Delegates

A delegate in C# is similar to a function pointer in C or C++. Delegate allows the programmer to encapsulate a reference to a method inside a delegate object. The delegate object can then be passed to code which can call the referenced method, without having to know at compile time which method will be invoked. Unlike function pointers in C or C++, delegates are object-oriented, type-safe, and secure.

A delegate is defined with a specific signature (return type, parameter type and order etc). To invoke a delegate object, one or more methods are required with the EXACT same signature. A delegate object is first created similar like a class object created. The delegate object will basically hold a reference of a function which has the same signature.

 

Syntax for Declaring a Delegate:

Public delegate <return type> <Delegate Name> (parameters);

Example: public delegate int Calculate (int x, int y);

We can use the delegates with out parameters also.

Delegates are of 2 types

1)     Single Cast Delegate

2)      Multi Cast  Delegate

Single Cast Delegate: It is a delegate which holds the reference of one method.

Example program for Single cast delegate

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace BloggerExamples

{

    //delegate declaration with two parameters integers.

    public delegate int MyDelegate(int x, int y);

    class SingleDelegateExample

    {

        public static int Add(int x, int y)

        {

            return x + y;

        }

        static void Main(string[] args)

        {

            MyDelegate del1 = new MyDelegate(Add);

            int addResult = del1(5, 5);

            Console.WriteLine("5 + 5 = {0}\n", addResult);

            Console.ReadLine();

        }

}

Multi Cast Delegate: It is a delegate which holds the reference of more than one method.

Delegate's multicast means that a delegate object can maintain a list of methods to call, rather than a single method.

If you want to add a method to the invocation list of a delegate object, you simply make use of the overloaded += operator, and if you want to remove a method from the invocation list you make use of the overloaded operator -=.

In Multi-casting you create a single delegate that will invoke multiple encapsulated methods. The return type of all the delegates should be same.

Note:  The question why are we using Multi-cast delegates when Single-cast delegates are enough?

 

Answer to this question is that if we want to call three methods when a button is clicked. In this situation we use the Multi-cast delegates instead of single cast delegate why because single cast can hold the reference of only one method. But Multi cast delegate can hold references of multiple methods 

The Multi-cast delegates are used with events where multiple calls to different methods are required.

The following program will demonstrate the Multi cast Delegate

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace Blogger Examples

{

    //delegate declaration with two parameters integers

    public delegate void MulticastDelegate(int x, int y);

    class MyMulticastDelegate

    {

        Public static void Add (int x, int y)

        {

            Console.WriteLine ("You are in Add () Method");

            Console.WriteLine ("{0} + {1} = {2}\n", x, y, x + y);

        }

        public static void Multiply (int x, int y)

        {

            Console.WriteLine ("You are in multiply () Method");

            Console.WriteLine("{0} X {1} = {2}", x, y, x * y);

        }

        static void Main(string[] args)

        {

            MulticastDelegate del = new MulticastDelegate (Add);

            del += new MulticastDelegate(Multiply);

            Console.WriteLine ("****calling Add() and Multiply() Methods.****\n\n");

            del(5, 5);

            del -= new MulticastDelegate(Add);

            Console.WriteLine("\n\n****Add() Method removed.****\n\n");

            del(5, 5);

            Console.ReadLine();

        }

    }

}

OutPut: