A Window.ShowDialog() overload method to set the onwer window.

WindowExtension.cs

public static class WindowExtension
{
  public static bool? ShowDialog(this Window window, Window parentWindow)
  {
    window.Owner = parentWindow;
    return window.ShowDialog();
  }
}

How to use:

new MyWindow.ShowDialog(this);

Instead:

var frm = new MyWindow();
frm.Owner = this;
frm.ShowDialog();