How to write callbacks (delegate) in C# and .NET

Copy from:
http://www.myelin.co.nz/notes/callbacks/cs-delegates.html

http://msdn.microsoft.com/en-us/library/Bb985784

http://msdn.microsoft.com/en-us/magazine/cc301810.aspx

If you're used to function pointers in C, a delegate is basically a pair of pointers rolled into one:

• A pointer to an object (optional)
• A pointer to a method of that object

That means a single delegate passes all the information needed to locate a function in your program, whether it's a static method or associated with an object.
You define them like this in C#:

public delegate void FooCallbackType( int a, int b, int c );

When you want to use them, you make delegate out of the function you want to call:

class CMyClass
{
 public void FunctionToCall( int a, int b, int c )
 {
  // This is the callback
 }

 public static void Foo()
 {
  FooCallbackType myDelegate = new FooCallbackType(
   this.FunctionToCall );
  // Now you can pass that to the function
  // that needs to call you back.
 }
}

If you want to make a delegate to point to a static method, it just looks the same:

class CMyClassWithStaticCallback
{
 public static void StaticFunctionToCall( int a, int b, int c )
 {
  // This is the callback
 }

 public static void Foo()
 {
  FooCallbackType myDelegate = new FooCallbackType(
   CMyClass.StaticFunctionToCall );
 }
}

All in all, they do the same thing as interface-based callbacks in C++, but cause a bit less trouble because you don't need to worry about naming your functions or making helper objects, and you can make delegates out of any method. They're more flexible.