class Singleton
    {
        private static Singleton instance;

private Singleton() { }

public static Singleton Instance
        {
            get
            {
                if (instance == null)
                {
                    instance = new Singleton();
                }
                return instance;
            }
        }
    }

class Test
    {
        public static void Main()
        {
            Singleton s = Singleton.Instance;
        }
    }

    class Singleton2
    {
        private static volatile Singleton2 instance = null;

private static object lockhelper = new object();

private Singleton2() { }

public static Singleton2 Instance
        {
            get
            {
                if (instance == null)
                {
                    lock (lockhelper)
                    {
                        if (instance == null)
                        {
                            instance = new Singleton2();
                        }
                    }
                }
                return instance;
            }
        }
    }

class Singleton3
    {
        public static readonly Singleton3 Instance = new Singleton3();

private Singleton3() { }
    }
    class Singleton3_1
    {
        public static readonly Singleton3_1 Instance = new Singleton3_1();

private Singleton3_1() { }

static Singleton3_1()
        {
            Instance = new Singleton3_1();
        }
    }