c# delegates tutorial

What is a Delegate?

In the English language one of the meanings of a delegate is "To commit or entrust to another". Thats precisely what a delegate does in C#. When called, it entrusts (delegates) the job to another method (function). If you are coming from the C++ world, you must note that in C++ delegate is implemented as a function pointer, but C# it is implemented as a class.

In the example below I define a delegate. Then I use instances of that delegate to call two different methods.

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace DelegateExample

{

    class Program

    {

        /*DECLARE THE DELEGATE WITH THE SAME SIGNATURE AS THE FUNCTION TO BE CALLED*/

        delegate void myDelegate();

      

        static void Main(string[] args)

        {

            /*INSTANTIATE THE DELEGATE INSTANCE TO THE FUNCTION THAT NEEDS TO BE CALLED*/

            myDelegate myDel1= SayHello;

            myDelegate myDel2 = SayGoodBye;

            myDel1();

            //Displays Hello!

            myDel2();

            //Displays Bye!

        }

        private static void SayHello()

        {

            Console.WriteLine("Hello!");

        }

        private static void SayGoodBye()

        {

            Console.WriteLine("Bye!");

        }

    } 

}

Output window

One important thing to remember about delegates is that, it can be passed as a parameter (which you cannot do with methods)


Using Delegate as a parameter to a method

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace Delagate

{

    class Program

    {

        delegate int myDelegate(int x, int y);

        static void Main(string[] args)

        {

            int a = 2;

            int b = 3;

            bool doYouWantToAdd = false;

            myDelegate myDel;

            if (doYouWantToAdd == true)

            {

                myDel = Add;

            }

            else

            {

                myDel = Subtract;

            }

            int z = AddOrSubtract(myDel, a, b);

            Console.WriteLine(z);

        }

        static int Add(int a, int b)

        {

            return (a + b);

        }

        static int Subtract(int a, int b)

        {

            return (a - b);

        }

        static int AddOrSubtract(myDelegate myDel, int a, int b)

        {

            //This method accepts a delegate as a parameter

            return myDel(a, b);

        }

    }

}

Using Delegate to create an anonymous method

Anonymous method is a shorthand way to assign a method to a delegate without having to declare a method separately. Anonymous method is an important (but simple) concept to be learned before you can learn Lambda expressions.

In this example below, instead of separately creating an add method, I will directly declare it when I instantiate the delegate.

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace DelegateExample

{

    class Program

    {

        delegate int myDelegate(int a,int b);

        static void Main(string[] args)

        {

            myDelegate myDel1 = new myDelegate(delegate(int a, int b) { return (a + b); });//the add method is an anonymous method here

            int addResult=myDel1(1, 2);

            Console.WriteLine(addResult);//displays 3

        }     

    }

}