Thread safe singleton design pattern

This pattern is one of the best known patterns. This pattern allows only a single instance of itself to be created and gives access to this instance.

Usually you can create it this way:

    public class Singleton
    {
        private Singleton()
        {
        }

        private static Singleton instance = null;

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

                return instance;
            }
        }

        public string SayHello()
        {
            return "Hello";
        }
    }

    public class test
    {
        public void Main()
        {
            Console.WriteLine(Singleton.Instance.SayHello());
        }
    }

But this is not thread safe. Because two threads could check the same if (instance == null) and create two instance of the Singleton class.

As a solution you can create Nested class and create instance of Singleton in it.

    public class Singleton
    {
        private Singleton()
        {
        }

        public static Singleton Instance
        {
            get
            {
                return Nested.instance;
            }
        }

        public string SayHello()
        {
            return "Hello";
        }

        private class Nested
        {
            // Explicit static constructor to tell C# compiler
            // not to mark type as beforefieldinit
            static Nested()
            {
            }

            internal static readonly Singleton instance = new Singleton();
        }
    }