protected void Session_Start(object sender, EventArgs e)
        {
            var sessionTimer = new System.Threading.Timer(Session_Timer, Session, 1000, 1000); //这里是可以拿到SessionID的，没有异常且值不为空
        }

protected void Session_Timer(object obj)
        {
            var session = (HttpSessionState)obj;
            var sessionID = "";
            try { sessionID = session.SessionID; } catch { }  //这里会引发异常            
            Debug.Print("{0}\tSession_Timer SessionID:{1}", DateTime.Now.ToString("yyyy-MM-DD HH:mm:ss") , sessionID);
        }