public partial class MainWindow : Window
    {
      
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            SubWindow s = new SubWindow();
            s.Closed += S_Closed;
            s.Show();
        }

        private void S_Closed(object sender, EventArgs e)
        {
            MessageBox.Show("子窗口要关闭啦。我们也要同步关闭了。");
            this.Close();
        }

    }