static void Main(string[] args)
        {
            // 也会打印An instance of Singleton4 is created.
            Singleton4.Print();

            // 不会打印An instance of Singleton5 is created.
            Singleton5.Print();
        }
        static void Main(string[] args)
        {
            Singleton5 xxx = Singleton5.Instance;

            xxx.doSomeWork();
        }