C# Delegates

A delegate is a type-safe function pointer — it holds a reference to a method and allows you to pass methods as arguments.

1 - Defining and Using

// Declare a delegate type
public delegate int MathOperation(int a, int b);

// Methods that match the signature
static int Add(int a, int b)      => a + b;
static int Multiply(int a, int b) => a * b;

// Use the delegate
MathOperation op = Add;
Console.WriteLine(op(3, 4));  // 7

op = Multiply;
Console.WriteLine(op(3, 4));  // 12

2 - Multicast Delegates

public delegate void Notify(string message);

static void Email(string msg) => Console.WriteLine($"Email: {msg}");
static void SMS(string msg)   => Console.WriteLine($"SMS: {msg}");

Notify notify = Email;
notify += SMS;  // both methods called
notify("Server is down!");
// Email: Server is down!
// SMS: Server is down!

3 - Func and Action (built-in delegates)

// Func — returns a value: Func<params..., returnType>
Func<int, int, int>    add  = (a, b) => a + b;
Func<string, string>   upper = s => s.ToUpper();
Func<int, bool>         isEven = n => n % 2 == 0;

// Action — returns void: Action<params...>
Action<string> print = msg => Console.WriteLine(msg);
Action greet = () => Console.WriteLine("Hello!");

Console.WriteLine(add(3, 4));        // 7
Console.WriteLine(upper("hello"));   // HELLO
print("World");

4 - Predicate<T>

// Special case: Func<T, bool>
Predicate<int> isPositive = n => n > 0;
var numbers = new List<int> { -3, -1, 0, 2, 5 };
var positive = numbers.FindAll(isPositive); // [2, 5]