Handling image load exception gracefully

Question

Hi,

I'm loading user images using Silverlight 3. Everything works fine and I can set the file stream to a BitmapImage and it gets rendered OK.

The problem is that if I try to load something that's not an image (like a .exe that's been renamed to .png) Silverlight crashes with a System.Exception that says "Catastrophic failure". The MSDN documentation unhelpfully says that it should be so there msdn link and I should listen to the ImageFailed event (which never gets fired).

Am I missing something there or is the library broken when loading from a stream?

The code I've got loading the image from the source:

        var openFileDialog = new OpenFileDialog();
        openFileDialog.Filter = "*.jpg;*.jpeg;*.png|*.jpg;*.jpeg;*.png";
        openFileDialog.Multiselect = false;
        var showDialog = openFileDialog.ShowDialog();

        if (showDialog.HasValue && showDialog.Value)
        {
            using (var reader = openFileDialog.File.OpenRead())
            {
                var picture = new BitmapImage();

                picture.DownloadProgress += (o, e) => System.Threading.SynchronizationContext.Current.Send((oo) => System.Windows.Browser.HtmlPage.Window.Alert("Download progress: " + e.Progress), null);
                picture.ImageFailed += (o, e) => System.Threading.SynchronizationContext.Current.Send((oo) => System.Windows.Browser.HtmlPage.Window.Alert("Image failed: " + e.ErrorException), null);
                picture.ImageOpened += (o, e) => System.Threading.SynchronizationContext.Current.Send((oo) => System.Windows.Browser.HtmlPage.Window.Alert("Image opened: " + e.OriginalSource), null);

                picture.SetSource(reader); // BANG! here without any of the alerts showing up
            }
        }

Answer 1

That's weird, but you're right, it does behave that way, even on Silverlight 4.

There may be a better option, but one way I've found to work around these exceptions that can't otherwise be handled is to modify the App.Application_UnhandledException() method. This would work for you:

private void Application_UnhandledException(object sender, ApplicationUnhandledExceptionEventArgs e)
{
    // Handle stupid image load exception.  Can't think of a better way to do it -- sorry.
    if (e.ExceptionObject is System.Exception && e.ExceptionObject.Message.Contains("HRESULT") && e.ExceptionObject.Message.Contains("E_UNEXPECTED"))
    {
        Deployment.Current.Dispatcher.BeginInvoke(() =>
            {
                MessageBox.Show("Error loading image.");
            });
        e.Handled = true;
        return;
    }

    // If the app is running outside of the debugger then report the exception using
    // the browser's exception mechanism. On IE this will display it a yellow alert 
    // icon in the status bar and Firefox will display a script error.
    if (!System.Diagnostics.Debugger.IsAttached)
    {

        // NOTE: This will allow the application to continue running after an exception has been thrown
        // but not handled. 
        // For production applications this error handling should be replaced with something that will 
        // report the error to the website and stop the application.
        e.Handled = true;
        Deployment.Current.Dispatcher.BeginInvoke(delegate { ReportErrorToDOM(e); });
    }
}

It's a hack and I don't like it, but it's better than an unhandled exception.

BTW, you really ought to file a Connect bug on this behavior. It quite certainly fails the "Law of Least Astonishment". See Tim Heuer's post on how to get the bug filed: http://timheuer.com/blog/archive/2010/05/03/ways-to-give-feedback-on-silverlight.aspx