How can I handle forms authentication timeout exceptions in ASP.NET?

If you're using Forms Authentication, the user will be redirected to the login page when the Forms Authentication ticket expires, which is not the same as the Session expiring.

You could consider increasing the Forms Authentication timeout if appropriate. Even to the extent of using a persistent cookie. But if it does expire, there's no real alternative to redirecting to the login page - anything else would be insecure.

One way to deal with Session timeouts is to use Session as a cache - and persist anything important to a backing store such as a database. Then check before accessing anything in Session and refresh if necessary:

MyType MyObject
{
    get
    {
        MyType myObject = Session["MySessionKey"] as MyType
        if (myObject == null)
        {
            myObject = ... get data from a backing store
            Session["MySessionKey"] = myObject;  
        }
        return myObject;
    }
    set
    {
        Session["MySessionKey"] = value;
        ... and persist it to backing store if appropriate
    }
}