Generics

using System;
using System.Collections.Generic;

// Generic method with constraint
T Max<T>(T a, T b) where T : IComparable<T>
    => a.CompareTo(b) >= 0 ? a : b;

// Generic class
public class Stack<T>
{
    private readonly List<T> _items = new();
    public int Count => _items.Count;

    public void Push(T item) => _items.Add(item);
    public T Pop()
    {
        if (_items.Count == 0) throw new InvalidOperationException("empty");
        var top = _items[^1];
        _items.RemoveAt(_items.Count - 1);
        return top;
    }
}

// Generic dictionary cache
public class Cache<TKey, TValue> where TKey : notnull
{
    private readonly Dictionary<TKey, TValue> _data = new();
    public TValue GetOrAdd(TKey key, Func<TKey, TValue> factory)
    {
        if (!_data.TryGetValue(key, out var value))
            _data[key] = value = factory(key);
        return value;
    }
}

Console.WriteLine(Max(3, 7));
Console.WriteLine(Max("apple", "pear"));

var s = new Stack<int>();
s.Push(1); s.Push(2);
Console.WriteLine(s.Pop());

var cache = new Cache<string, int>();
Console.WriteLine(cache.GetOrAdd("a", k => k.Length));