Calling ShowDialog in BackgroundWorker

Question

I have a WinForms application in which my background worker is doing a sync task, adding new files, removing old ones etc.

In my background worker code I want to show a custom form to user telling him what will be deleted and what will be added if he continues, with YES/NO buttons to get his feedback.

I was wondering if it is ok to do something like this in background worker's doWork method? If not, how should I do it?

Please advise..

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
   MyForm f = new MyForm();
   f.FilesToAddDelete(..);
   DialogResult result = f.ShowDialog();
   if(No...)
   return;
   else
   //keep working...
}

Answer 1

If you try this you will see for yourself that it will not work because the BackgroundWorker thread is not STA (it comes from the managed thread pool).

The essence of the matter is that you cannot show user interface from a worker thread¹, so you must work around it. You should pass a reference to a UI element of your application (the main form would be a good choice) and then use Invoke to marshal a request for user interaction to your UI thread. A barebones example:

class MainForm
{

    // all other members here

    public bool AskForConfirmation()
    {
        var confirmationForm = new ConfirmationForm();
        return confirmationForm.ShowDialog() == DialogResult.Yes;
    }
}

And the background worker would do this:

// I assume that mainForm has been passed somehow to BackgroundWorker
var result = (bool)mainForm.Invoke(mainForm.AskForConfirmation);
if (result) { ... }

---

¹ Technically, you cannot show user interface from a thread that is not STA. If you create a worker thread yourself you can choose to make it STA anyway, but if it comes from the thread pool there is no such possibility.

Answer 2

I usually create a method to execute a delegate on the UI thread:

  private void DoOnUIThread(MethodInvoker d) {
     if (this.InvokeRequired) { this.Invoke(d); } else { d(); }
  }

With this, you can change your code to:

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
   DialogResult result = DialogResult.No;
   DoOnUIThread(delegate() {
      MyForm f = new MyForm();
      f.FilesToAddDelete(..);
      result = f.ShowDialog();
   });

   if(No...)
   return;
   else
   //keep working...
}