Delegate in C#

Delegate

A delegate in C# is similar to a function pointer in C or C++. Using a 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.

1. Delegate is a type safe function pointer.

2. Delegate holds a reference (Pointer) to a function.

3. The signature of the delegate must match the signature of the function, the delegate points to, otherwise you get a compiler error.

4. Delegate is similar to a class.

5 You can create an instance of it.

Declaring Delegates

Without Parameter

public delegate void HelloDelegate();

With Parameter

public delegate void ParameteriseHelloDelegate(string Message);

Creating function that match the signature of the delegates

Without Parameter

public static void Hello()

{

       Console.WriteLine("Simple - Hello World");

}

With Parameter

public static void Hello(string Message)

{

       Console.WriteLine("Parameterise - " + Message);

}

Creating instance of delegates

Invoke Without Parameter Delegate

HelloDelegate simpleDelegate = new HelloDelegate(Hello);

simpleDelegate();

Invoke With Parameter Delegate

ParameteriseHelloDelegate ParameterDelegate = new ParameteriseHelloDelegate(Hello);

ParameterDelegate("Hello World");

Output

Without Parameter Delegate

With Parameter Delegate

Multicast delegate

1. A Multicast delegate is a delegate that has reference to more than one function

2. When you invoke a multicast delegate, all the functions the delegate is pointing to are invoked.

3. A multicast delegate, invokes the methods in the invocation list, in the same order in which they are added.

4. Register & Un-Register a method with the delegate

+ Or += to register a method with the delegate

- Or -= to unregister a method with the delegate

Declaring Delegate

public delegate void HelloDelegate();

Creating function that match the signature of the delegate

public static void HelloOne()

{

       Console.WriteLine("Hello World One");

}

public static void HelloTwo()

{

       Console.WriteLine("Hello World Two");

}

public static void HelloThree()

{

       Console.WriteLine("Hello World Three");

}

Creating instance of delegate

HelloDelegate multicastDelegate1 = new HelloDelegate(HelloOne);

HelloDelegate multicastDelegate2 = new HelloDelegate(HelloTwo);

HelloDelegate multicastDelegate3 = new HelloDelegate(HelloThree);

HelloDelegate multicastDelegate4 = multicastDelegate1 + multicastDelegate2 - multicastDelegate3;

multicastDelegate4();

OR

HelloDelegate multicastDelegate = new HelloDelegate(HelloOne);

multicastDelegate += HelloTwo;

multicastDelegate -= HelloThree;

multicastDelegate();

Output

Complete Code

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace Delegate

{

    class Program

    {

        /**************Simple Delegate***************/

        public delegate void HelloDelegate();

        //public delegate void HelloDelegate(string Message); // Error - Delegate can't be overloaded

        public static void Hello()

        {

            Console.WriteLine("Simple - Hello World");

        }

        /**************Simple Delegate***************/

        /**************Parameterise Delegate***************/

        public delegate void ParameteriseHelloDelegate(string Message);

        public static void Hello(string Message)

        {

            Console.WriteLine("Parameterise - " + Message);

        }

        /**************Parameterise Delegate***************/

        /**************Multicast Delegate***************/

        public static void HelloOne()

        {

            Console.WriteLine("Hello World One");

        }

        public static void HelloTwo()

        {

            Console.WriteLine("Hello World Two");

        }

        public static void HelloThree()

        {

            Console.WriteLine("Hello World Three");

        }

        /**************Multicast Delegate***************/

        public static void Main(string[] args)

        {

            /**************Simple Delegate***************/

            HelloDelegate simpleDelegate = new HelloDelegate(Hello);

            simpleDelegate();

            /**************Simple Delegate***************/

            /**************Parameterise Delegate***************/

            ParameteriseHelloDelegate ParameterDelegate = new ParameteriseHelloDelegate(Hello);

            ParameterDelegate("Hello World");

            /**************Parameterise Delegate***************/

            /**************Multicast Delegate***************/

            /*

                 + Or += to register a method with the delegate

                 - Or -= to unregister a method with the delegate

            */

            HelloDelegate multicastDelegate1 = new HelloDelegate(HelloOne);

            HelloDelegate multicastDelegate2 = new HelloDelegate(HelloTwo);

            HelloDelegate multicastDelegate3 = new HelloDelegate(HelloThree);

            HelloDelegate multicastDelegate4 = multicastDelegate1 + multicastDelegate2 + multicastDelegate3;

            multicastDelegate4();

            HelloDelegate multicastDelegate = new HelloDelegate(HelloOne);

            multicastDelegate += HelloTwo;

            multicastDelegate -= HelloThree;

            multicastDelegate();

            /**************Multicast Delegate***************/

            Console.ReadLine();

        }

    }

}