Question

In my asp.net web application i want to prevent multiple user login from the same user name on different machine or the same pc by using Cache.So, in my Global.asax.as page,

i put this line…

protected void Application_PreRequestHandlerExecute(Object sender, EventArgs e)
   {
     if (Session["user"] != null) // e.g. this is after an initial logon
     {
        string sKey = (string)Session["user"];
        string sUser = (string)HttpContext.Current.Cache[sKey];
     }
   }

and in my page login submit button click,

string userName=uName.Text;
string passWord=pwd.Text;

string sKey = uName.Text;
string sUser = Convert.ToString(Cache[sKey]);
if (sUser == null || sUser == String.Empty)
           {
               TimeSpan SessTimeOut = new TimeSpan(0, 0, HttpContext.Current.Session.Timeout, 0, 0);
               HttpContext.Current.Cache.Insert(sKey, sKey, null, DateTime.MaxValue, SessTimeOut, System.Web.Caching.CacheItemPriority.NotRemovable, null);
               Session["user"] = TextBox1.Text.Trim();

               if (userName.Equals(db_userName) && passWord.Equals(db_password))
               {
                   Response.Write("Welcome " + userName);
               }
               else
               {
                   Response.Write("Invalid Login");
               }
           }
           else
           {
               Response.Write("<Marquee><h1><font color=red>ILLEGAL LOGIN ATTEMPT!!!</font></h1></marquee>");
               return;
           }

But when run my application, It is raising Session state is not available in this context error message on Application_PreRequestHandlerExecute event…

I dont know what is the issue here…So, please guide me to get out of this issue…

Or tell me another good approach instead of this one…

Answer 1

Session state is not available during the initial request and may be null in subsequent requests. Check for null on session state object, not just the key/value.

if(HttpContext.Current.Session!=null){
//Proceed with xyz
}

Side note, Your use of Response.Write here…while not invalid, is atypical. I would suggest redirection to areas.