C# 4.0 Pocket Reference, 3rd Edition

Delegates

A delegate dynamically wires up a method caller to its target method. There are two aspects to a delegate: type and instance. A delegate type defines a protocol to which the caller and target will conform, comprising a list of parameter types and a return type. A delegate instance is an object that refers to one or more target methods conforming to that protocol.

A delegate instance literally acts as a delegate for the caller: the caller invokes the delegate, and then the delegate calls the target method. This indirection decouples the caller from the target method.

A delegate type declaration is preceded by the keyword delegate, but otherwise it resembles an (abstract) method declaration. For example:

delegate int Transformer (int x);

To create a delegate instance, you can assign a method to a delegate variable:

class Test
{
  static void Main()
  {
    Transformer t = Square;  // Create delegate instance
    int result = t(3);       // Invoke delegate
    Console.Write (result);  // 9
  }
  static int Square (int x) { return x * x; }
}

Invoking a delegate is just like invoking a method (since the delegate’s purpose is merely to provide a level of indirection):

t(3);

The statement Transformer t = Square is shorthand for:

Transformer t = new Transformer (Square);

And t(3) is shorthand for:

t.Invoke (3);

A delegate is similar to a callback, a general term that captures constructs such as C function pointers.

Writing Plug-in Methods with Delegates

A delegate variable is assigned a method dynamically. This is useful ...