C#有效防止同一账号多次登录（附三种方法）

/// <summary>
/// 防止多次登录
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void Button1_Click(object sender, System.EventArgs e)
{
string strUser = string.Empty;
string strCacheKey = this.TextBox1.Text;

strUser = Convert.ToString(Cache[strCacheKey]);

if (strUser == string.Empty)
{
TimeSpan SessTimeOut = new TimeSpan(0, 0, System.Web.HttpContext.Current.Session.Timeout, 0, 0);

Cache.Insert(strCacheKey, strCacheKey, null, DateTime.MaxValue, SessTimeOut, CacheItemPriority.NotRemovable, null);
Session["User"] = strCacheKey;
this.Label1.Text = Session["User"].ToString();
}
else
{
this.Label1.Text = "这个用户已经登录！";
}
}

//
//所有的登录信息
Map<String, String> loginUserMap = (Map<String, String>) super.getApplicationAttr(Constant.LOGIN_USER_MAP);
boolean isExist = false;
String sessionId = super.getSessionId(false);
if(loginUserMap==null){
loginUserMap = new HashMap<String, String>();
}
for (String username : loginUserMap.keySet()) {
//判断是否已经保存该登录用户的信息，是否为同一个用户进行重复登录
if(!username.equals(user.getFuUserName()) || loginUserMap.containsValue(sessionId)){
continue;
}
isExist = true;
break;
}
if(isExist){
//该用户已登录
//
}else {
//该用户没有登录
loginUserMap.put(result.getFuUserName(), sessionId);
//
}
//

//
public void sessionDestroyed(HttpSessionEvent event) { 
  //  
  //在session销毁的时候 把loginUserMap中保存的键值对清除 
  User user = (User)event.getSession().getAttribute("loginUser"); 
  if(user!=null){ 
    Map<String, String> loginUserMap = (Map<String, String>)event.getSession().getServletContext().getAttribute("loginUserMap"); 
    loginUserMap.remove(user.getFuUserName()); 
event.getSession().getServletContext().setAttribute("loginUserMap",loginUserMap); 
  } 
  //
} 
//

//在刷新或关闭时调用的事件
$(window).bind('beforeunload',function(){
 $.ajax({
  url:"${ctx}/system/user/user!logout.action",
  type:"post",
  success:function(){
   alert("您已退出登录");
  }
 });
});