WPF CrossThreadException in App.Xaml with BackgroundWorker

I have this in my App.Xaml:

public App()
{
    _backgroundWorker = new BackgroundWorker();
    _backgroundWorker.DoWork += new DoWorkEventHandler(DoBackgroundWork);
    _backgroundWorker.RunWorkerCompleted += new RunWorkerCompletedEventHandler(BackgroundCompleted);
    _backgroundWorker.RunWorkerAsync();

    _splashView = new SplashView();
    _splashView.Show();
}

The DoBackgroundWork method performs some database setup, and then the BackgroundCompleted event closes the _splashView and shows _mainView.

However, modifying anything in the _splashView from BackgroundCompleted causes a cross thread exception, which is what I though background workers were designed to fix. I'm guessing this has something to do with the way backgroundworker's work in App.Xaml. Maybe this is a bad way to do a splash screen?

Solution

There is no guarantee which thread the event handler of OnWorkCompleted will be used for execution.

You have to use the Invoke or BeginInvoke methods to modify visual elements from a background thread. You can call this directly on the object whose properties you are modifying or use the Dispatcher.

EDIT: As per conversation with Adam

The SynchronizationContext has the desired effect for the OnWorkCompleted event handler to be run on the initial thread (not the BackgroundWorker's). http://msdn.microsoft.com/en-us/magazine/gg598924.aspx. (See Figure 2)

If the BackgroundWorker is created and run prior to the SynchronizationContext initialization, then the OnWorkCompleted will execute on possibly the same thread as the BackgroundWorker.

Thanks Adam.