session problem in asp.net

Quote - HYD says:

... if the application is idle for sometime ...

Your problem is probably that the session timed out.
In this case you should revive your session parameters and in any case you'd better use a check for the session parameter if it exists, or if the session exists.

Session["mySessParam"] == null
If the above is True, it indicates that the session has ended (or that somewhere in your code you've removed the parameter).

NOTE: you can configure the session timeout in your web.config file to be more than the default 20 mins.

Cheers,
Edo

Quote:

if the application is idle for sometime

As stated by Edo Tzumer your problem seems to be of session time-out.You can maintain your session time out from web.config.

Similar thing can be done more easily by configuring the session state settings on your IIS where you can simply put in the session time out time or select the more appropriate session mode for your application.

There is no problem with your code just a small modification makes your program works correctly.
As mentioned above

protected void Page_Load(object sender, EventArgs e)
{
   if(Session["UserName"] != null)
     lblUser.Text = Session["UserName"].ToString();
   else
     Response.Redirect("SessionTimeoutMessagePage.aspx");
}

you have to change your code like this. Because when ever you clicked on some link on some aspx page in your website which associated with the masterpage it throws you an error.
Because every time the control loads your master page first and when executing the page_load event you are blindly looking for some session value.
Remember the ASP.NET doesn't show a null/empty value if the session was not created. It throws you an error for that reason.

So you have to check whether is there any session created or not before assigning the session value to any label to display eg: welcome XXXX

So kindly refer the above code now you can understand clearly why you are getting the error and not to get in your future.

I hope this will be useful to you. If it helps you kindly give 5 star rating and make it solved.
Thank you

Refer this : http://forums.asp.net/t/1283350.aspx[^]