Exception not caught when using BackgroundWorker

Question

I have a WPF window with a BackgroundWorker. I get an exception in Send() method here:

private void worker_DoWork(object sender, DoWorkEventArgs e)
{
     smtpClient.Send(mail);
}

which in turn is invoked in a Click Event for some button like this:

private async void SendClickAsync(object sender, RoutedEventArgs e)
{
    using (MessageServiceClient client = new MessageServiceClient())
    {
        try
        {
            [...]
            worker.RunWorkerAsync();

        }
        catch (Exception ex)
        {
            MessageBox.Show("Error! Check your sender data!", "!", MessageBoxButton.OK, MessageBoxImage.Error);
            [...]
        }
    }
}

Why does this exception doesn't get handled? When I was doing it not asynchronously (everything was in SendClickAsync() method) the message box popped up nicely.

Answer 1

When you call worker.RunWorkerAsync() method, your main thread continue execution and exits try..catch block. To handle exception use RunWorkerCompleted event handler. RunWorkerCompletedEventArgs arguement has property Error which will contain exception object:

void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    if (e.Error != null)
    {
       MessageBox.Show("Error", "!", MessageBoxButton.OK, MessageBoxImage.Error);
       return;
    }
}

Answer 2

The BackgroundWorker is using thread pool threads, so it doesn't want your code to be able to do something weird on the worker thread. That's why the BackgroundWorker swallows your exception that happened during DoWork and lets you know that it happened through RunWorkerCompletedEventArgs.Error.