WPF Multibiding how to enter ElementName when the target element is in a different UserControl

Question

In my WPF Window i have button with command

    <Button.CommandParameter>
        <MultiBinding Converter="{converters:Converter_MultipleCommandParameters}">
            <Binding ElementName="passwordBoxSQL" />
            <Binding ElementName="passwordBoxR2" />
        </MultiBinding>
    </Button.CommandParameter>

and passwordbox with x:Name

        <PasswordBox
            x:Name="passwordBoxSQL"
            Grid.Row="4"
            Grid.Column="1"
            Margin="0,0,10,0"
            VerticalAlignment="Center" />

It works well.

Then I moved Grid with passwordbox to another UserControl named Test in namespace XYZ.Control. In WPF windows i use

<Window
    xmlns:local="clr-namespace:XYZ.Control"

....
<local:Test />

After running the program, everything can be seen well, but there is an error:

XAML Binding Failures Description - Cannot find source: ElementName=passwordBoxSQL.

What should I give in Binding ElementName? Maybe it needs to be done somehow differently?

Answer 1

I warn you right away that I agree with @Clemens that it is incorrect to search for an element by name inside a child UserControl. But for educational purposes I will show you a solution based on throwing the element name into the Window visibility area.

    public partial class TestUserControl1 : UserControl
    {
        public TestUserControl1()
        {
            Loaded += OnLoaded;
            Unloaded += OnUnloaded;
            InitializeComponent();
        }

        private void OnUnloaded(object sender, RoutedEventArgs e)
        {
            Window window = Window.GetWindow(this);
            window.UnregisterName(nameof(passwordBoxSQL));
        }

        private void OnLoaded(object sender, RoutedEventArgs e)
        {
            Window window = Window.GetWindow(this);
            window?.RegisterName(nameof(passwordBoxSQL), passwordBoxSQL);
        }
    }