MessageBox in Windows Phone 8.1

Question

I want to show a MessageBox to the user when the back button has been pressed on the hardware, but it's simply doesn't work. I tried these variations, but I never see the MessageBox:

    // VARIATION 1
    IAsyncResult mbResult = Microsoft.Xna.Framework.GamerServices.Guide.BeginShowMessageBox("Warning", "Are you sure you want to leave this page?",
    new string[] { "Yes", "No" }, 0, Microsoft.Xna.Framework.GamerServices.MessageBoxIcon.None, null, null);

    mbResult.AsyncWaitHandle.WaitOne();

    int? yesNo = Microsoft.Xna.Framework.GamerServices.Guide.EndShowMessageBox(mbResult);
    if (yesNo.HasValue)
    {
        if (yesNo.Value == 0)
        {
            // Yes pressed
        }
        else
        {
            // No pressed
        }
    }

    // VARIATION 2
    MessageBoxResult mbr = MessageBox.Show("Are you sure you want to leave this page?", "Warning", MessageBoxButton.OKCancel);

    if(mbr == MessageBoxResult.OK)
    {
        // OK pressed
    }
    else
    {
        // Cancel pressed
    }

If I write e.Cancel = true to the OnBackKeyPress event then I can't leave the page, so the code is executing, but I never see the MessageBox:

protected override void OnBackKeyPress(System.ComponentModel.CancelEventArgs e)
{
    e.Cancel = true;
}

What can be the problem, or what I'm doing wrong?

Answer 1

As I can see you are targeting Windows Phone 8.1 Silverlight, then answer to this question is still actual, as at MSDN"

Applies to: Windows Phone 8 and Windows Phone Silverlight 8.1 | Windows Phone OS 7.1

In Windows Phone 8, if you call Show in OnBackKeyPress(CancelEventArgs) or a handler for the BackKeyPress event, the app will exit.

The solution is also given at MSDN. Shortly - run your Messeagebox.Show on Dispatcher:

protected override void OnBackKeyPress(System.ComponentModel.CancelEventArgs e)
{
    e.Cancel = true;
    Deployment.Current.Dispatcher.BeginInvoke(() =>
    {
      MessageBoxResult mbr = MessageBox.Show("Are you sure you want to leave this page?", "Warning", MessageBoxButton.OKCancel);

      if(mbr == MessageBoxResult.OK)
      {   OK pressed  } 
      else
      {   Cancel pressed  }
    });
}