Knowing if a DependencyProperty has not been set in XAML

Question

I have a control, which exposes a DP called PlacementTarget (it\'s bound to a child Popup FWIW).

What I want to do is if PlacementTarget is not set in XAML, then (

Answer 1

You don't need an extra placementTargetIsSet field and hence no PropertyChangedCallback.

In order to find out if a value has been set for the PlacementTarget property, you could just call the ReadLocalValue method and test if it returns DependencyProperty.UnsetValue:

bool placementTargetIsSet =
    ReadLocalValue(PlacementTargetProperty) != DependencyProperty.UnsetValue;