truyền thông giữa 2 cửa sổ

public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        public Window2 window2instance2 { get; set; }
        
      

        public void ChangeButton1Content()
        
        {
       
            

         if (window2instance2.key)
            {
                Button1.Content = "I Changed";

            }
        }

        private void Openwindows2_Click(object sender, RoutedEventArgs e)
        {
            Window2 window2instance = new Window2();
            window2instance.MainWindowInstance = this;
            window2instance.Show();

        }

    }

public partial class Window2 : Window
    {
        public Window2()
        {
            InitializeComponent();
        }

        public bool key = false;
        public MainWindow MainWindowInstance { get; set; }


       public void window2button_Click(object sender, RoutedEventArgs e)
        {
             MainWindow mainwindow = new MainWindow();
            mainwindow.window2instance2 = this;
            key = true;

           
            MainWindowInstance.ChangeButton1Content();

            this.Close();
        }
    }