Extension methods in C#

   What is Extension method in C# ?

• An extension method is a special kind of static method that allows you to add new methods to existing types without creating derived types.
• The extension methods are called as if they were instance methods from the extended type, For example: x is an object from int class and we called it as an instance method in int class.
• 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.Web;

namespace LINQPractice.HelperClass

{

  public static class String

  {

    public static string ConverToUpperCase(this string strg)

    {

      return strg.ToUpper();

    }

  }

}

Calling from Different class:

using System.Web.Mvc;

using LINQPractice.HelperClass;

namespace LINQPractice.Controllers

{

    public class LINQController : Controller

    {

        // GET: LINQ

        public ActionResult Index()

        {

          string str1= "vijay";

          string outputstr= str1.ConverToUpperCase();

            return View();

        }

    }

}

Important points for the use of extension methods:

• An extension method must be defined in a top-level static class.
• An extension method with the same name and signature as an instance method will not be called.
• Extension methods cannot be used to override existing methods.
• The concept of extension methods cannot be applied to fields, properties or events.
• Overuse of extension methods is not a good style of programming.

References:

https://www.codeproject.com/Articles/34209/Extension-Methods-in-C

http://www.c-sharpcorner.com/uploadfile/puranindia/extension-methods-in-C-Sharp-3-0/