Pass Data from Child Window to MainWindow TextBlock

Question

In this example, MainWindow has a button that opens Window2.

Window2 has a button that writes \"Hello, World!\" to MainWindo

Answer 1

The answer you're looking for is very implementation-based and depends heavily on what you want Window2 as a class to do.

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private MainWindow mainwindow;

This is acceptable.

public MainWindow mainwindow { get; private set; }

This would work but doesn't respect naming conventions because it's a property. Usually you'd use this for encapsulation of a field or for easy access to a computed value.

public Window MainWindow { get; set; }

This is not acceptable in your context because Window does not contain a textBlockMessage.

private object mainwindow { get; private set; };

This also wouldn't work for the same reason as above.

private MainWindow mainwindow = ((MainWindow)System.Windows.Application.Current.MainWindow);

This would work and would even let you not keep a field for the reference to the MainWindow instance in your Window2 instances. Still needs to get that MainWindow everytime you click the button however.

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Another interesting way to do what you're doing however is to simply pass the handler to the child windows at instanciation:

MainWindow

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

    // Open Window 2
    private void buttonWindow2_Click(object sender, RoutedEventArgs e)
    {
        Window2 window2 = new Window2(); // No need to give a reference to the child window anymore
        window2.setClickHandler((obj, ev) => {
            textBlockMessage.Text = "Hello, world!"; // Direct access to the textblock.
        });
        window2.Left = Math.Max(this.Left - window2.Width, 0);
        window2.Top = Math.Max(this.Top - 0, 0);
        window2.ShowDialog();
    }
}

Window2

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

    public void setClickHandler(RoutedEventHandler handler)
    {
        // The handler is given to the click event.
        buttonMessage.Click -= handler;
        buttonMessage.Click += handler;
    }
}

And with that your Window2 class has no need to know MainWindow.