Preventing Simultaneous Logons - The Solution

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Getting back on the track of our course, we would be using the cache object with Sliding expiration and Application_PreRequestHandlerExecute of Global.asax to tackle this multiple login issue. As we have seen enough of the theory part, I think its time to look at some source code. For the purpose of testing, let us set the session timeout as 1.

Sample: Global.asax.cs


protected void Application_PreRequestHandlerExecute(Object sender, EventArgs e)
{
 if(Session["UserDetails"]!=null) 
 {  
    string strCacheKey = Session["UserDetails"].ToString();
    string strUser = HttpContext.Current.Cache[strCacheKey].ToString();
 } 
}



Sample: Login Page


Private void BtnLogin_Click(object sender, System.EventArgs e)
{ 
string strConCat = TxtUserName.Text+TxtPassword.Text;
string strUser = Convert.ToString(Cache[strConCat]);
   
 if (strUser==null || strUser.Equals(String.Empty))
 {
    TimeSpan SessTimeOut=new TimeSpan(0,0,Session.Timeout,0,0);
    Cache.Insert(strConCat,strConCat,null,DateTime.MaxValue,SessTimeOut,
 CacheItemPriority.NotRemovable,null);
    Session["UserDetails"] = strConCat;
    Response.Write("Welcome!");    
 }
 else
 {
    Response.Write("Duplicate login not allowed !!");
    return;
 }
}



That’s it! Easy, isn’t it? (Don’t forget to include System.Web.Caching in the login page). The username is almost always unique so we could even avoid concatenating the username and password.

We can now compile the application and run it. After logging in, if you try to use the same username/password combination the application won’t authenticate you. The only way out is to wait until the Cache expires (I hope you now understand the reason for setting the session time out as 1). This holds good even if we try to login from two different browsers/machines.

Conclusion

The system requirement for testing this sample code is Windows 2000, XP or 2003 OS with IIS 5 (or 6) installed and .NET Framework 1.0. I have omitted the other necessary evils such as text editors! Happy programming!

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