泛型

public class Stack<T>
{
    private List<T> _items = new();
    public void Push(T x) => _items.Add(x);
    public T Pop() { var t = _items[^1]; _items.RemoveAt(_items.Count - 1); return t; }
    public int Count => _items.Count;
}

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

public static T Max<T>(T a, T b) where T : IComparable<T>
    => a.CompareTo(b) >= 0 ? a : b;

where T : class                    // 引用类型
where T : struct                   // 值类型
where T : new()                    // 有无参构造
where T : BaseClass                // 继承指定基类
where T : IComparable              // 实现指定接口
where T : notnull                  // 非空（C# 8+）
where T : unmanaged                // 非托管类型
where T : U                        // 类型参数依赖

public class Cache<T> where T : class, new()
{
    public T GetOrAdd() => new();
}

public delegate T Func<T, TResult>(T arg);

// .NET 内置
Action<string> log = msg => Console.WriteLine(msg);
Func<int, int, int> add = (a, b) => a + b;

List<int> l = new() { 1, 2, 3 };
Dictionary<string, int> d = new() { ["a"] = 1 };
HashSet<int> s = new() { 1, 2, 3 };
Queue<int> q = new();
Stack<int> st = new();

IEnumerable<string> strs = new List<string>();
IEnumerable<object> objs = strs;   // 协变

Action<object> printObj = o => Console.WriteLine(o);
Action<string> printStr = printObj;  // 逆变

var x = Max(1, 2);                  // 推断 int
var y = Max("a", "b");              // 推断 string

void Print<T>(T x) { Console.WriteLine(x); }

Print<int>(1);
Print("x");   // 推断

var listType = typeof(List<>);
var intListType = listType.MakeGenericType(typeof(int));
Console.WriteLine(intListType);    // List`1[Int]

public static T[] Where<T>(this T[] source, Func<T, bool> predicate);

public interface IRepository<out T>
{
    T Get(int id);
}

var users = new UserRepo();
IRepository<object> objs = users;   // OK，out 协变