Extention Methods

• This is a new feature in C# 3.0
• Extension methods allow an existing type to be extended with new methods without altering the definition of the original type.
• An extension method is a static method of a static class, where the this modifier is applied to the first parameter.
• The type of the first parameter will be the type that is extended.

Example 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Extention_Methods
{
    class Program
    {
        static void Main(string[] args)
        {
            string language = "malayalam";
            Console.WriteLine(language.MyCustomMethod());
            Console.ReadLine();
        }
    }

    static class StringUtils
    {
        public static int MyCustomMethod(this string input)
        {
            return (input.Length);
        }
    }
}

Ref : here