Can't bind an attached property to another dependency property

Question

I' writing a control library. In this library there are some custom panels which are populated with user UIElements. Since every child element in my lib must have a "Title" property, I wrote the following:

// Attached properties common to every UIElement
public static class MyLibCommonProperties
{
    public static readonly DependencyProperty TitleProperty =
        DependencyProperty.RegisterAttached( 
            "Title", 
            typeof(String),
            typeof(UIElement), 
            new FrameworkPropertyMetadata(
                "NoTitle", new PropertyChangedCallback(OnTitleChanged))
            );

    public static string GetTitle( UIElement _target )
    {
        return (string)_target.GetValue( TitleProperty );
    }

    public static void SetTitle( UIElement _target, string _value )
    {
        _target.SetValue( TitleProperty, _value );
    }

    private static void OnTitleChanged( DependencyObject _d, DependencyPropertyChangedEventArgs _e )
    {
       ...
    }
}

Then, if I write this:

<dl:HorizontalShelf>
    <Label dl:MyLibCommonProperties.Title="CustomTitle">1</Label>
    <Label>1</Label>
    <Label>2</Label>
    <Label>3</Label>
</dl:HorizontalShelf>

everything works fine and the property gets the specified value, but if I try to bind that property to some other UIElement DependencyProperty like this:

<dl:HorizontalShelf>
    <Label dl:MyLibCommonProperties.Title="{Binding ElementName=NamedLabel, Path=Name}">1</Label>
    <Label>1</Label>
    <Label>2</Label>
    <Label Name="NamedLabel">3</Label>
</dl:HorizontalShelf>

an exception would be thrown: "A 'Binding' cannot be set on the 'SetTitle' property of type 'Label'. A 'Binding' can only be set on a DependencyProperty of a DependencyObject."

What am I missing? Binding seems to work fine if instead of binding to "Name" I bind to some other attached property defined in MyLibCommonProperties.

Thanks in advance.

Answer 1

Replace the UIElement in your DependencyProperty definition to MyLibCommonProperties

public static readonly DependencyProperty TitleProperty =
    DependencyProperty.RegisterAttached( 
        "Title", 
        typeof(String),
        typeof(MyLibCommonProperties), // Change this line
        new FrameworkPropertyMetadata(
            "NoTitle", new PropertyChangedCallback(OnTitleChanged))
        );

I think it might be because the binding implicitly uses the parent class specified to call SetTitle() so it is calling Label.SetTitle() instead of MyLibCommonProperties.SetTitle()

I had the same issue with some custom TextBox properties. If I used typeof(TextBox) then I couldn't bind to the value, but if I used typeof(TextBoxHelpers) then I could