public static class NHibernateContextSessionFactory
 {
 private const string CurrentSessionKey = "nhibernate.current_session";
        private static ISessionFactory sessionFactory;//这个在程序运行中.不明白为什么会为空，他是静态的呀..
  
       //静态构造函数 这个构造函数 还会经常运行其中 sessionFactory就为NULL了
        static NHibernateContextSessionFactory()
        {
            if (sessionFactory == null)
            {
                Stopwatch sw = new Stopwatch();
                sw.Start();
                sessionFactory = new ConfigurationBuilder().Build().BuildSessionFactory();
                sw.Stop();
                Debug.WriteLine("获取session工厂耗时：" + sw.ElapsedMilliseconds + " 毫秒");
            }
        }
  
        public static ISession GetSession()
        {
            ISession currentSession = CallContext.GetData(CurrentSessionKey) as ISession;
            if (currentSession == null)
            {
                currentSession = sessionFactory.OpenSession();
                CallContext.SetData(CurrentSessionKey, currentSession);
            }
  
            return currentSession;
        }
}