//When we needed to add a new function to an existing class type without changing the original source of code and without the use of inheritance.

//https://www.c-sharpcorner.com/blogs/when-should-we-use-extension-methods-in-c-sharp   

using System;

namespace folder

{

    public class FunctionClass

    {

        public double Multiply(int a, int b)

        {

            return a*b;

        }

    }

    public static class Extension

    {

        public static double Discount(this FunctionClass obj, int i, int j)

        {

            return i*j-100;

        }

    }

    public class MainProg

    {

        public static void Main()

        {

            FunctionClass F = new FunctionClass();

            Console.WriteLine(F.Multiply(22,66));

            Console.WriteLine(F.Discount(44,55));

        }

    }

}