WPF ShowDialog returns null immediately on second call


I think this is a bug in the WPF framework, without going into depths of my program and why I am doing what I am doing, I wrote a simple test application to prove my theory.

Can this issue be confirmed? What are possible workarounds for a series of dialogs to be executed before putting the application into its run loop?

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Windows;

namespace ShowDialogWindow
  /// <summary>
  /// Interaction logic for App.xaml
  /// </summary>
  public partial class App : Application
    protected override void OnStartup(StartupEventArgs e)
       Window testWindow = new Window();
       Window testWindow2 = new Window();


Sure I can change my design to accommodate this behaviour. What I was trying to do was really simple however.

I have derived my MyApplication from Application. On the Main(), I initiate a series of start checks, for example, license information, splash screen, connectivity and configuration checks, ect. When I get my all clear, I call MyApplicatiom.Run(MyMainForm).

This design is based on a normal Windows application which works without a problem.

Thanks a lot for your help, and I will remember that you cannot call ShowDialog() before application.Run() as it simply innitializes a shutdown. I would have thought however that a shutdown sequence should only be initiated after a App.Run() instruction.

Please correct me if I am understanding this wrong.

Best Solution

Not a bug. The default ShutdownMode of Application is OnLastWindowClosed, so as soon as the first window is closed your application will start shutting down! Change to OnExplicitShutdown and it will work, but you'll have to manage the shutdown.

I think you probably just need to rethink what you're actually trying to achieve. Why would you display two subsequent dialog windows during the startup of your app?

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