According to the answer here, a workaround is to actually open an invisible window and use that as the parent of the MessageBox:
Window window = new Window()
{
Visibility = Visibility.Hidden,
// Just hiding the window is not sufficient, as it still temporarily pops up the first time. Therefore, make it transparent.
AllowsTransparency = true,
Background = System.Windows.Media.Brushes.Transparent,
WindowStyle = WindowStyle.None,
ShowInTaskbar = false
};
window.Show();
...then open the MessageBox with the appropriate parameter:
MessageBox.Show(window, "Titie", "Text");
...and don't forget to close the window when you're done (possibly on application exit):
window.close();
I tried this and it works well. It's undesirable to have to open an extra window, but it's better than making your own messagebox window just for the sake of making this work.