C# Delegates & Events Cheat Sheet

public delegate int MathOperation(int a, int b);   // Declare a delegate typeint Add(int a, int b) => a + b;int Multiply(int a, int b) => a * b;MathOperation op = Add;Console.WriteLine(op(3, 4));       // 7op = Multiply;Console.WriteLine(op(3, 4));       // 12// Multicast delegateAction<string> logger = Console.WriteLine;logger += msg => File.AppendAllText("log.txt", msg);logger("Hello");   // Invokes both handlers in order

Func<int, int, int> add = (a, b) => a + b;        // Takes params, returns a valueAction<string> print = msg => Console.WriteLine(msg);  // Takes params, returns voidPredicate<int> isEven = n => n % 2 == 0;          // Takes a param, returns boolint sum = add(2, 3);print("done");bool result = isEven(4);

public class Button{    public event EventHandler<EventArgs>? Clicked;   // Standard event pattern    public void SimulateClick()    {        Clicked?.Invoke(this, EventArgs.Empty);       // Null-safe raise    }}var button = new Button();button.Clicked += (sender, e) => Console.WriteLine("Button clicked!");button.SimulateClick();

public class OrderPlacedEventArgs : EventArgs{    public int OrderId { get; }    public decimal Total { get; }    public OrderPlacedEventArgs(int orderId, decimal total) => (OrderId, Total) = (orderId, total);}public class OrderProcessor{    public event EventHandler<OrderPlacedEventArgs>? OrderPlaced;    public void Place(int id, decimal total) =>        OrderPlaced?.Invoke(this, new OrderPlacedEventArgs(id, total));}var processor = new OrderProcessor();EventHandler<OrderPlacedEventArgs> handler = (s, e) => Console.WriteLine($"Order {e.OrderId}: {e.Total:C}");processor.OrderPlaced += handler;// ... later, before the subscriber's lifetime ends:processor.OrderPlaced -= handler;   // Prevents the publisher from keeping the subscriber alive

Action a1 = () => Console.WriteLine("first");Action a2 = () => Console.WriteLine("second");Delegate combined = Delegate.Combine(a1, a2);foreach (Action d in combined.GetInvocationList())    d();                                   // Invoke each subscriber individually, e.g. to isolate exceptions// Aggregating return values from a multicast Func requires GetInvocationList tooFunc<int> f1 = () => 1;Func<int> f2 = () => 2;Func<int> both = f1 + f2;int lastResult = both();                   // Only 2 — invoking directly discards all but the last resultint[] allResults = both.GetInvocationList().Select(d => ((Func<int>)d)()).ToArray(); // [1, 2]

// Covariant return type: a delegate expecting object can be satisfied by a method returning stringFunc<object> getObj = () => "hello";// Contravariant parameter type: a delegate expecting string can be satisfied by a method accepting objectAction<string> printStr = (object o) => Console.WriteLine(o);// Common real-world case: EventHandler<DerivedEventArgs> can be assigned where EventHandler<EventArgs> is expectedvoid Handler(object? sender, EventArgs e) => Console.WriteLine("handled");EventHandler<FileSystemEventArgs> fsHandler = Handler;  // object/EventArgs params are contravariant-compatible

var buttons = new List<Button>();for (int i = 0; i < 3; i++){    int captured = i;                    // Copy into a loop-scoped local    buttons[i].Clicked += (s, e) => Console.WriteLine($"Button {captured} clicked");    // Without the copy, C# >= 5 for-loops still create a new 'i' per iteration for 'for',    // but 'foreach' variables and mutable outer locals remain a common source of this bug    // in older code or when the loop variable is reused across nested delegates.}