单点登录

string sKey = username.Text.ToString().Trim(); // 得到Cache中的给定Key的值
            string sUser = Convert.ToString(Cache[sKey]); // 检查是否存在
            if (sUser == null || sUser == String.Empty)
            {
                
                TimeSpan SessTimeOut = new TimeSpan(0, 0, System.Web.HttpContext.Current.Session.Timeout, 0, 0);//取得Session的过期时间
                HttpContext.Current.Cache.Insert(sKey, sKey, null, DateTime.MaxValue, SessTimeOut, System.Web.Caching.CacheItemPriority.NotRemovable, null);//将值放入cache己方便单点登录
              //成功登录
            }
            else if (Cache[sKey].ToString() == sKey)//如果这个账号已经登录
            {
                ClientScript.RegisterStartupScript(GetType(), "提示", "<script>alert('对不起,当前用户已经登录');</script>");
                return;
            }
            else
            {
                Session.Abandon();//这段主要是为了避免不必要的错误导致不能登录
            }