File I/O

using System;
using System.IO;
using System.Text.Json;

// Write all text in one call
File.WriteAllText("note.txt", "hello world\n");

// Read all text
string text = File.ReadAllText("note.txt");
Console.WriteLine(text.Trim());

// Append
File.AppendAllText("note.txt", "second line\n");

// Read line by line lazily
foreach (var line in File.ReadLines("note.txt"))
    Console.WriteLine($"[line] {line}");

// JSON serialize/deserialize
var data = new { Name = "Alice", Age = 30 };
string json = JsonSerializer.Serialize(data);
File.WriteAllText("data.json", json);

var person = JsonSerializer.Deserialize<JsonElement>(json);
Console.WriteLine(person.GetProperty("Name").GetString());

// Async copy
async Task CopyAsync(string src, string dst)
{
    using var sIn = File.OpenRead(src);
    using var sOut = File.Create(dst);
    await sIn.CopyToAsync(sOut);
}
await CopyAsync("note.txt", "note.copy.txt");