How to bind ContextMenu that has Command to the attached property?

Question

I have an external control named DockSite. When displayed ContextMenu from the DockSite control, the MenuOpening event handler is called.

I wanted to add my ContextMenu to the default ContextMenu when the MenuOpening event is called and I created the attached property as below to extend the behavior of the DockSite.

    public static ContextMenu GetAddDocumentMenu(DependencyObject obj)
    {
        return (ContextMenu)obj.GetValue(AddDocumentMenuProperty);
    }

    public static void SetAddDocumentMenu(DependencyObject obj, ContextMenu value)
    {
        obj.SetValue(AddDocumentMenuProperty, value);
    }

    // Using a DependencyProperty as the backing store for AddDocumentMenu.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty AddDocumentMenuProperty =
        DependencyProperty.RegisterAttached("AddDocumentMenu", typeof(ContextMenu), typeof(DockSiteHook), new PropertyMetadata(new ContextMenu(), OnDocumentMenuChanged));

    private static void OnDocumentMenuChanged(object sender, DependencyPropertyChangedEventArgs e)
    {
        var dockSite = (sender as DockSite);
        if (dockSite == null) return;

        ContextMenu on = (ContextMenu)e.NewValue;
        if (on is null) dockSite.MenuOpening -= DockSite_MenuOpening;
        else dockSite.MenuOpening += DockSite_MenuOpening;
    }

    private static void DockSite_MenuOpening(object sender, DockingMenuEventArgs e)
    {
        e.Menu.Items.Add(DockSiteHook.GetAddDocumentMenu(sender as DockSite));
    }

I used the code above in my MainWindow as below.

<docking:DockSite Grid.Row="1" x:Name="dockSite">

    <ap:DockSiteHook.AddDocumentMenu>
        <ContextMenu>
            <MenuItem Command="{Binding TestCommand}"/>
        </ContextMenu>
    </ap:DockSiteHook.AddDocumentMenu>

<docking:DockSite/>

But the Visual Studio throws an error as below image.

The error message is "It can't bind the default value for AddDocumentMenu to the specific thread".

I want to bind the ContextMenu to the specific attached property.

Could someone tell me why fired the error above? and how to solve this problem?

Thanks for reading.

Answer 1

Set the default value to null (or default(ContextMenu)):

public static readonly DependencyProperty AddDocumentMenuProperty = 
    DependencyProperty.RegisterAttached("AddDocumentMenu", typeof(ContextMenu), 
        typeof(DockSiteHook), new PropertyMetadata(null, OnDocumentMenuChanged));