using System;

public class Program

{

    public static void Main()

    {

        var s1 = Single1.GetInstance();

        s1.Log();

        var s2 = Single2.GetInstance();

        s2.Log();

    }

    public class Single1

    {

        private static Single1 instance;

        private Single1() {}

        static Single1()

        {

            instance = new Single1();

        }

        public static Single1 GetInstance()

        {

            return instance;

        }

        public void Log()

        {

            Console.WriteLine("Hello World");

        }

    }

    public class Single2

    {

        private static object lockObj = new object();

        private static Single2 instance;

        private Single2() {}

        public static Single2 GetInstance()

        {

            if (instance != null)

                return instance;

            lock(lockObj)

            {

                if (instance != null)

                    return instance;

                instance = new Single2();

            }

            return instance;

        }

        public void Log()

        {

            Console.WriteLine("Hello World");

        }

    }

}