单例模式(C#)

 public sealed class At
    {
        private static At instance = null;
        public static At Instance
        {
            get
            {
                if (instance == null)
                {
                    instance = new At();
                }
                return instance;
            }
        }

        public void Hello()
        {
            Console.WriteLine("Hello");
        }

        public void Hi()
        {
            Console.WriteLine("Hi");
        }
    }