C# Delegates & Events Cheat Sheet

Covers delegates, multicast delegates, the built-in Func, Action, and Predicate types, and the C# event pattern for publish-subscribe notifications.

2 PagesIntermediateMar 30, 2026

Delegates

Type-safe references to methods, including multicast.

csharp

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, Action, Predicate

Built-in generic delegate types for common shapes.

csharp

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);

Events

The standard publish-subscribe pattern.

csharp

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();

Key Concepts

Terminology for delegates and events.

delegate- A type-safe reference to one or more methods with a matching signature
Multicast delegate- Combining delegates with += invokes each one in sequence when called
event keyword- Restricts external code to += / -= only, preventing direct invocation or overwrite
EventHandler / EventHandler<T>- Standard delegate signature: (object? sender, EventArgs e)
Func<T,...,TResult>- Built-in generic delegate for methods that return a value
Action<T,...>- Built-in generic delegate for methods that return void
Null-conditional invoke- handler?.Invoke(...) avoids a NullReferenceException when there are no subscribers

Pro Tip

Always raise events with the null-conditional operator (Clicked?.Invoke(...)) instead of a separate null check — it's safe against a subscriber unsubscribing between the check and the call.

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