C# 4.0 in a Nutshell, 4th Edition

Chapter 4. Advanced C#

In this chapter, we cover advanced C# topics that build on concepts explored in previous chapters. You should read the first four sections sequentially; you can read the remaining sections in any order.

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.WriteLine (result);      // 9
  }
  static int Square (int x) { return x * x; }
}

Note

Technically, we are specifying a method group when we refer to Square without brackets or arguments. If the method is overloaded, C# will pick the correct overload based on the signature of the delegate to which it’s being assigned.

Invoking ...