// global.asax
void Application_PreRequestHandlerExecute(object sender, EventArgs e)
{
    if (Session == null)
        return;

    // if special key is not present then session is expired
    if (Session["filled"] == null)
    {
        if (User.Identity.IsAuthenticated)
        {
            FillSession(Session, User.Identity.Name);
        }
    }
}