/// <summary>
    /// 单例模式
    /// sealed修饰符，不能被继承人
    /// 构造函数private 外面不可以实例化！
    /// </summary>
    public sealed class Singleton
    {
        private Singleton() {
            Console.WriteLine("初始化一次");
        }
        private static Singleton Instance = new Singleton();
        public static Singleton CreateInstance()
        {
            return Instance;
        }
    }