using System;

using System.Threading.Tasks;

public class Program

{

    public static void Main()

    {

        //Console.WriteLine("Hello World");

        //SingleTonExample s=new SingleTonExample(); //not allowed

        //SingleTonExample.GetInstance.M1();

        //SingleTonExample.GetInstance.M1();

        var t1=Task.Run(()=>SingleTonExample.GetInstance.M1());

        var t2=Task.Run(()=>SingleTonExample.GetInstance.M1());

        t1.Wait();

        t2.Wait();

    }

    public sealed class SingleTonExample

    {

       private static int counter=0;

       private static SingleTonExample instance=null;

       private static readonly object lockinc = new object();

       public static SingleTonExample GetInstance

        {

            get

            {

                if(instance==null)

                {

                  lock(lockinc)

                  {

                    if (instance == null)

                    {

                     instance = new SingleTonExample();

                    }

                  }  

                }

                return instance;

            }

        }

        public void M1()

        {

            Console.WriteLine("hello from M1");

        }

        private SingleTonExample()

        {

          counter++;

          Console.WriteLine("hello from ctor :{0}",counter);

        }

    }

}