C# Extension method | Extending methods of existing classes

Program

using System;

namespace SampleConsoleAppln

{

    public static class IntExtensions

    {

        public static bool IsInvalid(this int mark, int MaxMark)

        {

            return (mark < 0 || mark > MaxMark); //true or false

        }

    }

    //Extension-method must be defined in a top-level of static class;

    //The class containing Extension methods should not be nested inside      //the another class. Here the following class is a static class.

    class ExtensionMethodExample

    {

        public static void Main(String[] x)

        {

            Console.WriteLine("Enter the max mark");

            int MaxMark = int.Parse(Console.ReadLine());

            GetMark:

            Console.WriteLine("Enter scored mark");

            int ScoredMark = int.Parse(Console.ReadLine());

            bool result = ScoredMark.IsInvalid(MaxMark);

            if (result)

            {

                Console.WriteLine("Invalid mark entered");

                goto GetMark;

            }

            Console.WriteLine("You have scored : {0}", ScoredMark);

            Console.ReadKey();

        }

    }

}

Output

Enter the max mark
100

Enter scored mark
101
Invalid mark entered

Enter scored mark
-10
Invalid mark entered

Enter scored mark
95

You have scored: 95