WPF 解决弹出模态窗口关闭后，主窗口不在最前

     private void ButtonBase_OnClick(object sender, RoutedEventArgs e)
        {
            var w = new Window
            {
                Title = "ChildWindow",
                Owner = this,
                Width = 600,
                Height = 500,
                Content = "Step 1: Move a window of another appliation to over the MainWindow\r\n" +
                          "Step 2: Active this window from the Windows taskbar\r\n" +
                          "Step 3: Close this window (ChildWindow)\r\n" +
                          "\r\nNow you can see that the MainWindow drops down and coverd by another window.\r\n" +
                          "If you don't understand the description above, please refer to the video demo in this project."
            };
            w.Closing += W_Closed;
            w.Show();
        }

        private void W_Closed(object sender, CancelEventArgs e)
        {
            Activate();
        }