ArgumentException with an AttachedProperty in XAML

Hi all together!

I have a problem with an AttachedProperty in XAML that throws an exception in the designer. Because of not being able to change the design because of that I really want to get rid of that error.

The error is the following:

Error 5 The value "System.Windows.Data.Binding" is not of type "System.DateTime" and cannot be used in this generic collection.

The binding that causes the error is the attached property "local:SelectedDatesBehavior.MySelectedDates" in the following snippet:

<Calendar DataContext="{Binding Path=CalendarViewModels[2]}" DisplayDate="{Binding Path=DisplayDate}" Style="{StaticResource styleDatePickerCalendar}" DisplayDateStart="{Binding Path=StartDate}" DisplayDateEnd="{Binding Path=EndDate}" local:SelectedDatesBehavior.MySelectedDates="{Binding Path=SelectedDates}"></Calendar>

The ViewModel used for the calendar is the following:

    public class CalendarViewModel : DependencyObject
    {
        public DateTime StartDate
        {
            get
            {
                return (DateTime)this.GetValue(StartDateProperty);
            }

            set
            {
                this.SetValue(StartDateProperty, value);
            }
        }

        public static readonly DependencyProperty StartDateProperty =
            DependencyProperty.Register("StartDate", typeof(DateTime), typeof(CalendarViewModel));

        public DateTime EndDate
        {
            get
            {
                return (DateTime)this.GetValue(EndDateProperty);
            }

            set
            {
                this.SetValue(EndDateProperty, value);
            }
        }

        public static readonly DependencyProperty EndDateProperty =
            DependencyProperty.Register("EndDate", typeof(DateTime), typeof(CalendarViewModel));

        public DateTime DisplayDate
        {
            get
            {
                return (DateTime)this.GetValue(DisplayDateProperty);
            }

            set
            {
                this.SetValue(DisplayDateProperty, value);
            }
        }

        public static readonly DependencyProperty DisplayDateProperty =
            DependencyProperty.Register("DisplayDate", typeof(DateTime), typeof(CalendarViewModel));

        public bool IsTodayHighlighted
        {
            get
            {
                return (bool)this.GetValue(IsTodayHighlightedProperty);
            }

            set
            {
                this.SetValue(IsTodayHighlightedProperty, value);
            }
        }

        public static readonly DependencyProperty IsTodayHighlightedProperty =
            DependencyProperty.Register("IsTodayHighlighted", typeof(bool), typeof(CalendarViewModel), new UIPropertyMetadata(true));

        public ObservableCollection<DateTime> SelectedDates
        {
            get
            {
                return (ObservableCollection<DateTime>)this.GetValue(SelectedDatesProperty);
            }

            protected set
            {
                this.SetValue(SelectedDatesProperty, value);
            }
        }

        public static readonly DependencyProperty SelectedDatesProperty =
            DependencyProperty.Register("SelectedDates", typeof(ObservableCollection<DateTime>), typeof(CalendarViewModel));

        public CalendarViewModel()
        {
            this.SelectedDates = new ObservableCollection<DateTime>();
        }

        public void SetDates(IEnumerable<DateTime> dates)
        {
            ObservableCollection<DateTime> newDates = new ObservableCollection<DateTime>();

            foreach (DateTime item in dates)
            {
                newDates.Add(item);
            }

            this.SelectedDates = newDates;
        }

        public void ClearDates()
        {
            this.SelectedDates = new ObservableCollection<DateTime>();
        }
    }

And the static class with the logic for the attached property is this:

    public class SelectedDatesBehavior
    {
        public static readonly DependencyProperty MySelectedDatesProperty = DependencyProperty.RegisterAttached("MySelectedDates",
            typeof(ObservableCollection<DateTime>),
            typeof(SelectedDatesBehavior),
            new FrameworkPropertyMetadata(OnSelectedDatesPropertyChanged));

        public static void SetMySelectedDates(UIElement calendar, ObservableCollection<DateTime> value)
        {
            calendar.SetValue(MySelectedDatesProperty, value);
        }

        public static ObservableCollection<DateTime> GetMySelectedDates(UIElement calendar)
        {
             return (ObservableCollection<DateTime>)calendar.GetValue(MySelectedDatesProperty);
        }

        public static ObservableCollection<DateTime> GetMySelectedDates(Calendar calendar)
        {
            return (ObservableCollection<DateTime>)calendar.GetValue(MySelectedDatesProperty);
        }
}

Feel free to ask further questions if the structure is not clear. I'm thankful for every hint that could lead to solve the problem :)!

Best wishes,

Nico