How do I communicate between child and parent window in WPF?

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C#

Visual-Studio

I'm trying to build a program, where I have a simple keypad in child window. In my parent window I have a button that opens my keypad and two text boxes. When I click on my buttons on keypad (child) window, it should display values in text boxes, that are respresented by a specific button. What should I do to get this child/parent communication working?

What I have tried:

I found some good resources on delegates and I tried with this code, but it still doesn't work. The invoke function doesn't seem to work, because my text box is still empty, when I click on my keypad buttons.

My parent window:

public partial class MainWindow : MetroWindow
    {
       
        public MainWindow()

        {
            InitializeComponent();

        }

        public void WriteToTextBoxes(string a)
        {
            TextBox2.Text += a;
        }

        private void MetroWindow_Loaded(object sender, RoutedEventArgs e)
        {
            TextBox1.Text = String.Empty;
            TextBox2.Text = String.Empty;
        }

My child window:

public partial class KeyboardWindow : MetroWindow
{
    public delegate void someDelegate(string a);
    MainWindow _mainW = new MainWindow();

    public KeyboardWindow()
    {
        InitializeComponent();

    }

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {


    }

    private void Window_Closed(object sender, EventArgs e)
    {

    }

    private void KeypadOne_Click(object sender, RoutedEventArgs e)
    {
        someDelegate p = null;
        p = this._mainW.WriteToTextBoxes;
        p.Invoke("1");

    }