Question

I have noticed this very odd behavior in a WPF application.

I have a MainWindow, which is shown using Show() from App.OnStartup. Said MainWindow can open a (non-modal) SubWindow, also using Show(). SubWindow‘s Owner is set to MainWindow.

When SubWindow is closed, MainWindow is visible again (good).

Some actions can cause the SubWindow to open a third window as a modal dialog, using ShowDialog() (Owner is set to SubWindow). When that modal dialog is opened and closed at least once during the lifetime of a SubWindow, then the weird thing happens.

After closing SubWindow, MainWindow does not come into view. Instead, whatever random window is behind MainWindow comes into view. Can anyone explain to me why this happens, and how to fix it?

It makes no difference whether the modal dialog is a normal Window displayed using ShowDialog(), or a message box shown using MessageBox.Show().

---

Here is some minimal code to reproduce this. Create a new WPF application in visual studio, and paste this into the pre-generated MainWindow.xaml.cs

Then, press a key on the keyboard to open only one window, close it, behavior as expected. Press two keys, close both, then the very first window is behind Visual Studio (presumably).

public MainWindow()
{
    InitializeComponent();
    this.PreviewKeyDown += (sender, e) =>
    {
        if (this.Owner is MainWindow)
        {
            // we're the SubWindow

            MessageBox.Show("I am a modal dialog");

            // code below produces the exact same behavior as the message box

            //var dialog = new MainWindow();
            //dialog.Owner = this;
            //dialog.ShowDialog();
        }
        else
        {
            // we're the initial MainWindow created by App.
            var subWindow = new MainWindow();
            subWindow.Owner = this;
            subWindow.Show();
        }
    };
}

Answer 1

This is a pretty annoying WPF bug, I never did find the flaw in the code that causes it but there’s a heckofalot of “gotta figure this out” comments in the source code that deals with focusing. Just a workaround, a less than ideal one, you can solve it by explicitly giving the focus to the owner when the window is closing. Copy/paste this code in your SubWindow class;

    protected override void OnClosing(System.ComponentModel.CancelEventArgs e) {
        base.OnClosing(e);
        if (!e.Cancel && this.Owner != null) this.Owner.Focus();
    }