try
public class PageBase : System.Web.UI.Page
{
protected override void OnPreRender(EventArgs e)
{
base.OnPreRender(e);
AutoRedirect();
}
public void AutoRedirect()
{
int int_MilliSecondsTimeOut = (this.Session.Timeout * 60000);
string str_Script = @"
<script type='text/javascript'>
intervalset = window.setInterval('Redirect()'," + int_MilliSecondsTimeOut.ToString() + @");
function Redirect()
{
alert('Your session has been expired and system redirects to login page now.!\n\n');
window.location.href='/login.aspx';
}
</script>";
ClientScript.RegisterClientScriptBlock(this.GetType(), "Redirect", str_Script);
}
}
Read more How to do auto logout and redirect to login page when session expires using asp.net?