asp.net 单用户登录经典解决方案

             Hashtable h = (Hashtable)Application["online"];
             if (h[Session.SessionID] != null)
                 h.Remove(Session.SessionID);
             Application["online"] = h;

private void isLogin()
     {
         Hashtable h = (Hashtable)Application["online"];

         if (h == null)
         {
             h = new Hashtable();
         }

         //验证用户是否在Application中存在(是否在线)
         IDictionaryEnumerator e1 = h.GetEnumerator();
         bool flag = false;
         while (e1.MoveNext())
         {
             if (checkCookie(e1.Value.ToString()))
             {
                 flag = true;
                 break;
             }
         }
         if (flag)
         {
             Response.Write("<script defer language='javascript'>alert('This user is online!');history.go(-1);</script>");
         }
         else
         {
             loginLogic login = new loginLogic(this.txt_user_id.Text.Trim(), this.txt_password.Text.Trim());

             if (!login.getLoginStatus)
             {
                 Response.Write("<script defer language='javascript'>alert('Invalid UserID or password.Please try again.');</script>");
             }
             else
             {
                 //生成服务端标识值
                 DateTime now = DateTime.Now;
                 string cookieValue = now.Year.ToString() + now.Month.ToString() + now.Day.ToString() + now.Hour.ToString() + now.Minute.ToString() + now.Second.ToString() + now.Millisecond.ToString();
                 //把userid + 标识值写入全局变量表
                 h[Session.SessionID] = this.txt_user_id.Text.Trim() + "-" + cookieValue;
                 Application["Online"] = h;
                 //把标识值写入客户端cookie
                 Response.Cookies["hqs"].Value =cookieValue;
                 Response.Cookies["hqs"].Expires = DateTime.Now.AddDays(1);

                 //用户信息记入session
                 Session["userid"] = this.txt_user_id.Text.Trim();
                 Response.Redirect("Manage/index.aspx");
             }
         }
     }

private bool checkCookie(string appValue)
     {
         bool isExist = false;

         if (Request.Cookies["hqs"] != null)
         {
             string cookieValue = Request.Cookies["hqs"].Value;

             char[] sp = new char[1]{'-'};
             string appUserid = appValue.Split(sp)[0].ToString();
             string appCookie = appValue.Split(sp)[1].ToString();

             if (appUserid == this.txt_user_id.Text.Trim() && appCookie != cookieValue)
                 isExist = true;
         }
         return isExist;        
     }

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
             }