Extension Methods

Extension method enables to add new method(s) into a class (type / library) without modifying, recompiling or creating derived type.

Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type.

Scenario :
We've a class library 'Calculations' in which we have class 'Airithmatics' that contains method 'Add'.
Now we want to add 2 more methods 'Subtract' & 'Multiply' in this library without updating this library.

Limitation: Any extension method(s) with same signature to method(s) of main library / type will never be called

Steps to implement:

1. Create new library named 'ArithmaticAddons'
2. Add reference of main library 'Calculations'
3. Create a static class 'ArthAddons' in new library
4. Add new methods to this class as static method, you want to create as extension methods. Pass main library 'Airithmatics' in methods
5. Add reference of both the libraries 'Calculations', 'ArithmaticAddons' in client code
6. Create object of main library 'Airithmatics'
7. With the object of main library 'Airithmatics', you can see methods of main library & extension methods
8. Extension methods displays with a down arrow

Custom Library Code:

using  Calculations;  //Main library contains ‘Add’ function

namespace ArithmaticAddons

{

    public static class ArthAddons

    {

        public static int Subtract(this Airithmatics obj, int num1, int num2)

        {

            return num1 - num2;

        }

 public static int Multiply(this Airithmatics obj, int num1, int num2)

        {

            return num1 * num2;

        }

    }

}

Client Code:

using Calculations; //Main library contains ‘Add’ function

using ArithmaticAddons; //Custom library contains subtract & Multiply extension methods

namespace ExtensionMethods

{

  class Program

  {

    static void Main(string[] args)

    {

      Airithmatics obj = new Airithmatics();

      Console.WriteLine(obj.Add(20, 215).ToString());

      Console.ReadLine();

      //Extension method Subtract & Multiply call

      Console.WriteLine(obj.Subtract(220, 215).ToString());

      Console.WriteLine(obj.Multiply(2, 20).ToString());

      Console.ReadLine();

    }

  }

}