public class Singleton<T> where T : class    {        PRivate static T _instance;        private static readonly object _lock = new object();        public static T Instance        {            get            {                if (_instance == null)                {                    lock (_lock)                    {                        if (_instance == null)                        {                            _instance = (T)Activator.CreateInstance(typeof(T), true);                        }                    }                }                return _instance;            }        }    }

    public class User : Singleton<User>    {        private User() { }    }