ASP.NET Application Error Overriding 404?

Question

I use the Application_Error event to catch and log errors in my app. The Error is logged, then a friendly error screen is displayed:

Sub Application_Error(ByVal sender As Object, ByVal e As EventArgs)
        Dim ex As New Exception( _
            String.Format("Error on page: '{0}'.", HttpContext.Current.Request.Url), _
            Server.GetLastError())

        Dim uid As Guid = Log.FatalError(ex)
        Server.Transfer(String.Concat("~\\GlobalError.aspx?error=", uid))
End Sub

In my web.config I have:

<customErrors mode="On" defaultRedirect="GlobalError.aspx">
  <error statusCode="404" redirect="PageNotFound.aspx" />
</customErrors>

Whenever a user tries to load a page that doesn't exist, they get the GlobalError.aspx page, not the PageNotFound.aspx page. I looked in the Application_Error event and found that the Response StatusCode was 200, while the Server's last error was "Page 'foo.aspx' was not found."

What do I need to do to get this working correctly?

Answer 1

To begin with, you'll have to decide why they're getting GlobalError.aspx. You've shown us two possibilities; maybe there are more.

Change the name of one of those entries to GlobalErrorX.aspx and see what happens.

Answer 2

In the Application_Error function you should only Server.Transfer if the error isn't a page not found/404 error. In those cases leave it to ASP.NET to do the redirect using the settings in the web.config.

You could figure out if it's a 404 using the following:

Exception ex = Server.GetLastError();
if (ex.GetType() == typeof(HttpException))
{
  HttpException httpEx = (HttpException)ex;
  if(httpEx.GetHttpCode() == 404) {
    return;
  }
}
Server.Transfer("~/error.aspx");