An extension method is special types of static method of a static class, where the "this" keyword is used as a first parameter of this method. And Extension methods allow existing classes to be extended without inheritance or also allow to change the source code of existing class.

You can also use an extension method, in whole project by including the namespace of an extension method.

usingSystem;

using System.Text;

namespace ClassTest

{

    public class Class1

    {

        public string Display()

        {

            return ("Display Method Called");

        }

    }

}

usingSystem;

using System.Text;

using ClassTest;

{

    public static class ABCD

    {

         public static void NewExtensionMethod(this Class1 ob)

        {

            Console.WriteLine("Hello!...E xtended method called");

        }

    }

    class Program

    {

        static void Main(string[] args)

        {

            Class1 obj = new Class1();

            obj.Display();

            obj.NewMethod();

            Console.ReadKey();

        }

    }

}