asp.net 2.0防止同一用户同时登陆

                        string key = TextBox1.Text;   //用户名文本框设为cache关键字
                         string uer = Convert.ToString(Cache[key]);  //读取cache中用户相应的值
                         //判断cache中是否有用户的信息，如果没有相关的值，说明用户未登陆
                         if (uer == null || uer == String.Empty)
                         {
                            //定义cache过期时间
                            TimeSpan SessTimeout = new TimeSpan(0, 0, System.Web.HttpContext.Current.Session.Timeout, 0, 0);
                          //第一次登陆的时候插入一个用户相关的cache值，
                             HttpContext.Current.Cache.Insert(key, key, null, DateTime.MaxValue, SessTimeout, System.Web.Caching.CacheItemPriority.NotRemovable, null);
                             Session["ADMINID"] = TextBox1.Text;
                                                       Response.Redirect("main.aspx");
                         }
                         else
                         {
                             //重复登陆
                             Response.Write("<script>alert('您的账号已经登陆!');window.location='login.aspx';</script>");
                         }