C Sharp Extension Method

You probably came across the following declaration in C# and wonder what the hell does keyword this do.

namespace ConsoleApplication1
{
    public static class Numbers
    {
        public static int AddOne(this int a)
        {
            return a + 1;
        }

this keyword in this case makes the method a Extension Method, that means, instead of calling Numbers.AddOne(1) you can use 1.AddOne().

However, there are some restrictions:

1. this keyword must only appear before the first parameter (the modifier of the first parameter)

2. Extension method must be defined in a non-generic static class.

3. Extension method must be static.

The advantage of doing so is that you can see the declaration at the code-auto-completion dropdown list (Intellisense)

A full working example illustrates itself.

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

namespace ConsoleApplication1
{
    public static class Numbers // must be static class
    {
        public static int AddOne(this int a)
        {
            return a + 1;
        }

        // modifier 'this' must be on the first parameter
        public static int Add(this int a, int b)
        {
            return a + b;
        }

        public static string TestAdd(this string s, string b)
        {
            return s + b;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(1.AddOne());  // print 2
            Console.WriteLine(1.Add(2));    // print 3
            Console.WriteLine("1".TestAdd("2")); // print 12
            Console.ReadLine();
        }
    }
}

–EOF (The Ultimate Computing & Technology Blog) —