c# - How can I add logic to an existing dependency-property callback?

Question

I'm trying to add a PropertyChangedCallback to UIElement.RenderTransformOriginProperty. An exception is thrown when I try to override the PropertyMetadata.

I have searched MSDN and Google, and all I have been able to come up with is this. DependencyPropertyDescriptor.AddValueChanged is suggested at some point in that post, but that won't solve my problem since this is not a per-instance callback.

I don't understand what this exception means at all. Does anyone know what I am doing wrong?

public class foo : FrameworkElement
{
    private static void Origin_Changed( DependencyObject d,
                                        DependencyPropertyChangedEventArgs e)
    { }

    static foo()
    {
        PropertyMetadata OriginalMetaData =
            UIElement.RenderTransformOriginProperty.GetMetadata(
                typeof(FrameworkElement));



/*An exception is thrown when this line is executed:
 "Cannot change property metadata after it has been associated with a property"*/
        OriginalMetaData.PropertyChangedCallback +=
            new PropertyChangedCallback(Origin_Changed);



        UIElement.RenderTransformOriginProperty.OverrideMetadata(
            typeof(foo), OriginalMetaData);
    }
}

Answer 1

WPF will merge the property metadata for you when you call OverrideMetadata, no need to pass it the original Metadata object. So all you have to do is

UIElement.RenderTransformOriginProperty.OverrideMetadata(typeof(foo), new PropertyMetadata(new PropertyChangedCallback(Origin_Changed)));

One thing to be aware of is sometimes the code above throws an exception. The two cases where that happens are

1. The original metadata is a subclass of PropertyMetadata - I have seen FrameworkPropertyMetadata and UIPropertyMetadata. You just have to use the appropriate one in each case.

2. The dependency property is read only and you can't do anything about it.