C# Essentials

Delegates

attributes ? access-modifier ?

new?

delegate

[ void | type ]

delegate - name ( parameter-list );

A delegate is a type that defines a method signature, so that delegate instances can hold and invoke a method or list of methods that match its signature. A delegate declaration consists of a name and a method signature.^[1]

Here’s an example:

delegate bool Filter(string s);

This declaration lets you create delegate instances that can hold and invoke methods that return bool and have a single string parameter. In the following example a Filter is created that holds the FirstHalf-OfAlphabet method. You then pass the Filter to the Display method, which invokes the Filter:

class Test {
   static void Main( ) {
      Filter f = new Filter(FirstHalfOfAlphabet);
      Display(new String [] {"Ant","Lion","Yak"}, f);
   }
   static bool FirstHalfOfAlphabet(string s) {
      return "N".CompareTo(s) > 0;
   }
   static void Display(string[] names, Filter f) {
      int count = 0;
      foreach(string s in names)
         if(f(s)) // invoke delegate
            Console.WriteLine("Item {0} is {1}", count++, s);
   }
}

Multicast Delegates

If a delegate has a void return type, it is a multicast delegate that can hold and invoke multiple methods. In this example, we declare a simple delegate called MethodInvoker, which can hold and then invoke the Foo and Goo methods sequentially. The += method creates a new delegate by adding the right delegate operand to the left delegate operand.

delegate void MethodInvoker( ); class Test { static void Main( ) { new Test( ...