实现的不让同一个用户登陆

string  test  =   " username " ;
private   void  Page_Load( object  sender, System.EventArgs e)
{
   if(Application["User_" + test] == null || Application["User_" + test].ToString() != Request.UserHostAddress)
      Session["login"] = null;
   else
      Response.Write(Session["login"].ToString() + "已登录<br>");
}
private   void  Button1_Click( object  sender, System.EventArgs e)
{
   Session["login"] = test;
   Application["User_" + test] = Request.UserHostAddress;
}  

ASP程序中同一个用户不允许同时登陆两次 

登陆页login.asp:

<%
if  request.Form.count > 0  then 
session( " username " ) = request( " username " )
application(session( " username " )) = session.SessionID
response.Redirect( " index.asp " )
end  if
%>
< form method = post action = "" >
< input type = " text "  name = " username " >< input type = " submit " >
</ form >

其他需要认证的页面index.asp:

<%
if  application(session( " username " )) = session.SessionID then
response.Write( " 已经登陆 " )
else
response.Write( " 没有登陆 " )
end  if
%>