Main window disappears behind other application's windows after a sub window uses ShowDialog on a third window

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();
    }

Answer 2

I realize this is an old thread, but I tried all of these solutions (and many other more in other threads) and none of them worked. I didn't want to set my main window as the subwindow's Owner because I wanted to be able to minimize the subwindow.

What worked for me was to change the WindowState property in the main window instead.

This is how I do it, in my mainwindow.cs:

//create new window instance
subWindow = new SubWindow();

//event called on the mainwindow, after the subwindow has been closed. 
subWindow.Closed += (sender, args) =>
{
    subWindow = null; //clears the variable, so I can re-open the window later
    this.WindowState = WindowState.Normal; //this shows the main window!
};

//show subwindow
subWindow.Show();