Hi,
I have created a custom attached dependency property:

    public class CMProp : DependencyObject
    {
        public static readonly DependencyProperty ControlModeProperty;

        static CMProp()
        {
            FrameworkPropertyMetadata fm = new FrameworkPropertyMetadata();
            fm.Inherits = true;
            fm.DefaultValue = ControlMode.Display;
            ControlModeProperty = DependencyProperty.RegisterAttached("ControlMode", typeof(LLControls.ControlMode), typeof(CMProp), fm);
        }

        public static void SetControlMode(UIElement element, LLControls.ControlMode value)
        {
            element.SetValue(ControlModeProperty, value);
        }

        public static LLControls.ControlMode GetControlMode(UIElement element)
        {
            return (LLControls.ControlMode)element.GetValue(ControlModeProperty);
        }
     }

ControlMode is an enum with three values: Display, HeaderEdit and DataEdit. I am using the dependency property on a sub-classed canvas:

public static readonly DependencyProperty ControlModeProperty;
static LLCanvas()
{
FrameworkPropertyMetadata p5 = new FrameworkPropertyMetadata();
p5.Inherits = true;
ControlModeProperty = CMProp.ControlModeProperty.AddOwner(typeof(LLCanvas), p5);
}

In use these canvases can be nested so, for example:
CanvasA
-->CanvasB
---->CanvasC

If I set the ControlMode on CanvasA this flows through and is inherited by CanvasB and CanvasC as expected. If, however I set the ControlMode on CanvasB then, obviously, since I have set a local value there then subsequent changes on CanvasA are not inherited (which is fine).
But if I set the ControlMode of CanvasB to ControlMode.Display I want it to return to using the inherited value of CanvasA.
AFAICS I can't do this in the CoerceValue callback. I also tried using ClearValue() in the PropertyChanged callback but that doesn't appear to work either.

Any suggestions on how to accomplish this?
TIA,
Viv