Page.User.Identity.IsAuthenticated still true after FormsAuthentication.SignOut()

Page.User.Identity.IsAuthenticated gets its value from Page.User (obviously) which is unfortunately read-only and is not updated when you call FormsAuthentication.SignOut().

Luckily Page.User pulls its value from Context.User which can be modified:

// HttpContext.Current.User.Identity.IsAuthenticated == true;

FormsAuthentication.SignOut();
HttpContext.Current.User =
    new GenericPrincipal(new GenericIdentity(string.Empty), null);

// now HttpContext.Current.User.Identity.IsAuthenticated == false
// and Page.User.Identity.IsAuthenticated == false

This is useful when you sign out the current user and wish to respond with the actual page without doing a redirect. You can check IsAuthenticated where you need it within the same page request.