Return an object from a popup window

You can expose a property on the second window, so that the first window can retrieve it.

public class Window1 : Window
{
    ...

    private void btnPromptFoo_Click(object sender, RoutedEventArgs e)
    {
        var w = new Window2();
        if (w.ShowDialog() == true)
        {
            string foo = w.Foo;
            ...
        }
    }
}

public class Window2 : Window
{
    ...

    public string Foo
    {
        get { return txtFoo.Text; }
    }

}