带有BackgroundWorker的App.Xaml中的WPF CrossThreadException

Question

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?

Answer 1

The background worker uses the SynchronizationContext. This is going from memory, but I don't think it has been initialised in the App constructor.

Before you construct the BW check SynchronizationContext.Current is not null. Also check SynchronizationContext.Current is the same before construction of BW and in the completed method.

If it's not, you'll need to move the code later in the process... App.OnStartUp should be fine

Answer 2

Maybe this is a bad way to do a splash screen?

Unless I've misinterpreted your question, I don't think there's a need to do this to show a splash screen. Just select the image and in the Properties window click the BuildAction dropdown and select SplashScreen . Also, I think you can simplify your process by eliminating the BackgroundWorker , unless your data function takes a long time. I believe you added it to accomodate the showing/hiding of the splash screen.

Answer 3

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

See similar question BackgroundWorker OnWorkCompleted throws cross-thread exception

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.