一个泛型的单例模式

 public static class SingleInstanceFactory
        {
            private static Hashtable ht = new Hashtable();
            private static object syncObject = new object();

            public static T GetSingleInstance<T>() where T : new()
            {
                string key = typeof(T).ToString();

                T t = default(T);

                lock (syncObject)
                {

                    if (ht.ContainsKey(key))
                    {
                        t = (T)ht[key];
                    }
                    else
                    {
                        lock (syncObject)
                        {
                            t = Activator.CreateInstance<T>();
                            ht.Add(key, t);
                        }
                    }
                }

                return t;
            }
        }