Clear a dependencyProperty value from the xaml

Question

I have a control which has a default value for a property. When the control first gets its dataContext set, it assigns this property automatically.

In the xaml now, I want it to be possible to UNset this property. I've tried setting it to x:Null of just the empty string, but then I get an error because there's no converter for the property. How do I simply unassign this property from the xaml in the rare cases where I want the feature disabled?

code where it is originally set:

void OmniBox_DataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
{
    if( e.NewValue is BindingObjectBaseExtended )
    {
        BindingObjectBaseExtended value = (BindingObjectBaseExtended)e.NewValue;
        this.SetBinding(OmniBox.ContextValidationMessagesProperty, new Binding() { Source = value, Path = new PropertyPath("ValidationMessages") });
    }
}

xaml where I want to unset the property.

<Style TargetType="ui:OmniBox">
    <Setter Property="ContextValidationMessages" Value="" />
</Style>

Note that if I do not set up the binding automatically when the data context changes, then by default there are no validation messages and I have to do the following in the xaml to set them up:

<Style TargetType="ui:OmniBox">
    <Setter Property="ContextValidationMessages" Value="ValidationMessages" />
</Style>

What I'm trying to do is make the above binding the default for my custom OmniBox control, and allow the user to unset it or change it to something else.

Answer 1

DependencyProperty.UnsetValue cannot be used in XAML.

http://msdn.microsoft.com/en-us/library/system.windows.dependencyproperty.unsetvalue(v=vs.90).ASPX