单实例(Singleton) single instance 模式

public class pptest
    {
        private static pptest _instance = null;
        private static object lockobject = null;        private pptest()
        {        }
        public static pptest Instance()
        {
            if (_instance == null)
            {
                lock (lockobject)
                {
                    if (_instance == null)
                    {
                        _instance = new pptest();
                    }
                }
            }
            return _instance;
        }
    }