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I'm trying to use an attached property to trigger a style change on a UIElement when an event has fired. Here is the case scenario: A user sees a TextBox , and focuses then unfocuses it. Somewhere in an attached property it notices this LostFocus event and sets a property (somewhere?) to say that it HadFocus . The style on the TextBox then knows that it should style itself differently based on this HadFocus property. Here's how I imagine the markup to look... <TextBox Behaviors:UIElementBehaviors.ObserveFocus="True"> <TextBox.Style> <Style TargetType="TextBox"> <Style.Triggers> <Trigger Property="Behaviors:UIElementBehaviors.HadFocus" Value="True"> <Setter Property="Background" Value="Pink"/> </Trigger> </Style.Triggers> </Style> </TextBox.Style> I've tried a few combinations of the attached properties to get this working, my latest attempt throws a XamlParseException stating "Property can not be null on Trigger." public class UIElementBehaviors { public static readonly DependencyProperty ObserveFocusProperty = DependencyProperty.RegisterAttached("ObserveFocus", typeof (bool), typeof (UIElementBehaviors), new UIPropertyMetadata(false, OnObserveFocusChanged)); public static bool GetObserveFocus(DependencyObject obj) { return (bool) obj.GetValue(ObserveFocusProperty); } public static void SetObserveFocus(DependencyObject obj, bool value) { obj.SetValue(ObserveFocusProperty, value); } private static void OnObserveFocusChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)

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