Session.Timeout = 20;
if (Session["yourSessionname"] == null)
{
Response.Redirect("http://www.google.com");
}
if you write your code inside of class
if (HttpContext.Current.Session["SessionName"] == null)
{
Response.Redirect("http://www.google.com");
}
and also u can use this in the web.config
<system.web>
<sessionState timeout="2"></sessionState>