Application closes if any window is shown before the main window - wpf

Question

I have a very puzzling case. During the initialization of my application, before any windows are shown, this function deserializes application settings from XML file, but if deserialization function throws any errors, it displays an error message to the user (which is a custom-made WPF dialog), and once that message dialog is closed, it creates new instance of the settings, and continues the initialization:
Public Function LoadSettings()
Try
Return DeserializeFromXML(settingsPath)
Catch ex As Exception
Msg.ShowMessage(Msg.corruptedSettingsFile)
Return new AppSettings
End Try
End Function
ShowMessage is defined as:
Public Function ShowMessage(message As Message) As Boolean
Dim messageDialog As New MessageDialog(message.Title, message.Content, message.Buttons)
Return messageDialog.ShowDialog()
End Function
Now, the weird thing is, after that exception is caught, and error message is closed by the user, initialization continues, but when it arrives at mainWindow.Show(), nothing happens. Main window is not shown. And once initialization code finishes, the application closes immediately.
In my Application Properties, the Shutdown mode is set to "On main window close". StartupUri is not set, because I want to show that main window exactly when I want, not automatically. However, for testing, I tried to remove mainWindow.Show() altogether, and set StartupUri to MainWindow.XML, but it doesn't solve the issue either.
I have tested that if I do not display that error message to the user, the application loads correctly, either with mainWindow.Show() or with StartupUri.
Why is this happening?
EDIT: I forgot to mention, which is probably important, that before the mainWindow.Show(), I also call Application.Current.MainWindow = mainWindow. So in case WPF makes that message dialog main window, this call should override it... But it still doesn't work.
EDIT 2: I discovered that if I call Application.Current.ShutdownMode = ShutdownMode.OnExplicitShutdown before showing the message, and restore it with Application.Current.ShutdownMode = ShutdownMode.OnMainWindowClose and also call Application.Current.MainWindow = mainWindow, then the application does not close, and all works correctly. However, this is a horrible solution. It is obvious now that the message dialog hijacks the Application.Current.MainWindow. How do I stop this behavior on the application level, so that the Application.Current.MainWindow would only get set when I explicitly set it?

Answer 1

you must set MainWindow before calling ShowDialog methed of any other windows. The reason of this is: a window that was created in application thread, will set itself as main window if MainWindow was null; and after ShowDialog was called, you would have any chance to fix this because of modal mode.
// this will work.
private void Application_Startup(object sender, StartupEventArgs e)
{
var a = new MainWindow();
var b = new MessageDialog();
b.ShowDialog();
a.Show();
}
// this will not.
private void Application_Startup(object sender, StartupEventArgs e)
{
var b = new MessageDialog();
b.ShowDialog(); //app shutdown at this point
var a = new MainWindow();
a.Show();
}