using System;

public class Program

{

    public static void Main()

    {

        int i = 9;

        System.Console.WriteLine(i.MultiplyByTwo());

        System.Console.WriteLine("hello".AddIt());

    }

}

​

static class ExtensionMethods

{

    public static int MultiplyByTwo(this int value)

    {

    // This extension method multiplies an int by 2.

    return value * 2;

    }

    public static string AddIt(this string value)

    {

    // This extension method multiplies an int by 2.

    return value + " world";

    }

​

}