WPF showing dialog before main window

Question

How one can show dialog window (e.g. login / options etc.) before the main window?

Here is what I tried (it apparently has once worked, but not anymore):

Answer 1

Okay apologizes, here is the solution:

My original question worked almost, only one thing to add, remove the StartupUri from the Application XAML and after that add the Show to main window.

That is:

Above, StartupUri removed.

Add myMainWindow.Show() too:

public partial class App : Application
{

    private void Application_Startup(object sender, StartupEventArgs e)
    {
        Window1 myMainWindow = new Window1();
        DialogWindow myDialogWindow = new DialogWindow();
        myDialogWindow.ShowDialog();
        myMainWindow.Show();
    }

}