You can assign the Owner to the window that was created in your MainWindow.

window.Owner = this; //This is added to the code that use to create your Window

You can assign the Owner to the window that was created in your MainWindow.

window.Owner = this; //This is added to the code that use to create your Window

Then you should be able to access it something like this.

((MainWindow)this.Owner).Test();
MainWindow

Then you should be able to access it something like this.

((MainWindow)this.Owner).Test();
MainWindow

public partial class MainWindow : Window
{
    Window1 window = new Window1();
    public MainWindow()
    {
        InitializeComponent();
        window.Show();


    }

    public void Test()
    {
        label1.Content += " works";
    }

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        window.Owner = this;
    }


}

public partial class MainWindow : Window
{
    Window1 window = new Window1();
    public MainWindow()
    {
        InitializeComponent();
        window.Show();


    }

    public void Test()
    {
        label1.Content += " works";
    }

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        window.Owner = this;
    }


}

```public partial class Window1 : Window
{
    public Window1()
    {
        InitializeComponent();


    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        ((MainWindow)this.Owner).Test();
    }
}

```public partial class Window1 : Window
{
    public Window1()
    {
        InitializeComponent();


    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        ((MainWindow)this.Owner).Test();
    }
}

 public void LoadList()
        {
            ManageStock manageStock = new ManageStock();
            DataTable dt = new DataTable();
            string query = "select * from DesignTB";
            readDatathroughAdapter(query, dt);
            manageStock.lstBox.ItemsSource = dt.DefaultView;
            closeConn();

        }

 public void LoadList()
        {
            ManageStock manageStock = new ManageStock();
            DataTable dt = new DataTable();
            string query = "select * from DesignTB";
            readDatathroughAdapter(query, dt);
            manageStock.lstBox.ItemsSource = dt.DefaultView;
            closeConn();

        }