本例完成的功能就是防止用戶重復(fù)登錄！若用戶已經(jīng) 登錄，則當(dāng)其再次登錄時(shí)，彈出提示框后返回！

實(shí)現(xiàn)思路：用戶登錄成功后，將用戶登錄信息存放到 Hashtable類型的Application["Online"]里面，其鍵 值為SessionID，其Value值為用戶ID；當(dāng)用戶注銷時(shí) ，調(diào)用Session.Abandon；在Global.asax里面的 SessionEnd事件中，將用戶ID從Hashtable中刪除；在 用戶訪問(wèn)頁(yè)面時(shí)，察看Hashtable中是否有對(duì)應(yīng)的用戶 ID如果沒(méi)有則判斷用戶不在線（用戶不在線的原因可 能是按了注銷按鈕、網(wǎng)頁(yè)超時(shí)等）

?

// 1、公用類中判斷用戶是否在線的函數(shù)（供用戶調(diào)用）?
/**/ /// ? <summary> ?
/// ?判斷用戶strUserID是否包含在Hashtable?h中?
/// ? </summary> ?
/// ? <param?name="strUserID"></param> ?
/// ? <param?name="h"></param> ?
/// ? <returns></returns> ?
public ? static ? bool ?AmIOnline( string ?strUserID,Hashtable?h)?
{?
???? if (strUserID? == ? null )?
???? return ? false ;?

???? // 繼續(xù)判斷是否該用戶已經(jīng)登陸?
???? if (h? == ? null )?
???? return ? false ;?

???? // 判斷哈希表中是否有該用戶?
????IDictionaryEnumerator?e1? = ?h.GetEnumerator();?
???? bool ?flag? = ? false ;?
???? while (e1.MoveNext())?
???? {?
???????? if (e1.Value.ToString().CompareTo(strUserID)? == ? 0 )?
???????? {?
????????????flag? = ? true ;?
???????????? break ;?
????????} ?
????} ?
???? return ?flag;?
} ?

// 2、用戶登錄事件處理：?
private ? void ?btnlogin_Click( object ?sender,?System.Web.UI.ImageClickEventArgs?e)?
{? /**/ /// /User為自定義的類，其中包含Login方法?
????User?CurUser? = ? new ?User();?
????CurUser.UserID? = ? this .username.Text.Trim();?

???? if (MyUtility.AmIOnline(CurUser.UserID,?(Hashtable)Application[ " Online " ]))?
???? {?
????????JScript.Alert( " 您所使用的登錄ID已經(jīng)在線了！您不能重復(fù)登錄！ " );?
???????? return ;?
????} ?

????CurUser.LoginPsw? = ?FormsAuthentication.HashPasswordForStoringInConfigFile( this .password.Text.Trim(), " SHA1 " );?
???? int ?ii? = ?CurUser.Login();?
????StringBuilder?sbPmt? = ? new ?StringBuilder();?

???? switch (ii)?
???? {?
???????? case ? 0 :? // 如果登錄成功，則將UserID加入Application["Online"]中?
????????????Hashtable?h? = ?(Hashtable)Application[ " Online " ];?
???????????? if (h? == ? null )?
????????????h? = ? new ?Hashtable();?
????????????h[Session.SessionID]? = ?CurUser.UserID;?
????????????Application[ " Online " ]? = ?h;?

????????????Session[ " UserID " ]? = ?CurUser.UserID;?
????????????Session[ " UserNM " ]? = ?CurUser.UserNM;?
????????????Session[ " RoleMap " ]? = ?CurUser.RoleMap;?
????????????Session[ " LoginPsw " ]? = ?CurUser.LoginPsw;?
????????????Session[ " LoginTime " ]? = ?DateTime.Now;?
????????????Response.Redirect( " ChooseRole.aspx " );?
???????? break ;?
???????? case ? - 1 :?
????????????JScript.Alert( " 用戶名錯(cuò)誤！ " );?
???????? break ;?
???????? case ? - 2 :?
????????????JScript.Alert( " 密碼錯(cuò)誤！ " );?
???????? break ;?
???????? default :?
????????????sbPmt.Append( " 登錄過(guò)程中發(fā)生未知錯(cuò)誤！ " );?
????????????JScript.Alert(sbPmt.ToString());?
???????? break ;?
????} ?
???? return ;?
} ?

// 3、在Global.asax中的Session_End事件：?
protected ? void ?Session_End(Object?sender,?EventArgs?e)?
{?
????Hashtable?h = (Hashtable)Application[ " Online " ];?

???? if (h[Session.SessionID] != null )?
????h.Remove(Session.SessionID);?

????Application[ " Online " ] = h;?
} ?

// 4、在每一個(gè)頁(yè)面需要刷新的地方，調(diào)用如下代碼：?
try ?
{?
???? if ( ! common.MyUtility.AmIOnline(Session[ " UserID " ].ToString(),(Hashtable)Application[ " OnLine " ]))?
???? {?
???????? // 用戶沒(méi)有在線?，轉(zhuǎn)到登錄界面?
????????Response.Write( " <script>parent.document.location.href='Login.aspx';</script> " );? /**/ /// /有框架時(shí)用?
???????? // Response.Redirect("login.aspx");? //// 無(wú)框架時(shí)用?
???????? return ;?
????} ?
} ?
catch ?
{?
???? // 會(huì)話過(guò)期?，轉(zhuǎn)到登錄界面?
????Response.Write( " <script>parent.document.location.href='Login.aspx';</script> " );? /**/ /// /有框架時(shí)所用?
???? // Response.Redirect("login.aspx");? //// 無(wú)框架時(shí)用?
???? return ;?
} ?

?

深入思考：
由本例的解決方法可以加以延伸，比如，在存儲(chǔ) UserID的時(shí)候，將UserID+客戶端IP地址一起存進(jìn)去， 則在將相應(yīng)信息取出來(lái)分析的時(shí)候，可以做到：當(dāng)用 戶在不同的計(jì)算機(jī)上先后登錄的時(shí)候，則允許最近一 次的登錄，而將之前的登錄刪除！等等等等


--這個(gè)里面是具體的詳細(xì)信息

http://www.cnblogs.com/panshuiqing/articles/951779.html