Анимация при изменении видимости: свойство уже было зарегистрировано «FrameworkElement»

Я использую этот код для установки анимации при изменении видимости.

public class VisibilityAnimation : DependencyObject
{
    public enum AnimationType
    {
        None,
        Fade
    }

    private const int AnimationDuration = 1000;

    private static readonly Dictionary<FrameworkElement, bool> _hookedElements =
        new Dictionary<FrameworkElement, bool>();

    public static AnimationType GetAnimationType(DependencyObject obj)
    {
        return (AnimationType)obj.GetValue(AnimationTypeProperty);
    }

    public static void SetAnimationType(DependencyObject obj, AnimationType value)
    {
        obj.SetValue(AnimationTypeProperty, value);
    }

    public static readonly DependencyProperty AnimationTypeProperty =
        DependencyProperty.RegisterAttached(
            "AnimationType",
            typeof(AnimationType),
            typeof(VisibilityAnimation),
            new FrameworkPropertyMetadata(AnimationType.None,
                new PropertyChangedCallback(OnAnimationTypePropertyChanged)));
    private static void OnAnimationTypePropertyChanged(
        DependencyObject dependencyObject,
        DependencyPropertyChangedEventArgs e)
    {
        FrameworkElement frameworkElement = dependencyObject as FrameworkElement;

        if (frameworkElement == null)
        {
            return;
        }

        // If AnimationType is set to True on this framework element, 
        if (GetAnimationType(frameworkElement) != AnimationType.None)
        {
            // Add this framework element to hooked list
            HookVisibilityChanges(frameworkElement);
        }
        else
        {
            // Otherwise, remove it from the hooked list
            UnHookVisibilityChanges(frameworkElement);
        }
    }
    private static void HookVisibilityChanges(FrameworkElement frameworkElement)
    {
        _hookedElements.Add(frameworkElement, false);
    }
    private static void UnHookVisibilityChanges(FrameworkElement frameworkElement)
    {
        if (_hookedElements.ContainsKey(frameworkElement))
        {
            _hookedElements.Remove(frameworkElement);
        }
    }
    static VisibilityAnimation()
    {
        // Here we "register" on Visibility property "before change" event
        UIElement.VisibilityProperty.AddOwner(
            typeof(FrameworkElement),
            new FrameworkPropertyMetadata(
                Visibility.Visible,
                VisibilityChanged,
                CoerceVisibility));

    }
    private static void VisibilityChanged(
        DependencyObject dependencyObject,
        DependencyPropertyChangedEventArgs e)
    {
        // Ignore
    }
    private static object CoerceVisibility(
        DependencyObject dependencyObject,
        object baseValue)
    {
        // Make sure object is a framework element
        FrameworkElement frameworkElement = dependencyObject as FrameworkElement;
        if (frameworkElement == null)
        {
            return baseValue;
        }

        // Cast to type safe value
        Visibility visibility = (Visibility)baseValue;

        // If Visibility value hasn't change, do nothing.
        // This can happen if the Visibility property is set using data binding 
        // and the binding source has changed but the new visibility value 
        // hasn't changed.
        if (visibility == frameworkElement.Visibility || visibility == Visibility.Collapsed) //Aggiungo da cri..x fare l'effetto solo sul fade in
        {
            return baseValue;
        }

        // If element is not hooked by our attached property, stop here
        if (!IsHookedElement(frameworkElement))
        {
            return baseValue;
        }

        // Update animation flag
        // If animation already started, don't restart it (otherwise, infinite loop)
        if (UpdateAnimationStartedFlag(frameworkElement))
        {
            return baseValue;
        }

        // If we get here, it means we have to start fade in or fade out animation. 
        // In any case return value of this method will be Visibility.Visible, 
        // to allow the animation.
        DoubleAnimation doubleAnimation = new DoubleAnimation
        {
            Duration = new Duration(TimeSpan.FromMilliseconds(AnimationDuration))
        };

        // When animation completes, set the visibility value to the requested 
        // value (baseValue)
        doubleAnimation.Completed += (sender, eventArgs) =>
        {
            if (visibility == Visibility.Visible)
            {
                // In case we change into Visibility.Visible, the correct value 
                // is already set, so just update the animation started flag
                UpdateAnimationStartedFlag(frameworkElement);
            }
            else
            {
                // This will trigger value coercion again 
                // but UpdateAnimationStartedFlag() function will reture true 
                // this time, thus animation will not be triggered. 
                if (BindingOperations.IsDataBound(frameworkElement,
                    UIElement.VisibilityProperty))
                {
                    // Set visiblity using bounded value
                    Binding bindingValue =
                        BindingOperations.GetBinding(frameworkElement,
                            UIElement.VisibilityProperty);
                    BindingOperations.SetBinding(frameworkElement,
                        UIElement.VisibilityProperty, bindingValue);
                }
                else
                {
                    // No binding, just assign the value
                    frameworkElement.Visibility = visibility;
                }
            }
        };

        if (visibility == Visibility.Collapsed || visibility == Visibility.Hidden)
        {
            // Fade out by animating opacity
            doubleAnimation.From = 1.0;
            doubleAnimation.To = 0.0;
        }
        else
        {
            // Fade in by animating opacity
            doubleAnimation.From = 0.0;
            doubleAnimation.To = 1.0;
        }

        // Start animation
        frameworkElement.BeginAnimation(UIElement.OpacityProperty, doubleAnimation);

        // Make sure the element remains visible during the animation
        // The original requested value will be set in the completed event of 
        // the animation
        return Visibility.Visible;
    }
    private static bool IsHookedElement(FrameworkElement frameworkElement)
    {
        return _hookedElements.ContainsKey(frameworkElement);
    }
    private static bool UpdateAnimationStartedFlag(FrameworkElement frameworkElement)
    {
        bool animationStarted = (bool)_hookedElements[frameworkElement];
        _hookedElements[frameworkElement] = !animationStarted;

        return animationStarted;
    }

В xaml мне нужно установить: VisibilityAnimation.AnimationType="Fade"

Анимация работает нормально, но проблема в том, что у меня ошибка в заголовке.

Как я могу это исправить?

Stack Overflow хочет больше деталей, чтобы вставить этот код, но это все... напишите эту фразу, надеясь, что я смогу ее вставить.