using System;

public class Program

{

    class Singleton

    {

        private static Singleton instance = null;

        protected Singleton()

        {           

        }

        //Fabric method

        public static Singleton Instance()

        {

            if (instance == null)

            {

                instance = new Singleton();

            }

            return instance;

        }

    }

    public static void Main()

    {

        Singleton instance1 = Singleton.Instance();

        Singleton instance2 = Singleton.Instance();

        if (instance1 == instance2)

        {

            Console.WriteLine("Reference mark in similar Class");

        }

    }

}