Salary)

For Each g In byDept
    Console.WriteLine($"{g.Dept}: {g.Count} people, avg ${g.Avg}")
Next

' method syntax grouping
Dim grouped = people.GroupBy(Function(p) p.Dept)
For Each g In grouped
    Console.WriteLine($"{g.Key}: {g.Count()} people")
Next

' group with multiple aggregates
Dim stats = people.GroupBy(Function(p) p.Dept).
    Select(Function(g) New With {
        .Dept = g.Key,
        .Count = g.Count(),
        .Total = g.Sum(Function(p) p.Salary),
        .Min = g.Min(Function(p) p.Salary)
    })

Dim employees = New List(Of Employee) From {
    New Employee With {.Id = 1, .Name = "Al", .DeptId = 10},
    New Employee With {.Id = 2, .Name = "Bo", .DeptId = 20}
}
Dim departments = New List(Of Department) From {
    New Department With {.Id = 10, .Name = "Eng"},
    New Department With {.Id = 20, .Name = "Sales"},
    New Department With {.Id = 30, .Name = "HR"}
}

' inner join
Dim result = From e In employees
             Join d In departments On e.DeptId Equals d.Id
             Select e.Name, Dept = d.Name

' group join (left join with grouped results)
Dim leftJoin = From d In departments
               Group Join e In employees On d.Id Equals e.DeptId Into Group
               From e In Group.DefaultIfEmpty()
               Select Dept = d.Name, Emp = If(e Is Nothing, "Vacant", e.Name)

' method syntax join
Dim joined = employees.Join(departments,
    Function(e) e.DeptId,
    Function(d) d.Id,
    Function(e, d) New With { .Name = e.Name, .Dept = d.Name })

' zip (pair elements)
Dim nums = {1, 2, 3}
Dim words = {"one", "two", "three"}
Dim pairs = nums.Zip(words, Function(n, w) $"{n}={w}")

' set operations
Dim a = {1, 2, 3, 4}
Dim b = {3, 4, 5, 6}
Dim union = a.Union(b)        ' 1,2,3,4,5,6
Dim intersect = a.Intersect(b) ' 3,4
Dim except = a.Except(b)       ' 1,2
Dim concat = a.Concat(b)       ' 1,2,3,4,3,4,5,6
Dim distinct = concat.Distinct() ' 1,2,3,4,5,6

Dim nums = {5, 2, 8, 1, 9, 3, 7, 4, 6}

' sorting
Dim asc = nums.OrderBy(Function(n) n)
Dim desc = nums.OrderByDescending(Function(n) n)
Dim multi = people.OrderBy(Function(p) p.Dept).
                  ThenBy(Function(p) p.Name)
Dim multiDesc = people.OrderByDescending(Function(p) p.Dept).
                       ThenByDescending(Function(p) p.Salary)

' reverse
Dim reversed = nums.Reverse()

' paging (skip + take)
Dim page = nums.Skip(5).Take(3)   ' elements 6,7,8 (0-indexed)
Dim firstPage = nums.Take(5)       ' first 5

' element operations
Dim first = nums.First()           ' throws if empty
Dim firstOr = nums.FirstOrDefault() ' default (0) if empty
Dim last = nums.Last()
Dim single = nums.Single(Function(n) n = 5)  ' throws if not exactly one
Dim singleOr = nums.SingleOrDefault(Function(n) n = 99) ' 0 if not found

' element at position
Dim third = nums.ElementAt(2)       ' throws if out of range
Dim thirdOr = nums.ElementAtOrDefault(99)  ' 0 if out of range

' find index (not LINQ, but related)
Dim idx = Array.FindIndex(nums, Function(n) n > 5)

' partition
Dim large = nums.Where(Function(n) n > 5)
Dim small = nums.TakeWhile(Function(n) n < 8)  ' stops at first false
Dim skipSmall = nums.SkipWhile(Function(n) n < 8)

' custom extension method
Imports System.Runtime.CompilerServices

<Extension()>
Public Function WhereNot(Of T)(source As IEnumerable(Of T),
                                predicate As Func(Of T, Boolean)) As IEnumerable(Of T)
    Return source.Where(Function(x) Not predicate(x))
End Function

' usage
Dim odds = numbers.WhereNot(Function(n) n Mod 2 = 0)

' custom aggregation
<Extension()>
Public Function Median(source As IEnumerable(Of Double)) As Double
    Dim sorted = source.OrderBy(Function(x) x).ToList()
    Dim count = sorted.Count
    If count = 0 Then Return 0
    If count Mod 2 = 0 Then
        Return (sorted(count  2 - 1) + sorted(count  2)) / 2
    Else
        Return sorted(count  2)
    End If
End Function

' usage
Dim med = numbers.Median()

' IEnumerable vs IQueryable
' IEnumerable: LINQ to Objects (in-memory, delegates)
' IQueryable: LINQ to SQL/EF (translates to SQL, expression trees)

' force evaluation
Dim list = query.ToList()      ' List(Of T)
Dim array = query.ToArray()    ' T()
Dim dict = query.ToDictionary(Function(x) x.Id)
Dim lookup = query.ToLookup(Function(x) x.Category)

' PLINQ (parallel)
Dim parallel = nums.AsParallel().
    Where(Function(n) IsPrime(n)).
    Select(Function(n) n * 2).
    ToList()

Imports System.Threading.Tasks

' async function
Async Function DownloadAsync(url As String) As Task(Of String)
    Using client As New Net.Http.HttpClient()
        Return Await client.GetStringAsync(url)
    End Function
End Function

' call async from event handler
Private Async Sub btnDownload_Click(sender As Object, e As EventArgs) Handles btnDownload.Click
    btnDownload.Enabled = False
    Try
        Dim content = Await DownloadAsync("https://example.com")
        txtResult.Text = content
    Catch ex As Exception
        MessageBox.Show(ex.Message)
    Finally
        btnDownload.Enabled = True
    End Try
End Sub

' run CPU-bound work on background thread
Async Function ProcessDataAsync(data As Byte()) As Task(Of Result)
    Return Await Task.Run(Function()
        ' CPU-intensive work
        Return Analyze(data)
    End Function)
End Function

' Task.Run overloads
Dim t1 = Task.Run(Function() Compute(1))
Dim t2 = Task.Run(Async Function() Await FetchAsync())

' wait for multiple tasks
Dim tasks = New List(Of Task) From {t1, t2, t3}
Await Task.WhenAll(tasks)

' wait for any
Dim firstDone = Await Task.WhenAny(t1, t2, t3)

' task continuation
Dim result = Await t1.ContinueWith(Function(t) t.Result * 2)

Imports System.Threading

' lock (mutual exclusion)
Private ReadOnly _lock As New Object
Private _counter As Integer = 0

Sub Increment()
    SyncLock _lock
        _counter += 1
    End SyncLock
End Sub

' Monitor (equivalent to SyncLock)
Monitor.Enter(_lock)
Try
    _counter += 1
Finally
    Monitor.Exit(_lock)
End Try

' Interlocked (atomic operations, no lock needed)
Interlocked.Increment(_counter)
Interlocked.Add(_counter, 10)
Interlocked.Exchange(_counter, 0)
Interlocked.CompareExchange(_counter, 1, 0)  ' if 0, set to 1

' Mutex (cross-process)
Dim mutex As New Mutex(False, "GlobalMyAppMutex")
mutex.WaitOne()
Try
    ' critical section
Finally
    mutex.ReleaseMutex()
End Try

' SemaphoreSlim (limit concurrent access)
Dim sem As New SemaphoreSlim(3)  ' max 3 concurrent
Await sem.WaitAsync()
Try
    ' limited concurrency
Finally
    sem.Release()
End Try

' CountdownEvent
Dim cde As New CountdownEvent(5)
Parallel.For(0, 5, Sub(i)
    ' work
    cde.Signal()
End Sub)
cde.Wait()

Imports System.Threading.Tasks

' Parallel.For (CPU-bound parallelism)
Parallel.For(0, 1000, Sub(i)
    Process(i)
End Sub)

' Parallel.ForEach
Parallel.ForEach(files, Sub(f)
    Compress(f)
End Sub)

' with options
Dim opts As New ParallelOptions With {.MaxDegreeOfParallelism = 4}
Parallel.For(0, 1000, opts, Sub(i) Process(i))

' PLINQ
Dim results = numbers.AsParallel().
    Where(Function(n) IsPrime(n)).
    Select(Function(n) n * 2).
    ToArray()

' cancel parallel work
Dim cts As New CancellationTokenSource()
Parallel.For(0, 1000,
    New ParallelOptions With {.CancellationToken = cts.Token},
    Sub(i, state)
        If SomeCondition Then state.Break()
        Process(i)
    End Sub)

' BackgroundWorker (legacy, but simple for WinForms)
Dim bgw As New BackgroundWorker With {.WorkerReportsProgress = True, .WorkerSupportsCancellation = True}
AddHandler bgw.DoWork, Sub(sender, e)
    For i = 1 To 100
        If bgw.CancellationPending Then
            e.Cancel = True
            Return
        End If
        bgw.ReportProgress(i)
        Thread.Sleep(50)
    Next
End Sub
AddHandler bgw.ProgressChanged, Sub(sender, e)
    ProgressBar1.Value = e.ProgressPercentage
End Sub
AddHandler bgw.RunWorkerCompleted, Sub(sender, e)
    MessageBox.Show("Done!")
End Sub
bgw.RunWorkerAsync()

' Windows.Forms.Timer (UI thread, simple)
Dim timer1 As New Windows.Forms.Timer With {.Interval = 1000}
AddHandler timer1.Tick, Sub(sender, e)
    Label1.Text = Date.Now.ToString()
End Sub
timer1.Start()

' System.Threading.Timer (thread pool, not UI thread)
Dim timer2 As New Threading.Timer(
    Sub(state)
        ' runs on thread pool thread!
        ' must marshal to UI thread to update controls:
        Me.Invoke(Sub() Label1.Text = Date.Now.ToString())
    End Sub,
    Nothing, 0, 1000)

' System.Timers.Timer (component, fires on thread pool)
Dim timer3 As New Timers.Timer With {.Interval = 1000, .AutoReset = True}
AddHandler timer3.Elapsed, Sub(sender, e)
    Me.Invoke(Sub() Label1.Text = Date.Now.ToString())
End Sub
timer3.Start()

' UI thread marshaling
If Label1.InvokeRequired Then
    Label1.Invoke(Sub() Label1.Text = "updated")
Else
    Label1.Text = "updated"
End If

' async with ConfigureAwait
Await SomeAsync().ConfigureAwait(True)   ' capture context (default)
Await SomeAsync().ConfigureAwait(False)  ' don't capture (faster, but no UI access)

' SynchronizationContext for custom marshaling
Dim uiContext = SynchronizationContext.Current
uiContext.Post(Sub() Label1.Text = "done", Nothing)

Imports System.Collections.Concurrent

' ConcurrentDictionary (thread-safe dictionary)
Dim dict As New ConcurrentDictionary(Of String, Integer)
dict.TryAdd("a", 1)
dict.AddOrUpdate("a", 1, Function(key, old) old + 1)  ' atomic
Dim val = dict.GetOrAdd("b", Function(key) ComputeValue(key))

' ConcurrentQueue (FIFO, thread-safe)
Dim queue As New ConcurrentQueue(Of String)
queue.Enqueue("first")
Dim item As String
If queue.TryDequeue(item) Then
    Console.WriteLine(item)
End If

' ConcurrentStack (LIFO)
Dim stack As New ConcurrentStack(Of Integer)
stack.Push(1)
Dim popped As Integer
stack.TryPop(popped)

' ConcurrentBag (unordered, fast for producer=consumer)
Dim bag As New ConcurrentBag(Of Integer)
bag.Add(42)

' BlockingCollection (producer-consumer pattern)
Dim bc As New BlockingCollection(Of Integer)(boundedCapacity:=100)
' producer
Task.Run(Sub()
    For i = 1 To 1000
        bc.Add(i)  ' blocks if full
    Next
    bc.CompleteAdding()
End Sub)
' consumer
Task.Run(Sub()
    For Each x In bc.GetConsumingEnumerable()  ' blocks if empty
        Process(x)
    Next
End Sub)

' Channels (modern, high-performance)
' NuGet: System.Threading.Channels
Dim channel = System.Threading.Channels.Channel.CreateBounded(Of String)(100)
Await channel.Writer.WriteAsync("message")
Dim msg = Await channel.Reader.ReadAsync()

Imports Microsoft.Win32

' read a value
Dim value As String = Registry.GetValue(
    "HKEY_CURRENT_USERSoftwareMyApp",
    "Setting1",
    "default")  ' default if not found
Console.WriteLine(value)

' write a value
Registry.SetValue(
    "HKEY_CURRENT_USERSoftwareMyApp",
    "Setting1",
    "my value")

' using RegistryKey (more control)
Using key As RegistryKey = Registry.CurrentUser.OpenSubKey("SoftwareMyApp", writable:=True)
    If key IsNot Nothing Then
        Dim setting = key.GetValue("Setting1")
        key.SetValue("Setting2", 42, RegistryValueKind.DWord)
        key.DeleteValue("Setting2", throwOnMissingValue:=False)
    End If
End Using

' create subkey
Using key As RegistryKey = Registry.CurrentUser.CreateSubKey("SoftwareMyAppSub")
    key.SetValue("Name", "Test")
End Using

' enumerate subkeys and values
Using key As RegistryKey = Registry.LocalMachine.OpenSubKey("SOFTWARE")
    For Each subkeyName In key.GetSubKeyNames()
        Console.WriteLine(subkeyName)
    Next
    For Each valueName In key.GetValueNames()
        Console.WriteLine($"{valueName}: {key.GetValue(valueName)}")
    Next
End Using

' run on startup (add to Run key)
Registry.SetValue("HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionRun",
    "MyApp", Application.ExecutablePath)

Imports System.Runtime.InteropServices

Public Class NativeMethods
    ' MessageBox from user32.dll
    <DllImport("user32.dll", CharSet:=CharSet.Auto)>
    Public Shared Function MessageBox(hWnd As IntPtr, text As String,
        caption As String, type As Integer) As Integer
    End Function

    ' GetSystemMetrics
    <DllImport("user32.dll")>
    Public Shared Function GetSystemMetrics(nIndex As Integer) As Integer
    End Function

    ' Beep
    <DllImport("kernel32.dll")>
    Public Shared Function Beep(frequency As Integer, duration As Integer) As Boolean
    End Function

    ' SendMessage (for custom control messages)
    <DllImport("user32.dll", CharSet:=CharSet.Auto)>
    Public Shared Function SendMessage(hWnd As IntPtr, msg As Integer,
        wParam As IntPtr, lParam As IntPtr) As IntPtr
    End Function

    ' structures
    <StructLayout(LayoutKind.Sequential)>
    Public Structure POINT
        Public X As Integer
        Public Y As Integer
    End Structure

    <DllImport("user32.dll")>
    Public Shared Function GetCursorPos(ByRef lpPoint As POINT) As Boolean
    End Function
End Class

' usage
NativeMethods.MessageBox(IntPtr.Zero, "Hello from API!", "Test", 0)
NativeMethods.Beep(440, 500)  ' 440 Hz for 500ms

Dim p As NativeMethods.POINT
NativeMethods.GetCursorPos(p)
Console.WriteLine($"Cursor at {p.X}, {p.Y}")

' screen dimensions
Dim width = NativeMethods.GetSystemMetrics(0)   ' SM_CXSCREEN
Dim height = NativeMethods.GetSystemMetrics(1)  ' SM_CYSCREEN

Imports System.Diagnostics

' start a process
Process.Start("notepad.exe")
Process.Start("notepad.exe", "file.txt")
Process.Start("https://example.com")  ' opens default browser
Process.Start("mailto:[email protected]")

' with ProcessStartInfo (more control)
Dim psi As New ProcessStartInfo With {
    .FileName = "cmd.exe",
    .Arguments = "/c dir",
    .UseShellExecute = False,
    .RedirectStandardOutput = True,
    .CreateNoWindow = True
}
Using p As Process = Process.Start(psi)
    Dim output = p.StandardOutput.ReadToEnd()
    p.WaitForExit()
    Console.WriteLine(output)
End Using

' async read output
Using p As Process = Process.Start(psi)
    Dim output = Await p.StandardOutput.ReadToEndAsync()
    Await Task.Run(Sub() p.WaitForExit())
End Using

' get running processes
For Each proc In Process.GetProcesses()
    Console.WriteLine($"{proc.ProcessName} (PID {proc.Id})")
Next

' find specific process
Dim excel = Process.GetProcessesByName("EXCEL").FirstOrDefault()
If excel IsNot Nothing Then
    Console.WriteLine($"Excel memory: {excel.WorkingSet64  1024} KB")
End If

' run as admin
psi.Verb = "runas"  ' triggers UAC prompt
Process.Start(psi)

' environment variables
Dim path = Environment.GetEnvironmentVariable("PATH")
Environment.SetEnvironmentVariable("MY_VAR", "value")
Dim allVars = Environment.GetEnvironmentVariables()

' special folders
Dim appData = Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData)
Dim myDocs = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments)
Dim temp = Environment.GetFolderPath(Environment.SpecialFolder.Temp)
Dim programFiles = Environment.GetFolderPath(Environment.SpecialFolder.ProgramFiles)

' system info
Console.WriteLine($"Machine: {Environment.MachineName}")
Console.WriteLine($"User: {Environment.UserName}")
Console.WriteLine($"OS: {Environment.OSVersion}")
Console.WriteLine($"64-bit: {Environment.Is64BitOperatingSystem}")
Console.WriteLine($"Processors: {Environment.ProcessorCount}")
Console.WriteLine($".NET: {Environment.Version}")
Console.WriteLine($"Uptime: {Environment.TickCount64  1000} seconds")

' command line args
Dim args = Environment.GetCommandLineArgs()
' args(0) is the executable path

' current directory
Environment.CurrentDirectory = "C:	emp"

' system power
Console.WriteLine($"System uptime: {TimeSpan.FromMilliseconds(Environment.TickCount64)}")

' WMI for detailed hardware info
Imports System.Management
Dim searcher As New ManagementObjectSearcher("SELECT * FROM Win32_Processor")
For Each obj In searcher.Get()
    Console.WriteLine($"CPU: {obj("Name")}")
Next

' screen info
Dim screens = Screen.AllScreens
For Each s In screens
    Console.WriteLine($"{s.DeviceName}: {s.Bounds.Width}x{s.Bounds.Height}")
Next

' INI files (legacy but still common)
Imports System.Runtime.InteropServices

Public Class IniFile
    Private path As String

    <DllImport("kernel32.dll", CharSet:=CharSet.Unicode)>
    Private Shared Function WritePrivateProfileString(section As String,
        key As String, value As String, filePath As String) As Boolean
    End Function

    <DllImport("kernel32.dll", CharSet:=CharSet.Unicode)>
    Private Shared Function GetPrivateProfileString(section As String,
        key As String, defaultVal As String,
        <Out> ByVal retVal As StringBuilder, size As Integer,
        filePath As String) As Integer
    End Function

    Public Sub New(filePath As String)
        path = filePath
    End Sub

    Public Function Read(section As String, key As String, Optional defaultVal As String = "") As String
        Dim sb As New StringBuilder(255)
        GetPrivateProfileString(section, key, defaultVal, sb, 255, path)
        Return sb.ToString()
    End Function

    Public Sub Write(section As String, key As String, value As String)
        WritePrivateProfileString(section, key, value, path)
    End Sub
End Class

' usage
Dim ini As New IniFile("config.ini")
ini.Write("Settings", "Theme", "Dark")
Dim theme = ini.Read("Settings", "Theme", "Light")

' modern: app.config / appsettings.json
' app.config:
' <appSettings>
'   <add key="Theme" value="Dark" />
' </appSettings>
Dim theme2 = Configuration.ConfigurationManager.AppSettings("Theme")

' JSON config (with System.Configuration or Microsoft.Extensions.Configuration)
' appsettings.json:
' { "Theme": "Dark", "MaxItems": 100 }

Imports System.Collections.Generic

' List(Of T)
Dim list As New List(Of String) From {"a", "b", "c"}
list.Add("d")
list.AddRange({"e", "f"})
list.Insert(0, "first")
list.Remove("b")
list.RemoveAt(0)
list.Sort()
list.Reverse()
Dim count = list.Count
Dim hasA = list.Contains("a")
Dim idx = list.IndexOf("c")
Dim slice = list.GetRange(0, 2)  ' first 2 elements
Dim arr = list.ToArray()

' Dictionary(Of TKey, TValue)
Dim dict As New Dictionary(Of String, Integer)
dict.Add("one", 1)
dict("two") = 2
dict.TryGetValue("one", Dim val As Integer)  ' val = 1, returns True
For Each kvp In dict
    Console.WriteLine($"{kvp.Key} = {kvp.Value}")
Next
Dim keys = dict.Keys.ToList()
Dim vals = dict.Values.ToList()

' HashSet(Of T) (unique elements, O(1) lookup)
Dim set1 As New HashSet(Of Integer) From {1, 2, 3}
Dim set2 As New HashSet(Of Integer) From {3, 4, 5}
set1.Add(3)  ' no effect (already exists)
Dim union = New HashSet(Of Integer)(set1) : union.UnionWith(set2)  ' 1,2,3,4,5
Dim inter = New HashSet(Of Integer)(set1) : inter.IntersectWith(set2)  ' 3
Dim diff = New HashSet(Of Integer)(set1) : diff.ExceptWith(set2)  ' 1,2

' SortedDictionary / SortedList (sorted by key)
Dim sorted As New SortedDictionary(Of String, Integer)
sorted("zebra") = 1
sorted("apple") = 2  ' iterates in sorted order

' LinkedList (fast insert/remove at ends)
Dim ll As New LinkedList(Of Integer)
ll.AddFirst(1)
ll.AddLast(2)

Imports System.Collections.Generic
Imports System.Collections.ObjectModel

' Queue(Of T) (FIFO)
Dim queue As New Queue(Of String)
queue.Enqueue("first")
queue.Enqueue("second")
Dim next1 = queue.Dequeue()  ' "first"
Dim peek = queue.Peek()       ' "second" (doesn't remove)
Dim hasItems = queue.Count > 0

' Stack(Of T) (LIFO)
Dim stack As New Stack(Of Integer)
stack.Push(1)
stack.Push(2)
stack.Push(3)
Dim top = stack.Pop()   ' 3
Dim peek = stack.Peek() ' 2

' ObservableCollection (notifies on changes - for data binding)
Dim people As New ObservableCollection(Of Person)()
AddHandler people.CollectionChanged, Sub(sender, e)
    If e.Action = NotifyCollectionChangedAction.Add Then
        Console.WriteLine($"Added: {CType(e.NewItems(0), Person).Name}")
    End If
End Sub
people.Add(New Person With {.Name = "Alice"})  ' triggers event

' BindingList(Of T) (editable, for DataGridView)
Dim list As New BindingList(Of Person)()
list.AddingNew = Function() New Person()
list.RaiseListChangedEvents = True

' ReadOnlyCollection (wrapper, prevents modification)
Dim ro As New ReadOnlyCollection(Of String)(list)
' ro.Add("x")  ' throws NotSupportedException

' KeyedCollection (abstract, dictionary-like by key)
Public Class PersonCollection
    Inherits KeyedCollection(Of String, Person)
    Protected Overrides Function GetKeyForItem(item As Person) As String
        Return item.Name
    End Function
End Class

' generic method
Public Function Max(Of T As IComparable(Of T))(a As T, b As T) As T
    If a.CompareTo(b) > 0 Then Return a
    Return b
End Function

Dim m1 = Max(3, 5)          ' Integer
Dim m2 = Max("apple", "banana")  ' String
Dim m3 = Max(3.14, 2.71)    ' Double

' multiple type parameters
Public Function Pair(Of T1, T2)(first As T1, second As T2) As Tuple(Of T1, T2)
    Return Tuple.Create(first, second)
End Function

' constraints
Public Class Repository(Of T As Class, New)
    ' T must be a reference type with parameterless constructor
    Public Function Create() As T
        Return New T()
    End Function
End Class

' constraint types:
' Class      - reference type
' Structure  - value type
' New        - has parameterless constructor
' IComparable(Of T) - implements interface
' BaseClass  - inherits from base class

' generic method with interface constraint
Public Sub Sort(Of T As IComparable(Of T))(ByRef arr As T())
    Array.Sort(arr)
End Sub

' multiple constraints
Public Function Process(Of T As {Class, IComparable(Of T), New})(items As List(Of T)) As T
    Dim item As New T()
    items.Sort()
    Return items(0)
End Function

' generic class with method
Public Class Cache(Of TKey, TValue)
    Private dict As New Dictionary(Of TKey, TValue)
    Public Sub Add(key As TKey, value As TValue)
        dict.Add(key, value)
    End Sub
    Public Function Get(key As TKey) As TValue
        Return dict(key)
    End Function
End Class

Imports System.Collections
Imports System.Collections.Generic

' implement IEnumerable (foreach support)
Public Class NumberSequence
    Implements IEnumerable(Of Integer)

    Private start, count As Integer

    Public Sub New(start As Integer, count As Integer)
        Me.start = start
        Me.count = count
    End Sub

    Public Function GetEnumerator() As IEnumerator(Of Integer) _
        Implements IEnumerable(Of Integer).GetEnumerator
        For i = 0 To count - 1
            Yield start + i
        Next
    End Function

    Private Function GetEnumerator1() As IEnumerator _
        Implements IEnumerable.GetEnumerator
        Return GetEnumerator()
    End Function
End Class

' usage
Dim seq As New NumberSequence(10, 5)
For Each n In seq
    Console.WriteLine(n)  ' 10, 11, 12, 13, 14
Next

' iterator with Yield (lazy evaluation)
Public Iterator Function Evens(max As Integer) As IEnumerable(Of Integer)
    For i = 0 To max
        If i Mod 2 = 0 Then Yield i
    Next
End Function

' custom collection class
Public Class PersonList
    Inherits CollectionBase

    Default Public Property Item(index As Integer) As Person
        Get
            Return CType(List(index), Person)
        End Get
        Set(value As Person)
            List(index) = value
        End Set
    End Property

    Public Function Add(p As Person) As Integer
        Return List.Add(p)
    End Function
End Class

' implement ICollection(Of T) for full collection semantics
Public Class CircularBuffer(Of T)
    Implements ICollection(Of T)
    ' ... implement all members
End Class

' Tuple (reference type, up to 8 items)
Dim t1 As New Tuple(Of Integer, String, Boolean)(1, "hello", True)
Console.WriteLine(t1.Item1)  ' 1
Console.WriteLine(t1.Item2)  ' "hello"

' ValueTuple (value type, lightweight, named fields)
Dim vt1 As (Id As Integer, Name As String, Active As Boolean) =
    (1, "hello", True)
Console.WriteLine(vt1.Id)      ' 1
Console.WriteLine(vt1.Name)    ' "hello"

' without names (Item1, Item2, ...)
Dim vt2 = (1, "hello", True)
Console.WriteLine(vt2.Item1)

' return multiple values
Public Function FindMinMax(numbers As Integer()) As (Min As Integer, Max As Integer)
    Return (numbers.Min(), numbers.Max())
End Function

Dim result = FindMinMax({3, 1, 4, 1, 5, 9})
Console.WriteLine($"Min: {result.Min}, Max: {result.Max}")

' deconstruction
Dim (id, name, active) = GetRecord()
Dim (min, max) = FindMinMax({1, 2, 3})

' tuple in LINQ
Dim stats = From p In people
            Group By p.Dept Into Group
            Select Dept,
                   Count = Group.Count(),
                   AvgSalary = Group.Average(Function(x) x.Salary)

' tuple comparison
Dim a = (1, 2)
Dim b = (1, 2)
Console.WriteLine(a = b)  ' True (element-wise comparison)

' discard
Dim (first, _, last) = GetThreeValues()  ' ignore middle

Imports System.Diagnostics

' Debug class (Debug builds only)
Debug.WriteLine("Debug message")
Debug.Assert(balance >= 0, "Balance is negative!")
Debug.WriteLineIf(verbose, "Verbose output")
Debug.Indent()
Debug.WriteLine("Indented")
Debug.Unindent()

' Trace class (Release builds too)
Trace.WriteLine("Trace message")
Trace.TraceInformation("Info: {0} at {1}", value, Date.Now)
Trace.TraceWarning("Warning: low memory")
Trace.TraceError("Error: connection failed")

' trace listeners
Trace.Listeners.Add(New TextWriterTraceListener("trace.log"))
Trace.Listeners.Add(New ConsoleTraceListener())
Trace.AutoFlush = True

' conditional tracing
#If DEBUG Then
    Debug.WriteLine("Only in debug")
#End If

<Conditional("DEBUG")>
Public Sub DebugLog(msg As String)
    Console.WriteLine(msg)
End Sub

' TraceSwitch (configurable level)
Dim ts As New TraceSwitch("MySwitch", "Application tracing")
' app.config:
' <switches><add name="MySwitch" value="3" /></switches>
' 0=Off, 1=Error, 2=Warning, 3=Info, 4=Verbose
If ts.TraceInfo Then Trace.WriteLine("Info message")
If ts.TraceVerbose Then Trace.WriteLine("Verbose message")

' TraceSource (named, granular)
Dim ts2 As New TraceSource("MyApp")
ts2.TraceEvent(TraceEventType.Error, 0, "Something failed")
ts2.Switch.Level = SourceLevels.All

Imports System.Diagnostics

' break into debugger
Debugger.Break()  ' if attached, pauses; otherwise prompts

' check if debugger attached
If Debugger.IsAttached Then
    Console.WriteLine("Running in debugger")
End If

' launch debugger
Debugger.Launch()

' DebuggerDisplay attribute (custom display in watch window)
<DebuggerDisplay("Name = {Name}, Age = {Age}")>
Public Class Person
    Public Property Name As String
    Public Property Age As Integer

    <DebuggerBrowsable(DebuggerBrowsableState.RootHidden)>
    Public Property Items As Integer() = {1, 2, 3}
End Class

' DebuggerStepThrough (skip in F11 step-into)
<DebuggerStepThrough>
Public Sub TrivialMethod()
    ' debugger won't step into this
End Sub

' DebuggerHidden (not visible in debugger)
<DebuggerHidden>
Public Sub InternalHelper()
End Sub

' conditional breakpoints in IDE:
' Right-click breakpoint > Settings > Condition
' e.g., i == 42 or str.Contains("error")

' tracepoints (log message without code):
' Right-click > Actions > Log message
' "i = {i}, value = {value}"

' edit and continue:
' modify code while paused, continue (limited support)

' immediate window (while debugging):
' ? variableName
' ? methodCall()
' variableName = newValue

' try/catch/finally
Try
    RiskyOperation()
Catch ex As FileNotFoundException
    ' specific exception first
    Logger.Error(ex, "File not found")
    Throw New UserFriendlyException("Please check the file path", ex)
Catch ex As IOException
    Logger.Error(ex, "IO error")
    Throw
Catch ex As Exception
    ' generic last resort
    Logger.Error(ex, "Unexpected error")
    Throw
Finally
    ' always runs (cleanup)
    resource?.Dispose()
End Try

' exception filters (VB 14+)
Catch ex As HttpException When ex.StatusCode = 404
    ' only catch 404s
End Try

' custom exception
<Serializable>
Public Class BusinessRuleException
    Inherits Exception

    Public Property RuleName As String

    Public Sub New(message As String, ruleName As String)
        MyBase.New(message)
        RuleName = ruleName
    End Sub

    Public Sub New(message As String, ruleName As String, inner As Exception)
        MyBase.New(message, inner)
        RuleName = ruleName
    End Sub

    Protected Sub New(info As Runtime.Serialization.SerializationInfo,
                     context As Runtime.Serialization.StreamingContext)
        MyBase.New(info, context)
        RuleName = info.GetString("RuleName")
    End Sub

    Public Overrides Sub GetObjectData(info As Runtime.Serialization.SerializationInfo,
                                       context As Runtime.Serialization.StreamingContext)
        MyBase.GetObjectData(info, context)
        info.AddValue("RuleName", RuleName)
    End Sub
End Class

Imports System.Diagnostics

' write to Windows Event Log
If Not EventLog.SourceExists("MyApp") Then
    EventLog.CreateEventSource("MyApp", "Application")
End If
EventLog.WriteEntry("MyApp", "Application started", EventLogEntryType.Information)
EventLog.WriteEntry("MyApp", "Disk full!", EventLogEntryType.Error, 1001)
EventLog.WriteEntry("MyApp", "Retrying operation", EventLogEntryType.Warning, 2001)

' read from Event Log
Dim log As New EventLog("Application")
For Each entry In log.Entries.Cast(Of EventLogEntry)().
    Where(Function(e) e.Source = "MyApp").
    Take(10)
    Console.WriteLine($"{entry.TimeGenerated}: {entry.Message}")
Next

' structured logging with Serilog (NuGet)
' Install-Package Serilog.Sinks.File
' Dim logger = New LoggerConfiguration().
'     WriteTo.File("log.txt", rollingInterval:=RollingInterval.Day).
'     CreateLogger()
' logger.Information("User {UserId} logged in", userId)
' logger.Error(ex, "Failed to process {OrderId}", orderId)

' log4net (alternative)
' <log4net>
'   <appender name="FileAppender" type="log4net.Appender.FileAppender">
'     <file value="log.txt" />
'   </appender>
' </log4net>

' NLog (alternative)
' Dim logger = LogManager.GetCurrentClassLogger()
' logger.Info("Processing started")
' logger.Error(ex, "Processing failed")

' performance counters
Dim pc As New PerformanceCounter("Processor", "% Processor Time", "_Total")
Dim cpuUsage = pc.NextValue()
Thread.Sleep(1000)
cpuUsage = pc.NextValue()  ' actual value (needs two samples)

' MSTest (built-in)
Imports Microsoft.VisualStudio.TestTools.UnitTesting

<TestClass>
Public Class CalculatorTests
    Private calc As Calculator

    <TestInitialize>
    Public Sub Setup()
        calc = New Calculator()
    End Sub

    <TestCleanup>
    Public Sub Cleanup()
        ' cleanup after each test
    End Sub

    <TestMethod>
    Public Sub Add_TwoNumbers_ReturnsSum()
        Dim result = calc.Add(2, 3)
        Assert.AreEqual(5, result)
    End Sub

    <TestMethod>
    Public Sub Divide_ByZero_ThrowsException()
        Try
            calc.Divide(10, 0)
            Assert.Fail("Should have thrown")
        Catch ex As DivideByZeroException
            ' expected
        End Try
    End Sub

    <TestMethod, ExpectedException(GetType(DivideByZeroException))>
    Public Sub Divide_ByZero_Throws()
        calc.Divide(10, 0)
    End Sub

    <DataTestMethod>
    <DataRow(1, 2, 3)>
    <DataRow(10, 20, 30)>
    <DataRow(-1, 1, 0)>
    Public Sub Add_VariousInputs(a As Integer, b As Integer, expected As Integer)
        Assert.AreEqual(expected, calc.Add(a, b))
    End Sub
End Class

' xUnit (alternative)
' <Fact> instead of <TestMethod>
' <Theory> with <InlineData> instead of <DataTestMethod>/<DataRow>

' Moq for mocking
' Dim mockRepo = New Mock(Of IUserRepository)()
' mockRepo.Setup(Function(r) r.GetById(1)).Returns(New User With {.Name = "Test"})
' Dim service = New UserService(mockRepo.Object)

' ArrayList (loosely typed, legacy)
Dim arr As New ArrayList()
arr.Add("hello")
arr.Add(42)
arr.Add(3.14)
arr.Remove("hello")
arr.RemoveAt(0)
For Each item In arr
    Console.WriteLine(item)
Next

' List(Of T) — strongly typed, preferred
Dim nums As New List(Of Integer) From {1, 2, 3, 4, 5}
nums.Add(6)
nums.AddRange({7, 8, 9})
nums.Remove(3)
nums.RemoveAt(0)
nums.Sort()
nums.Reverse()

' find and convert
Dim evens = nums.FindAll(Function(n) n Mod 2 = 0)
Dim firstEven = nums.Find(Function(n) n Mod 2 = 0)
Dim arr2 = nums.ToArray()

' index
Dim idx = nums.IndexOf(5)
Console.WriteLine($"Count: {nums.Count}, Contains 5: {nums.Contains(5)}")

' Dictionary(Of TKey, TValue)
Dim ages As New Dictionary(Of String, Integer) From {
    {"Alice", 30},
    {"Bob", 25}
}

' add and update
ages("Carol") = 42                  ' add or update
ages.Add("Dave", 35)                ' throws if key exists
ages.Remove("Bob")

' access
If ages.ContainsKey("Alice") Then
    Console.WriteLine(ages("Alice"))
End If

' safe access
Dim age As Integer
If ages.TryGetValue("Eve", age) Then
    Console.WriteLine($"Eve is {age}")
End If

' iterate
For Each kvp In ages
    Console.WriteLine($"{kvp.Key}: {kvp.Value}")
Next

' keys and values
Dim names = ages.Keys.ToList()
Dim allAges = ages.Values.ToList()

' LINQ on dictionaries
Dim adults = ages.Where(Function(kvp) kvp.Value >= 30).ToDictionary(Function(kvp) kvp.Key, Function(kvp) kvp.Value)

' HashSet(Of T) — unique elements, fast lookup
Dim set1 As New HashSet(Of Integer) From {1, 2, 3, 4, 5}
Dim set2 As New HashSet(Of Integer) From {3, 4, 5, 6, 7}

set1.Add(6)                         ' adds only if not present
set1.Remove(1)

' set operations
Dim union = New HashSet(Of Integer)(set1)
union.UnionWith(set2)               ' {1,2,3,4,5,6,7}

Dim intersect = New HashSet(Of Integer)(set1)
intersect.IntersectWith(set2)       ' {3,4,5,6}

Dim diff = New HashSet(Of Integer)(set1)
diff.ExceptWith(set2)               ' {1,2}

Dim symDiff = New HashSet(Of Integer)(set1)
symDiff.SymmetricExceptWith(set2)   ' {1,2,7}

' checks
set1.IsSubsetOf(set2)
set1.IsSupersetOf(set2)
set1.Overlaps(set2)                 ' any common elements

' SortedSet keeps elements sorted
Dim sorted As New SortedSet(Of Integer) From {5, 2, 8, 1, 9}
' {1, 2, 5, 8, 9}
Console.WriteLine(sorted.Min)
Console.WriteLine(sorted.Max)

' Queue(Of T) — FIFO
Dim queue As New Queue(Of String)()
queue.Enqueue("first")
queue.Enqueue("second")
queue.Enqueue("third")

Dim next1 = queue.Dequeue()         ' "first"
Dim peek = queue.Peek()             ' "second" (doesn't remove)
Console.WriteLine($"Count: {queue.Count}")

' process queue
Do While queue.Count > 0
    Dim item = queue.Dequeue()
    Console.WriteLine(item)
Loop

' Stack(Of T) — LIFO
Dim stack As New Stack(Of Integer)()
stack.Push(1)
stack.Push(2)
stack.Push(3)

Dim top = stack.Pop()               ' 3
Dim peek = stack.Peek()             ' 2

' reverse using stack
Dim arr = {1, 2, 3, 4, 5}
Dim revStack As New Stack(Of Integer)(arr)
Dim reversed = revStack.ToArray()   ' {5, 4, 3, 2, 1}

Imports System.Collections.Concurrent

' thread-safe collections for parallel programming

' ConcurrentDictionary
Dim cd As New ConcurrentDictionary(Of String, Integer)()
cd("a") = 1
cd.AddOrUpdate("a", 1, Function(key, oldVal) oldVal + 1)
cd.GetOrAdd("b", Function(key) 42)

' ConcurrentQueue (lock-free FIFO)
Dim cq As New ConcurrentQueue(Of Integer)()
cq.Enqueue(1)
Dim val As Integer
If cq.TryDequeue(val) Then
    Console.WriteLine(val)
End If

' ConcurrentStack (lock-free LIFO)
Dim cs As New ConcurrentStack(Of Integer)()
cs.Push(1)
cs.TryPop(val)

' ConcurrentBag (unordered, fast for same-thread adds)
Dim cb As New ConcurrentBag(Of Integer)()
cb.Add(1)
cb.TryTake(val)

' BlockingCollection (producer-consumer)
Dim bc As New BlockingCollection(Of Integer)(100)  ' bounded
' Producer:
Task.Run(Sub()
    For i = 1 To 100
        bc.Add(i)
    Next
    bc.CompleteAdding()
End Sub)
' Consumer:
Task.Run(Sub()
    For Each item In bc.GetConsumingEnumerable()
        Console.WriteLine(item)
    Next
End Sub)

Imports System.IO

' StreamReader
Using reader As New StreamReader("input.txt")
    Dim line As String
    Do While reader.Peek() >= 0
        line = reader.ReadLine()
        Console.WriteLine(line)
    Loop
End Using

' read all at once
Dim allText = File.ReadAllText("input.txt")
Dim allLines = File.ReadAllLines("input.txt")

' StreamWriter
Using writer As New StreamWriter("output.txt")
    writer.WriteLine("First line")
    writer.WriteLine("Second line")
    writer.Write("No newline")
End Using

' append
File.AppendAllText("log.txt", $"{Date.Now}: message{Environment.NewLine}")

' async I/O (don't block UI)
Async Function ReadAsync(path As String) As Task(Of String)
    Using reader As New StreamReader(path)
        Return Await reader.ReadToEndAsync()
    End Using
End Function

' write async
Using writer As New StreamWriter("out.txt")
    Await writer.WriteLineAsync("async line")
End Using

Imports System.IO
Imports Microsoft.VisualBasic.FileIO

' BinaryWriter/Reader
Using fs As New FileStream("data.bin", FileMode.Create),
      bw As New BinaryWriter(fs)
    bw.Write(42I)                    ' Integer
    bw.Write(3.14R)                  ' Double
    bw.Write("hello")                ' String (length-prefixed)
End Using

Using fs As New FileStream("data.bin", FileMode.Open),
      br As New BinaryReader(fs)
    Dim n = br.ReadInt32()
    Dim d = br.ReadDouble()
    Dim s = br.ReadString()
End Using

' CSV with TextFieldParser (handles quoted fields)
Using parser As New TextFieldParser("data.csv")
    parser.TextFieldType = FieldType.Delimited
    parser.SetDelimiters(",")
    parser.HasFieldsEnclosedInQuotes = True

    ' skip header
    If Not parser.EndOfData Then parser.ReadFields()

    While Not parser.EndOfData
        Dim fields = parser.ReadFields()
        Console.WriteLine($"Name: {fields(0)}, Age: {fields(1)}")
    End While
End Using

' write CSV
Dim lines = {"name,age", "Alice,30", "Bob,25"}
File.WriteAllLines("out.csv", lines)

Imports System.IO

' path manipulation (cross-platform)
Dim full = Path.Combine("C:", "Users", "alice", "data.txt")
' C:Usersalicedata.txt

Dim dir = Path.GetDirectoryName(full)      ' C:Usersalice
Dim file = Path.GetFileName(full)           ' data.txt
Dim name = Path.GetFileNameWithoutExtension(full)  ' data
Dim ext = Path.GetExtension(full)           ' .txt

' temp files
Dim tempFile = Path.GetTempFileName()       ' creates empty file
Dim tempDir = Path.GetTempPath()

' check extension
If Path.GetExtension(path).Equals(".csv", StringComparison.OrdinalIgnoreCase) Then
    ' process CSV
End If

' directory operations
Directory.CreateDirectory("path/to/dir")
If Directory.Exists("path") Then
    Dim files = Directory.GetFiles("path", "*.txt", SearchOption.AllDirectories)
    Dim dirs = Directory.GetDirectories("path")
End If

' file info
Dim fi As New FileInfo("data.txt")
Console.WriteLine($"Size: {fi.Length}, Modified: {fi.LastWriteTime}")

' safe file name
Dim invalid = Path.GetInvalidFileNameChars()
Dim safe = New String(name.Select(Function(c) If(invalid.Contains(c), "_"c, c)).ToArray())

Imports System.IO

' FileSystemWatcher monitors directory changes
Dim watcher As New FileSystemWatcher() With {
    .Path = "C:Watch",
    .Filter = "*.txt",
    .IncludeSubdirectories = True,
    .NotifyFilter = NotifyFilters.FileName Or NotifyFilters.LastWrite Or NotifyFilters.Size
}

' event handlers
AddHandler watcher.Created, Sub(s, e)
    Console.WriteLine($"Created: {e.FullPath}")
End Sub

AddHandler watcher.Changed, Sub(s, e)
    Console.WriteLine($"Changed: {e.FullPath}")
End Sub

AddHandler watcher.Deleted, Sub(s, e)
    Console.WriteLine($"Deleted: {e.FullPath}")
End Sub

AddHandler watcher.Renamed, Sub(s, e)
    Console.WriteLine($"Renamed: {e.OldFullPath} -> {e.FullPath}")
End Sub

AddHandler watcher.Error, Sub(s, e)
    Console.WriteLine($"Error: {e.GetException().Message}")
End Sub

watcher.EnableRaisingEvents = True

' common gotcha: multiple events fire for one save
' debounce by tracking last event time
Dim lastEvent As DateTime
AddHandler watcher.Changed, Sub(s, e)
    If (DateTime.Now - lastEvent).TotalMilliseconds > 500 Then
        lastEvent = DateTime.Now
        ' process
    End If
End Sub

Imports System.IO
Imports System.Xml.Serialization
Imports System.Text.Json

' class to serialize
Public Class Person
    Public Property Name As String
    Public Property Age As Integer
    Public Property Email As String
End Class

' JSON (System.Text.Json — modern, fast)
Dim p As New Person With {.Name = "Alice", .Age = 30, .Email = "[email protected]"}
Dim json = JsonSerializer.Serialize(p)
File.WriteAllText("p.json", json)

Dim jsonOpts As New JsonSerializerOptions With {.WriteIndented = True}
File.WriteAllText("p_pretty.json", JsonSerializer.Serialize(p, jsonOpts))

Dim loaded = JsonSerializer.Deserialize(Of Person)(File.ReadAllText("p.json"))

' XML
Dim xs As New XmlSerializer(GetType(Person))
Using fs As New FileStream("p.xml", FileMode.Create)
    xs.Serialize(fs, p)
End Using

Using fs As New FileStream("p.xml", FileMode.Open)
    Dim loadedXml = DirectCast(xs.Deserialize(fs), Person)
End Using

' collections
Dim people As New List(Of Person) From {p, New Person With {.Name = "Bob"}}
Dim jsonList = JsonSerializer.Serialize(people)
Dim loadedList = JsonSerializer.Deserialize(Of List(Of Person))(jsonList)

Try
    Dim result = 10 / x
    File.WriteAllText("out.txt", result.ToString())
Catch ex As DivideByZeroException
    Console.WriteLine($"Cannot divide by zero: {ex.Message}")
Catch ex As IOException
    Console.WriteLine($"File error: {ex.Message}")
Catch ex As UnauthorizedAccessException
    Console.WriteLine($"Permission denied: {ex.Message}")
Catch ex As Exception
    ' catch-all (use sparingly)
    Console.WriteLine($"Unexpected: {ex.Message}")
    Throw                          ' re-throw
Finally
    ' always runs (even on exception or return)
    ' cleanup: close files, release resources
End Try

' exception filters (VB 14+)
Try
    ProcessData()
Catch ex As InvalidOperationException When ex.Message.Contains("collection")
    Console.WriteLine("Collection was modified")
Catch ex As InvalidOperationException
    Console.WriteLine("Other invalid op")
End Try

' nested try
Try
    Try
        RiskyOperation()
    Catch ex As Exception
        Throw New InvalidOperationException("Inner failed", ex)  ' wrap
    End Try
Catch ex As Exception
    Console.WriteLine(ex.InnerException?.Message)
End Try

' inherit from Exception (or a more specific base)
<Serializable>
Public Class ValidationException
    Inherits Exception

    Public Property FieldName As String

    Public Sub New(message As String)
        MyBase.New(message)
    End Sub

    Public Sub New(message As String, inner As Exception)
        MyBase.New(message, inner)
    End Sub

    Public Sub New(fieldName As String, message As String)
        MyBase.New(message)
        Me.FieldName = fieldName
    End Sub

    Public Overrides Function ToString() As String
        Return $"Validation error in '{FieldName}': {Message}"
    End Function
End Class

' usage
Sub ValidateAge(age As Integer)
    If age < 0 Then
        Throw New ValidationException("age", "Age cannot be negative")
    ElseIf age > 150
        Throw New ValidationException("age", "Age seems unrealistic")
    End If
End Sub

' catch custom
Try
    ValidateAge(-5)
Catch ex As ValidationException
    Console.WriteLine($"Field '{ex.FieldName}': {ex.Message}")
End Try

' 1. Don't catch what you can't handle
' BAD: swallows errors silently
Try
    SaveData()
Catch ex As Exception
    ' nothing
End Try

' GOOD: log and rethrow, or handle meaningfully
Try
    SaveData()
Catch ex As IOException
    Logger.Warn(ex, "Save failed, will retry")
    RetrySave()
Catch ex As Exception
    Logger.Error(ex, "Unexpected error saving")
    Throw
End Try

' 2. Use Using instead of try/finally for IDisposable
Using fs As New FileStream("f.txt", FileMode.Open)
    ' use fs
End Using  ' automatically disposes

' 3. Global exception handlers
AddHandler AppDomain.CurrentDomain.UnhandledException, Sub(s, e)
    Logger.Fatal(DirectCast(e.ExceptionObject, Exception), "App crash")
End Sub

AddHandler Application.ThreadException, Sub(s, e)
    Logger.Error(e.Exception, "UI thread error")
End Sub

' 4. AggregateException (parallel)
Try
    Parallel.ForEach(items, Sub(item) ProcessItem(item))
Catch ex As AggregateException
    For Each inner In ex.InnerExceptions
        Logger.Error(inner, "Task failed")
    Next
End Try

' Alternative to exceptions for expected failures
Public Class Result(Of T)
    Public ReadOnly Property Value As T
    Public ReadOnly Property [Error] As String
    Public ReadOnly Property IsSuccess As Boolean

    Private Sub New(value As T, [error] As String, isSuccess As Boolean)
        Me.Value = value
        Me.Error = [error]
        Me.IsSuccess = isSuccess
    End Sub

    Public Shared Function Ok(value As T) As Result(Of T)
        Return New Result(Of T)(value, Nothing, True)
    End Function

    Public Shared Function Fail([error] As String) As Result(Of T)
        Return New Result(Of T)(Nothing, [error], False)
    End Function
End Class

' usage
Function ParseInt(s As String) As Result(Of Integer)
    Dim n As Integer
    If Integer.TryParse(s, n) Then
        Return Result(Of Integer).Ok(n)
    Else
        Return Result(Of Integer).Fail($"Cannot parse '{s}'")
    End If
End Function

' caller
Dim result = ParseInt("42")
If result.IsSuccess Then
    Console.WriteLine($"Got: {result.Value}")
Else
    Console.WriteLine($"Error: {result.Error}")
End If

' chain (extension methods)
' result.OnSuccess(Function(x) x * 2).OnFailure(Function(e) 0)

Imports NLog  ' or log4net, Serilog

' configure (in app.config or code)
' NLog.config:
' <targets><target name="file" type="File" fileName="app.log" /></targets>
' <rules><logger name="*" minlevel="Info" writeTo="file" /></rules>

Public Class MyService
    Private Shared ReadOnly Logger As Logger = LogManager.GetCurrentClassLogger()

    Public Sub ProcessData(data As String)
        Logger.Info($"Processing: {data}")

        Try
            Logger.Debug("Starting parse")
            Dim result = Parse(data)
            Logger.Info($"Done: {result}")
        Catch ex As FormatException
            Logger.Warn(ex, "Parse failed")
            Throw
        Catch ex As Exception
            Logger.Error(ex, "Unexpected error")
            Throw
        End Try
    End Sub
End Class

' structured logging (Serilog)
' Log.Information("User {UserId} logged in from {IP}", userId, ip)

' log levels (in order):
'   Trace < Debug < Info < Warn < Error < Fatal
' set minimum level in config to filter

' exception with context
Try
    SaveUser(user)
Catch ex As Exception
    Logger.Error(ex, "Failed to save user {UserId}", user.Id)
    Throw
End Try

Imports System.Data.SqlClient  ' or Microsoft.Data.SqlClient for .NET+
' or MySql.Data.MySqlClient, System.Data.SQLite, etc.

Dim connStr As String = "Server=localhost;Database=mydb;Integrated Security=True;"

' basic query
Using conn As New SqlConnection(connStr)
    conn.Open()
    Using cmd As New SqlCommand("SELECT Id, Name FROM Users WHERE Age > @age", conn)
        cmd.Parameters.AddWithValue("@age", 18)
        Using reader = cmd.ExecuteReader()
            While reader.Read()
                Console.WriteLine($"{reader("Id")}: {reader("Name")}")
            End While
        End Using
    End Using
End Using

' insert with parameters
Using conn As New SqlConnection(connStr), cmd As New SqlCommand(
    "INSERT INTO Users (Name, Age) VALUES (@name, @age); SELECT SCOPE_IDENTITY();", conn)
    cmd.Parameters.AddWithValue("@name", "Alice")
    cmd.Parameters.AddWithValue("@age", 30)
    conn.Open()
    Dim newId = CInt(cmd.ExecuteScalar())
End Using

' update/delete
Using cmd As New SqlCommand("UPDATE Users SET Age = @age WHERE Id = @id", conn)
    cmd.Parameters.AddWithValue("@age", 31)
    cmd.Parameters.AddWithValue("@id", newId)
    Dim rowsAffected = cmd.ExecuteNonQuery()
End Using

Dim connStr = "Server=localhost;Database=mydb;Integrated Security=True;"

' basic transaction
Using conn As New SqlConnection(connStr)
    conn.Open()
    Using tran = conn.BeginTransaction()
        Try
            Using cmd As New SqlCommand("", conn, tran)
                cmd.CommandText = "UPDATE Accounts SET Balance = Balance - 100 WHERE Id = 1"
                cmd.ExecuteNonQuery()

                cmd.CommandText = "UPDATE Accounts SET Balance = Balance + 100 WHERE Id = 2"
                cmd.ExecuteNonQuery()

                tran.Commit()
            End Using
        Catch ex As Exception
            tran.Rollback()
            Console.WriteLine($"Transaction rolled back: {ex.Message}")
            Throw
        End Try
    End Using
End Using

' TransactionScope (distributed transactions)
Using scope As New TransactionScope()
    Using conn1 As New SqlConnection(connStr1)
        ' work on db1
    End Using
    Using conn2 As New SqlConnection(connStr2)
        ' work on db2
    End Using
    scope.Complete()  ' commit all
End Using  ' rollback if Complete not called

' isolation levels
Using tran = conn.BeginTransaction(IsolationLevel.Serializable)
    ' ...
End Using

Dim connStr = "Server=localhost;Database=mydb;Integrated Security=True;"

' fill DataTable
Dim dt As New DataTable()
Using adapter As New SqlDataAdapter("SELECT * FROM Users", connStr)
    adapter.Fill(dt)
End Using

' access rows
For Each row As DataRow In dt.Rows
    Console.WriteLine($"{row("Id")}: {row("Name")}")
Next

' filter and sort (in-memory)
Dim view As New DataView(dt) With {
    .Sort = "Name ASC",
    .RowFilter = "Age > 25"
}
For Each row As DataRowView In view
    Console.WriteLine(row("Name"))
Next

' modify and update
Dim newRow = dt.NewRow()
newRow("Name") = "Carol"
newRow("Age") = 28
dt.Rows.Add(newRow)

dt.Rows(0)("Age") = 31              ' modify
dt.Rows(1).Delete()                 ' mark for deletion

' push changes back to DB
Using adapter As New SqlDataAdapter("SELECT * FROM Users", connStr)
    Dim builder As New SqlCommandBuilder(adapter)
    adapter.Update(dt)              ' generates INSERT/UPDATE/DELETE
End Using

' LINQ on DataTable
Dim young = From row In dt.AsEnumerable()
            Where row.Field(Of Integer)("Age") < 30
            Select row.Field(Of String)("Name")

Imports Dapper  ' NuGet: Install-Package Dapper

' Dapper is a lightweight ORM that extends IDbConnection

Dim connStr = "Server=localhost;Database=mydb;Integrated Security=True;"
Using conn As New SqlConnection(connStr)
    ' query
    Dim users = conn.Query(Of User)("SELECT * FROM Users WHERE Age > @age",
                                     New With {Key .age = 18}).ToList()

    ' single
    Dim user = conn.QuerySingle(Of User)("SELECT * FROM Users WHERE Id = @id",
                                          New With {Key .id = 5})

    ' execute
    Dim rows = conn.Execute("INSERT INTO Users (Name, Age) VALUES (@name, @age)",
                            New With {Key .name = "Alice", Key .age = 30})

    ' batch insert
    Dim newUsers = {
        New With {Key .name = "Bob", Key .age = 25},
        New With {Key .name = "Carol", Key .age = 28}
    }
    conn.Execute("INSERT INTO Users (Name, Age) VALUES (@name, @age)", newUsers)

    ' multi-result (one-to-many)
    Dim sql = "SELECT * FROM Companies; SELECT * FROM Employees;"
    Using multi = conn.QueryMultiple(sql)
        Dim companies = multi.Read(Of Company)().ToList()
        Dim employees = multi.Read(Of Employee)().ToList()
    End Using

    ' stored procedure
    Dim result = conn.Query(Of User)("sp_GetUsers", New With {Key .role = "admin"},
                                       commandType:=CommandType.StoredProcedure)
End Using

Imports Microsoft.EntityFrameworkCore

' DbContext
Public Class AppDbContext
    Inherits DbContext

    Public Property Users As DbSet(Of User)
    Public Property Orders As DbSet(Of Order)

    Protected Overrides Sub OnConfiguring(options As DbContextOptionsBuilder)
        options.UseSqlServer("Server=localhost;Database=mydb;Integrated Security=True;")
    End Sub

    Protected Overrides Sub OnModelCreating(modelBuilder As ModelBuilder)
        modelBuilder.Entity(Of User)().
            HasKey(Function(u) u.Id).
            HasMany(Function(u) u.Orders).
            WithOne(Function(o) o.User).
            HasForeignKey(Function(o) o.UserId)
    End Sub
End Class

' CRUD
Using db As New AppDbContext()
    ' create
    db.Users.Add(New User With {.Name = "Alice", .Age = 30})
    db.SaveChanges()

    ' read
    Dim alice = db.Users.First(Function(u) u.Name = "Alice")
    Dim adults = db.Users.Where(Function(u) u.Age >= 18).ToList()

    ' update
    alice.Age = 31
    db.SaveChanges()

    ' delete
    db.Users.Remove(alice)
    db.SaveChanges()

    ' eager loading
    Dim usersWithOrders = db.Users.Include(Function(u) u.Orders).ToList()

    ' raw SQL
    Dim raw = db.Users.FromSqlRaw("SELECT * FROM Users WHERE Age > {0}", 18).ToList()

    ' transactions
    Using tran = db.Database.BeginTransaction()
        Try
            ' work
            db.SaveChanges()
            tran.Commit()
        Catch
            tran.Rollback()
            Throw
        End Try
    End Using
End Using

Dim nums = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}

' query syntax (SQL-like)
Dim evens = From n In nums
            Where n Mod 2 = 0
            Select n
            ' List: 2, 4, 6, 8, 10

Dim squares = From n In nums
              Select n, Square = n * n

Dim pairs = From n In nums
            Where n > 3
            Order By n Descending
            Select n

' with multiple sources
Dim names = {"Alice", "Bob", "Carol"}
Dim ages = {30, 25, 42}
Dim people = From name In names
             From age In ages
             Where age > 26
             Select name, age

' group by
Dim orders = GetOrders()
Dim byCustomer = From o In orders
                 Group o By o.CustomerId Into Group
                 Select CustomerId, Total = Group.Sum(Function(o) o.Amount)

' join
Dim result = From u In users
             Join o In orders On u.Id Equals o.UserId
             Select u.Name, o.Amount

Dim nums = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}

' method syntax (fluent)
Dim evens = nums.Where(Function(n) n Mod 2 = 0).ToList()
Dim squares = nums.Select(Function(n) n * n).ToList()
Dim sorted = nums.OrderBy(Function(n) n).ToList()
Dim desc = nums.OrderByDescending(Function(n) n).ToList()

' aggregation
Dim sum = nums.Sum()
Dim avg = nums.Average()
Dim min = nums.Min()
Dim max = nums.Max()
Dim count = nums.Count()
Dim countEven = nums.Count(Function(n) n Mod 2 = 0)

' element access
Dim first = nums.First()
Dim firstEven = nums.First(Function(n) n Mod 2 = 0)
Dim maybeFirst = nums.FirstOrDefault(Function(n) n > 100)  ' 0 if not found
Dim single = nums.Single(Function(n) n = 5)

' take/skip
Dim top3 = nums.Take(3).ToList()
Dim skip3 = nums.Skip(3).ToList()
Dim page = nums.Skip(10).Take(5).ToList()  ' pagination

' distinct
Dim unique = nums.Distinct().ToList()
Dim union = nums.Union({11, 12}).ToList()
Dim intersect = nums.Intersect({5, 6, 7}).ToList()
Dim except = nums.Except({5, 6, 7}).ToList()

Public Class Order
    Public Property Id As Integer
    Public Property CustomerId As Integer
    Public Property Amount As Decimal
    Public Property Date As DateTime
End Class

Dim orders = GetOrders()

' group by
Dim byCustomer = orders.GroupBy(Function(o) o.CustomerId)
For Each grp In byCustomer
    Console.WriteLine($"Customer {grp.Key}: {grp.Count()} orders, total {grp.Sum(Function(o) o.Amount)}")
Next

' group with projection
Dim summary = orders.GroupBy(Function(o) o.CustomerId).
    Select(Function(g) New With {
        Key .CustomerId = g.Key,
        Key .OrderCount = g.Count(),
        Key .TotalAmount = g.Sum(Function(o) o.Amount),
        Key .AvgAmount = g.Average(Function(o) o.Amount),
        Key .MaxAmount = g.Max(Function(o) o.Amount)
    }).ToList()

' multi-level grouping
Dim byMonth = orders.GroupBy(Function(o) New With {Key o.Date.Year, Key o.Date.Month})
For Each grp In byMonth
    Console.WriteLine($"{grp.Key.Year}-{grp.Key.Month}: {grp.Sum(Function(o) o.Amount)}")
Next

' lookup (like a dictionary of lists)
Dim lookup = orders.ToLookup(Function(o) o.CustomerId)
Dim customerOrders = lookup(5).ToList()  ' all orders for customer 5

' aggregate with seed
Dim total = orders.Aggregate(0D, Function(acc, o) acc + o.Amount)

Public Class User
    Public Property Id As Integer
    Public Property Name As String
    Public Property DepartmentId As Integer
End Class

Public Class Department
    Public Property Id As Integer
    Public Property Name As String
End Class

Dim users = GetUsers()
Dim departments = GetDepartments()

' inner join
Dim result = From u In users
             Join d In departments On u.DepartmentId Equals d.Id
             Select u.Name, Department = d.Name

' method syntax
Dim result2 = users.Join(departments,
                          Function(u) u.DepartmentId,
                          Function(d) d.Id,
                          Function(u, d) New With {Key .Name = u.Name, Key .Dept = d.Name}).ToList()

' group join (left join)
Dim leftJoin = From d In departments
               Group Join u In users On d.Id Equals u.DepartmentId Into Group
               From u In Group.DefaultIfEmpty()
               Select Department = d.Name, User = If(u Is Nothing, "(none)", u.Name)

' multiple joins
Dim detail = From u In users
             Join d In departments On u.DepartmentId Equals d.Id
             Join o In orders On u.Id Equals o.UserId
             Select u.Name, d.Name, o.Amount

' zip (pair two sequences element-by-element)
Dim nums = {1, 2, 3}
Dim words = {"one", "two", "three"}
Dim pairs = nums.Zip(words, Function(n, w) $"{n}={w}").ToList()
' {"1=one", "2=two", "3=three"}

Dim nums = New List(Of Integer) From {1, 2, 3, 4, 5}

' query is NOT executed yet
Dim query = nums.Where(Function(n) n > 2).Select(Function(n) n * 10)

' executes when iterated
For Each n In query
    Console.WriteLine(n)            ' 30, 40, 50
Next

' executes AGAIN on each iteration
For Each n In query
    Console.WriteLine(n)            ' 30, 40, 50 again
Next

' force immediate execution
Dim list = query.ToList()           ' executes once, caches
Dim arr = query.ToArray()
Dim count = query.Count()           ' executes
Dim first = query.First()           ' executes

' side effects in query — be careful!
Dim i = 0
Dim bad = nums.Where(Function(n)
                        i += 1
                        Return n > 2
                     End Function)
' i changes every time you iterate bad

' eager vs deferred operators
'   Deferred: Where, Select, OrderBy, Skip, Take, Distinct
'   Eager:    ToList, ToArray, Count, First, Sum, Max, Any, All

' check if any/all match
Dim hasEven = nums.Any(Function(n) n Mod 2 = 0)
Dim allPositive = nums.All(Function(n) n > 0)

Imports System.Threading.Tasks

' Task creation
Dim t1 = Task.Run(Function() DoWork())
Dim t2 = Task.Run(Function() 42)  ' returns Integer
Dim result = Await t2  ' 42

' async function
Async Function FetchDataAsync() As Task(Of String)
    Using client As New HttpClient()
        Return Await client.GetStringAsync("https://api.example.com")
    End Using
End Function

' call it
Dim data = Await FetchDataAsync()

' parallel execution
Dim tasks = {
    Task.Run(Function() ComputePart1()),
    Task.Run(Function() ComputePart2()),
    Task.Run(Function() ComputePart3())
}
Dim results = Await Task.WhenAll(tasks)

' when any (first to finish)
Dim allTasks = urls.Select(Function(u) FetchAsync(u))
Dim firstDone = Await Task.WhenAny(allTasks)
Dim firstResult = Await firstDone

' continuation
Dim t = Task.Run(Function() 42).
    ContinueWith(Function(prev) prev.Result * 2)
Console.WriteLine(t.Result)  ' 84

' cancellation
Dim cts As New CancellationTokenSource()
Dim token = cts.Token
Dim task = Task.Run(Function()
    For i = 1 To 100
        token.ThrowIfCancellationRequested()
        Thread.Sleep(100)
    Next
End Function, token)
cts.CancelAfter(500)  ' cancel after 500ms

Imports System.Threading.Tasks

' Parallel.For
Parallel.For(0, 100, Sub(i)
    ProcessItem(i)
End Sub)

' with options
Dim opts As New ParallelOptions With {
    .MaxDegreeOfParallelism = Environment.ProcessorCount
}
Parallel.For(0, 100, opts, Sub(i)
    ProcessItem(i)
End Sub)

' Parallel.ForEach
Dim items = GetItems()
Parallel.ForEach(items, Sub(item)
    ProcessItem(item)
End Sub)

' with state (thread-local)
Dim total As Integer = 0
Dim lockObj As New Object()
Parallel.For(0, 1000,
    Function() 0,  ' local init
    Function(i, state, localTotal)
        localTotal += Compute(i)
        Return localTotal
    End Function,  ' body
    Sub(localTotal)
        SyncLock lockObj
            total += localTotal
        End SyncLock
    End Sub  ' local finally
)

' break vs stop
Parallel.For(0, 100, Sub(i, state)
    If i = 50 Then state.Break()  ' stop iterations > 50
    If i = 25 Then state.Stop()   ' stop all immediately
End Sub)

' PLINQ
Dim nums = Enumerable.Range(1, 1000)
Dim squares = nums.AsParallel().
    Where(Function(n) n Mod 2 = 0).
    Select(Function(n) n * n).
    ToList()

Imports System.Threading

' SyncLock (Monitor.Enter/Exit)
Private ReadOnly lockObj As New Object()
Private counter As Integer = 0

Sub Increment()
    SyncLock lockObj
        counter += 1
    End SyncLock
End Sub

' Interlocked (atomic operations)
Interlocked.Increment(counter)
Interlocked.Decrement(counter)
Interlocked.Add(counter, 10)
Interlocked.Exchange(counter, 0)
Dim oldVal = Interlocked.CompareExchange(counter, 42, 0)  ' if 0, set to 42

' Mutex (cross-process)
Using mtx As New Mutex(False, "Global\MyAppMutex")
    If mtx.WaitOne(0) Then
        ' got the mutex — only one instance runs
    Else
        ' another instance is running
    End If
End Using

' Semaphore (limit concurrent access)
Dim sem As New Semaphore(3, 3)  ' 3 concurrent
sem.WaitOne()
Try
    ' work
Finally
    sem.Release()
End Try

' ReaderWriterLockSlim
Dim rwLock As New ReaderWriterLockSlim()
rwLock.EnterReadLock()
Try
    ' multiple readers
Finally
    rwLock.ExitReadLock()
End Try
rwLock.EnterWriteLock()
Try
    ' exclusive write
Finally
    rwLock.ExitWriteLock()
End Try

Imports System.Threading.Channels

' create a channel (bounded for backpressure)
Dim channel = Channel.CreateBounded(Of Integer)(100)

' producer
Async Function ProducerAsync() As Task
    For i = 1 To 1000
        Await channel.Writer.WriteAsync(i)
    Next
    channel.Writer.Complete()
End Function

' consumer
Async Function ConsumerAsync() As Task
    Await ForEachAsync(channel.Reader.ReadAllAsync(),
                       Environment.ProcessorCount,
                       Async Function(item)
                           Await ProcessItemAsync(item)
                       End Function)
End Function

' multiple producers/consumers
Dim producers = Enumerable.Range(0, 3).Select(Function(i) ProducerAsync())
Dim consumers = Enumerable.Range(0, 3).Select(Function(i) ConsumerAsync())
Await Task.WhenAll(producers.Concat(consumers))

' BlockingCollection (older API, still useful)
Dim bc As New BlockingCollection(Of Integer)(100)
' producer
Task.Run(Sub()
    For i = 1 To 100
        bc.Add(i)
    Next
    bc.CompleteAdding()
End Sub)
' consumer
Task.Run(Sub()
    For Each item In bc.GetConsumingEnumerable()
        ProcessItem(item)
    Next
End Sub)

' thread-safe queue
Dim queue As New ConcurrentQueue(Of Integer)()
queue.Enqueue(1)
Dim val As Integer
queue.TryDequeue(val)

Imports System.Threading

' System.Threading.Timer (background thread)
Dim timer As New Timer(Sub(state)
    Console.WriteLine($"Tick at {DateTime.Now}")
End Sub, Nothing, TimeSpan.Zero, TimeSpan.FromSeconds(5))

' change interval
timer.Change(TimeSpan.Zero, TimeSpan.FromSeconds(10))

' dispose
timer.Dispose()

' System.Timers.Timer (events, can use SynchronizingObject)
Dim eventTimer As New Timers.Timer With {
    .Interval = 5000,
    .AutoReset = True
}
AddHandler eventTimer.Elapsed, Sub(s, e)
    Console.WriteLine("Elapsed")
End Sub
eventTimer.Start()

' UI timer (WinForms — runs on UI thread)
Dim uiTimer As New Windows.Forms.Timer With {.Interval = 1000}
AddHandler uiTimer.Tick, Sub(s, e)
    Label1.Text = DateTime.Now.ToString()
End Sub
uiTimer.Start()

' one-shot delay
Async Function DoLaterAsync() As Task
    Await Task.Delay(5000)  ' 5 seconds
    Console.WriteLine("Done waiting")
End Function

' periodic with cancellation
Async Function PeriodicAsync(token As CancellationToken) As Task
    While Not token.IsCancellationRequested
        DoWork()
        Await Task.Delay(1000, token)
    End While
End Function

Imports System.Text.Json
Imports System.Text.Json.Serialization

Public Class Person
    Public Property Id As Integer
    Public Property Name As String
    <JsonPropertyName("email")>
    Public Property Email As String
    <JsonIgnore>
    Public Property Password As String
    Public Property CreatedAt As DateTime
End Class

' serialize
Dim p As New Person With {.Id = 1, .Name = "Alice", .Email = "[email protected]", .CreatedAt = Date.Now}
Dim json As String = JsonSerializer.Serialize(p)

' with options
Dim opts As New JsonSerializerOptions With {
    .WriteIndented = True,
    .PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    .DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
}
json = JsonSerializer.Serialize(p, opts)

' deserialize
Dim loaded = JsonSerializer.Deserialize(Of Person)(json)

' collections
Dim people As New List(Of Person) From {p}
Dim jsonList = JsonSerializer.Serialize(people)
Dim loadedList = JsonSerializer.Deserialize(Of List(Of Person))(jsonList)

' dynamic (JsonDocument)
Using doc As JsonDocument = JsonDocument.Parse(json)
    Dim name = doc.RootElement.GetProperty("name").GetString()
    Console.WriteLine(name)
End Using

' stream (large files)
Using fs As New FileStream("data.json", FileMode.Create)
    JsonSerializer.Serialize(fs, p)
End Using

Imports System.Xml.Serialization
Imports System.IO

<XmlRoot("person")>
Public Class Person
    <XmlElement("id")>
    Public Property Id As Integer
    <XmlElement("name")>
    Public Property Name As String
    <XmlAttribute("role")>
    Public Property Role As String
    <XmlIgnore>
    Public Property Password As String
    <XmlArray("orders")>
    <XmlArrayItem("order")>
    Public Property Orders As List(Of Order)
End Class

' serialize
Dim xs As New XmlSerializer(GetType(Person))
Using fs As New FileStream("p.xml", FileMode.Create)
    xs.Serialize(fs, p)
End Using

' with settings
Dim settings As New XmlWriterSettings With {
    .Indent = True,
    .IndentChars = "  ",
    .Encoding = Encoding.UTF8
}
Using fs As New FileStream("p.xml", FileMode.Create),
      xw As XmlWriter = XmlWriter.Create(fs, settings)
    xs.Serialize(xw, p)
End Using

' deserialize
Using fs As New FileStream("p.xml", FileMode.Open)
    Dim loaded = DirectCast(xs.Deserialize(fs), Person)
End Using

' collections
Dim xsList As New XmlSerializer(GetType(List(Of Person)))
Using fs As New FileStream("people.xml", FileMode.Create)
    xsList.Serialize(fs, people)
End Using

Imports Microsoft.VisualBasic.FileIO

' read CSV (handles quoted fields)
Using parser As New TextFieldParser("data.csv")
    parser.TextFieldType = FieldType.Delimited
    parser.SetDelimiters(",")
    parser.HasFieldsEnclosedInQuotes = True

    While Not parser.EndOfData
        Dim fields = parser.ReadFields()
        Console.WriteLine($"{fields(0)}, {fields(1)}")
    End While
End Using

' write CSV (manual, with proper escaping)
Function CsvEscape(s As String) As String
    If s.Contains(",") OrElse s.Contains("""") OrElse s.Contains(vbCrLf) Then
        Return $"""{s.Replace("""", """""")}"""
    End If
    Return s
End Function

Dim lines As New List(Of String)
lines.Add("name,age,email")
For Each p In people
    lines.Add($"{CsvEscape(p.Name)},{p.Age},{CsvEscape(p.Email)}")
Next
File.WriteAllLines("out.csv", lines)

' CsvHelper (NuGet) — production-grade
' Install-Package CsvHelper
Imports CsvHelper
Using reader As New StreamReader("data.csv"),
      csv As New CsvReader(reader, CultureInfo.InvariantCulture)
    Dim records = csv.GetRecords(Of Person)().ToList()
End Using

Using writer As New StreamWriter("out.csv"),
      csv As New CsvWriter(writer, CultureInfo.InvariantCulture)
    csv.WriteRecords(people)
End Using

Imports System.Runtime.Serialization
Imports System.IO

' DataContract (version-tolerant)
<DataContract>
Public Class Person
    <DataMember(Order:=1)>
    Public Property Id As Integer
    <DataMember(Order:=2)>
    Public Property Name As String
    <DataMember(Order:=3, IsRequired:=False)>
    Public Property Email As String
End Class

' serialize to XML
Dim ser As New DataContractSerializer(GetType(Person))
Using fs As New FileStream("p.xml", FileMode.Create)
    ser.WriteObject(fs, p)
End Using

' deserialize
Using fs As New FileStream("p.xml", FileMode.Open)
    Dim loaded = DirectCast(ser.ReadObject(fs), Person)
End Using

' Protocol Buffers (protobuf-net, NuGet)
' Install-Package protobuf-net
<ProtoContract>
Public Class Person
    <ProtoMember(1)>
    Public Property Id As Integer
    <ProtoMember(2)>
    Public Property Name As String
End Class

' serialize
Using fs As New FileStream("p.bin", FileMode.Create)
    ProtoBuf.Serializer.Serialize(fs, p)
End Using

' deserialize
Using fs As New FileStream("p.bin", FileMode.Open)
    Dim loaded = ProtoBuf.Serializer.Deserialize(Of Person)(fs)
End Using

' MessagePack (MessagePack-CSharp, NuGet)
' compact, fast, schema-less
Dim bytes = MessagePack.MessagePackSerializer.Serialize(p)
Dim loaded = MessagePack.MessagePackSerializer.Deserialize(Of Person)(bytes)

Imports System.Text.Json
Imports System.Text.Json.Serialization

' custom converter for a type
Public Class DateOnlyConverter
    Inherits JsonConverter(Of DateOnly)

    Public Overrides Function Read(reader As ByRef Utf8JsonReader,
                                    type As Type, options As JsonSerializerOptions) As DateOnly
        Return DateOnly.Parse(reader.GetString())
    End Function

    Public Overrides Sub Write(writer As Utf8JsonWriter, value As DateOnly, options As JsonSerializerOptions)
        writer.WriteStringValue(value.ToString("yyyy-MM-dd"))
    End Sub
End Class

' register globally
Dim opts As New JsonSerializerOptions()
opts.Converters.Add(New DateOnlyConverter())

' or per-property
Public Class Event
    <JsonConverter(GetType(DateOnlyConverter))>
    Public Property Date As DateOnly
End Class

' polymorphic serialization
<JsonDerivedType(GetType(Dog), "dog")>
<JsonDerivedType(GetType(Cat), "cat")>
Public MustInherit Class Animal
    Public Property Name As String
End Class

Public Class Dog
    Inherits Animal
    Public Property Breed As String
End Class

Dim a As Animal = New Dog With {.Name = "Rex", .Breed = "Lab"}
Dim json = JsonSerializer.Serialize(a, GetType(Animal))
' {"$type":"dog","name":"Rex","breed":"Lab"}

' conditional serialization
Public Class User
    Public Property Name As String
    Public Property Password As String

    Public Function ShouldSerializePassword() As Boolean
        Return False  ' never serialize password
    End Function
End Class

Imports System.Reflection

Dim t As Type = GetType(String)
' or: Dim t = obj.GetType()

' basic info
Console.WriteLine($"Name: {t.Name}")
Console.WriteLine($"Full: {t.FullName}")
Console.WriteLine($"Base: {t.BaseType}")
Console.WriteLine($"IsClass: {t.IsClass}")
Console.WriteLine($"IsEnum: {t.IsEnum}")
Console.WriteLine($"IsGenericType: {t.IsGenericType}")

' interfaces
For Each i In t.GetInterfaces()
    Console.WriteLine($"Implements: {i.Name}")
Next

' properties
For Each prop In t.GetProperties()
    Console.WriteLine($"Property: {prop.Name} ({prop.PropertyType.Name})")
Next

' methods
For Each m In t.GetMethods(BindingFlags.Public Or BindingFlags.Instance)
    Console.WriteLine($"Method: {m.Name}({String.Join(", ", m.GetParameters().Select(Function(p) p.ParameterType.Name))})")
Next

' fields, constructors, events
t.GetFields()
t.GetConstructors()
t.GetEvents()

' attributes
For Each attr In t.GetCustomAttributes(False)
    Console.WriteLine($"Attribute: {attr.GetType().Name}")
Next

' generic type args
If t.IsGenericType Then
    For Each arg In t.GetGenericArguments()
        Console.WriteLine($"Type arg: {arg.Name}")
    Next
End If

Imports System.Reflection

' create instance
Dim t As Type = GetType(List(Of String))
Dim obj As Object = Activator.CreateInstance(t)

' with constructor args
Dim dt As DateTime = Activator.CreateInstance(GetType(DateTime), {2024, 12, 31})

' invoke method
Dim mi As MethodInfo = t.GetMethod("Add")
mi.Invoke(obj, New Object() {"hello"})

' property get/set
Dim pi As PropertyInfo = t.GetProperty("Count")
Dim count As Integer = CInt(pi.GetValue(obj))
pi.SetValue(obj, 10)  ' may throw if read-only

' field get/set
Dim fi As FieldInfo = GetType(MyClass).GetField("_private", BindingFlags.NonPublic Or BindingFlags.Instance)
Dim val = fi.GetValue(instance)
fi.SetValue(instance, newValue)

' generic method
Dim method As MethodInfo = GetType(Enumerable).GetMethod("Where")
Dim generic = method.MakeGenericMethod(GetType(Integer))
Dim filtered = generic.Invoke(Nothing, New Object() {nums, predicate})

' create delegate from method
Dim del As [Delegate] = [Delegate].CreateDelegate(GetType(Func(Of String, Boolean)), mi)

' load assembly
Dim asm As Assembly = Assembly.LoadFrom("plugin.dll")
Dim pluginType As Type = asm.GetType("MyPlugin")
Dim plugin As Object = Activator.CreateInstance(pluginType)

Imports System

' define custom attribute
<AttributeUsage(AttributeTargets.Class Or AttributeTargets.Method, AllowMultiple:=True)>
Public Class DescriptionAttribute
    Inherits Attribute

    Public ReadOnly Property Text As String
    Public Sub New(text As String)
        Me.Text = text
    End Sub
End Class

' apply
<Description("User service")>
Public Class UserService
    <Description("Get user by ID")>
    Public Function GetUser(id As Integer) As User
        ' ...
    End Function
End Class

' read attributes
Dim t As Type = GetType(UserService)
Dim classAttr = CType(Attribute.GetCustomAttribute(t, GetType(DescriptionAttribute)), DescriptionAttribute)
Console.WriteLine(classAttr?.Text)

For Each mi In t.GetMethods()
    Dim methodAttr = CType(Attribute.GetCustomAttribute(mi, GetType(DescriptionAttribute)), DescriptionAttribute)
    If methodAttr IsNot Nothing Then
        Console.WriteLine($"{mi.Name}: {methodAttr.Text}")
    End If
Next

' built-in attributes
<Obsolete("Use NewMethod instead", True)>  ' True = error, False = warning
Sub OldMethod()
End Sub

<Conditional("DEBUG")>
Sub LogDebug(msg As String)
    Console.WriteLine(msg)
End Sub

<Serializable>
Public Class MyData
End Class

Imports System.Reflection
Imports System.Reflection.Emit

' build a dynamic assembly/module/type/method
Dim asmName As New AssemblyName("DynamicAsm")
Dim asm As AssemblyBuilder = AssemblyBuilder.DefineDynamicAssembly(asmName, AssemblyBuilderAccess.Run)
Dim mod As ModuleBuilder = asm.DefineDynamicModule("MainModule")
Dim type As TypeBuilder = mod.DefineType("Calculator", TypeAttributes.Public)

' add a method
Dim mb As MethodBuilder = type.DefineMethod("Add",
    MethodAttributes.Public Or MethodAttributes.Static,
    GetType(Integer), {GetType(Integer), GetType(Integer)})

Dim il As ILGenerator = mb.GetILGenerator()
il.Emit(OpCodes.Ldarg_0)            ' load first arg
il.Emit(OpCodes.Ldarg_1)            ' load second arg
il.Emit(OpCodes.Add)                ' add
il.Emit(OpCodes.Ret)                ' return

Dim createdType As Type = type.CreateType()
Dim result As Integer = CInt(createdType.GetMethod("Add").Invoke(Nothing, {5, 3}))
Console.WriteLine(result)  ' 8

' expression trees (higher-level)
Imports System.Linq.Expressions

Dim xParam As ParameterExpression = Expression.Parameter(GetType(Integer), "x")
Dim yParam As ParameterExpression = Expression.Parameter(GetType(Integer), "y")
Dim body As BinaryExpression = Expression.Add(xParam, yParam)
Dim addFunc As Func(Of Integer, Integer, Integer) =
    Expression.Lambda(Of Func(Of Integer, Integer, Integer))(body, {xParam, yParam}).Compile()
Console.WriteLine(addFunc(5, 3))  ' 8

Imports System.Reflection

' reflection is slow — cache and optimize

' BAD: re-fetch on every call
For Each item In items
    Dim prop = item.GetType().GetProperty("Name")
    Dim name = CStr(prop.GetValue(item))
Next

' GOOD: cache PropertyInfo
Dim propInfo As PropertyInfo = GetType(Item).GetProperty("Name")
For Each item In items
    Dim name = CStr(propInfo.GetValue(item))
Next

' BEST: compile to delegate (10-100x faster)
Dim getter = CreateGetter(Of Item, String)("Name")
For Each item In items
    Dim name = getter(item)
Next

Function CreateGetter(Of T, TResult)(propName As String) As Func(Of T, TResult)
    Dim param = Expression.Parameter(GetType(T), "obj")
    Dim body = Expression.Property(param, propName)
    Return Expression.Lambda(Of Func(Of T, TResult))(body, param).Compile()
End Function

' source generators (modern alternative)
' compile-time reflection — no runtime cost
' Partial Class MyService
'     <GenerateReflection>
'     Public Shared Sub PrintMethods()
'         ' generated code knows the types at compile time
'     End Sub
' End Class

' when to use reflection:
'   - serializers (JSON, XML)
'   - ORMs (mapping columns to properties)
'   - DI containers (constructor injection)
'   - plugin systems
'   - testing frameworks
' avoid for hot paths — cache or compile

Imports System.Runtime.InteropServices

Public Class NativeMethods
    ' MessageBox from user32.dll
    <DllImport("user32.dll", CharSet:=CharSet.Unicode, SetLastError:=True)>
    Public Shared Function MessageBox(hWnd As IntPtr, text As String, caption As String, type As UInteger) As Integer
    End Function

    ' Beep from kernel32
    <DllImport("kernel32.dll")>
    Public Shared Function Beep(freq As UInteger, duration As UInteger) As Boolean
    End Function

    ' GetSystemMetrics
    <DllImport("user32.dll")>
    Public Shared Function GetSystemMetrics(nIndex As Integer) As Integer
    End Function
End Class

' usage
NativeMethods.MessageBox(IntPtr.Zero, "Hello from native!", "Test", 0)
NativeMethods.Beep(440, 500)  ' 440 Hz for 500ms

' constants
Public Const SM_CXSCREEN As Integer = 0
Public Const SM_CYSCREEN As Integer = 1
Dim screenWidth = NativeMethods.GetSystemMetrics(SM_CXSCREEN)

' structures
<StructLayout(LayoutKind.Sequential)>
Public Structure POINT
    Public X As Integer
    Public Y As Integer
End Structure

<DllImport("user32.dll")>
Public Shared Function GetCursorPos(ByRef lpPoint As POINT) As Boolean
End Function

Dim p As POINT
GetCursorPos(p)
Console.WriteLine($"Cursor at ({p.X}, {p.Y})")

Imports System.Runtime.InteropServices
Imports Excel = Microsoft.Office.Interop.Excel

' add reference: Microsoft.Office.Interop.Excel (or via NuGet)

Sub ExportToExcel(data As DataTable)
    Dim app As Excel.Application = Nothing
    Dim wb As Excel.Workbook = Nothing
    Dim ws As Excel.Worksheet = Nothing

    Try
        app = New Excel.Application()
        app.Visible = False
        wb = app.Workbooks.Add()
        ws = CType(wb.Worksheets(1), Excel.Worksheet)

        ' write headers
        For c = 0 To data.Columns.Count - 1
            ws.Cells(1, c + 1) = data.Columns(c).ColumnName
        Next

        ' write data
        For r = 0 To data.Rows.Count - 1
            For c = 0 To data.Columns.Count - 1
                ws.Cells(r + 2, c + 1) = data.Rows(r)(c).ToString()
            Next
        Next

        ws.SaveAs("C:	empexport.xlsx")
    Finally
        ' ALWAYS release COM objects
        If ws IsNot Nothing Then Marshal.ReleaseComObject(ws)
        If wb IsNot Nothing Then
            wb.Close(False)
            Marshal.ReleaseComObject(wb)
        End If
        If app IsNot Nothing Then
            app.Quit()
            Marshal.ReleaseComObject(app)
        End If
    End Try
End Sub

' late binding (no reference needed)
Dim lateApp As Object = CreateObject("Excel.Application")
lateApp.Visible = True
lateApp.Quit()

' VB and C# interoperate seamlessly in the same project (multi-targeting)
' Add a C# project reference, or use a shared library

' C# code (in CSharpLib.dll):
'   namespace Utils
'   {
'       public class Calculator
'       {
'           public int Add(int a, int b) => a + b;
'           public static int Multiply(int a, int b) => a * b;
'       }
'   }

' VB usage:
Imports Utils

Dim calc As New Calculator()
Dim sum = calc.Add(2, 3)              ' 5
Dim product = Calculator.Multiply(4, 5)  ' 20

' differences to watch:
'   - VB is case-insensitive; C# is case-sensitive
'   - VB uses () for indexing; C# uses []
'   - VB And/Or are bitwise (AndAlso/OrElse for short-circuit)
'   - C# uses camelCase often; VB PascalCase by convention
'   - C# has unsafe code and pointers; VB doesn't
'   - C# has yield return; VB has Yield (similar)

' consuming async C# methods
Dim result = Await csharpObj.GetDataAsync()

' ref/out parameters
' C#: void TryParse(string s, out int result)
' VB: Integer.TryParse(s, result)  ' result is ByRef

' using C# extension methods
Imports CSharpExtensions
Dim s = "hello".Capitalize()  ' extension method from C# lib

' Python via Python.NET (NuGet: Python.Runtime)
' Install-Package Python.NET

' After setup:
' PythonEngine.Initialize()
' Using Py.GIL()
'     Dim np As PyObject = Py.Import("numpy")
'     Dim arr = np.array(New Integer() {1, 2, 3, 4, 5})
'     Dim mean = np.mean(arr)
'     Console.WriteLine(mean)
' End Using

' Or run Python script via process
Dim psi As New ProcessStartInfo With {
    .FileName = "python.exe",
    .Arguments = "script.py arg1 arg2",
    .UseShellExecute = False,
    .RedirectStandardOutput = True,
    .CreateNoWindow = True
}
Using p As Process = Process.Start(psi)
    Dim output = p.StandardOutput.ReadToEnd()
    p.WaitForExit()
    Console.WriteLine(output)
End Using

' R via R.NET (NuGet)
' Install-Package R.NET
' Dim engine = REngine.GetInstance()
' engine.Evaluate("x <- c(1, 2, 3, 4, 5)")
' Dim mean = engine.Evaluate("mean(x)").AsNumeric()[0]

' JavaScript via ClearScript (V8)
' Install-Package Microsoft.ClearScript.V8
' Dim engine As New V8ScriptEngine()
' engine.Execute("function add(a, b) { return a + b; }")
' Dim result = engine.Script.add(2, 3)

' command-line tools
Dim result = Process.Start("ffmpeg", "-i input.mp4 output.wav")

Imports System.Runtime.InteropServices

' allocate native memory
Dim ptr As IntPtr = Marshal.AllocHGlobal(1024)  ' 1KB
Try
    ' copy data
    Marshal.WriteByte(ptr, 0, 42)
    Marshal.WriteInt32(ptr, 4, 1234)

    ' copy array
    Dim arr As Integer() = {1, 2, 3, 4}
    Marshal.Copy(arr, 0, ptr, arr.Length)

    ' read back
    Dim b = Marshal.ReadByte(ptr, 0)
    Dim i = Marshal.ReadInt32(ptr, 4)

    Dim arr2(3) As Integer
    Marshal.Copy(ptr, arr2, 0, 4)
Finally
    Marshal.FreeHGlobal(ptr)
End Try

' string marshaling
Dim strPtr As IntPtr = Marshal.StringToHGlobalUni("hello")
Try
    ' pass to native function
Finally
    Marshal.FreeHGlobal(strPtr)
End Try

' pointer-sized fields
<StructLayout(LayoutKind.Sequential)>
Public Structure HandleInfo
    Public Handle As IntPtr
    Public Size As IntPtr
End Structure

' unsafe code (C# only — VB doesn't support unsafe)
' in C#: unsafe { int* p = &x; *p = 42; }
' in VB: use Marshal class or write a C# helper

' Span(Of T) — modern memory-safe pointer
Dim span As Span(Of Byte) = New Byte(1023) {}
span(0) = 42
Dim intSpan = MemoryMarshal.Cast(Of Byte, Integer)(span)