WPF转换器转换导致Visual Studio设计器异常

如下所示的转换器将导致2008 Visual Studio设计器不显示XAML,并以“指定的强制转换无效”出错。异常。

public class ItemsVisibilityToGridColumnWidthConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        //THE TWO OFFENDING LINES...
        var itemsVisibility = (Visibility)values[0];
        var orientation = (Orientation)values[1];

        if (orientation == Orientation.Horizontal && itemsVisibility != Visibility.Visible)
        {
            return new GridLength(0);
        }

        return new GridLength(4, GridUnitType.Star);
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

将强制转换更改为使用这样的方法可以解决以下问题:

static class EnumCaster
{
    internal static Orientation CastAsOrientation(object value)
    {
        if (value is Enum)
        {
            return (Orientation)value;
        }
        return Orientation.Horizontal;
    }
    internal static Visibility CastAsVisibility(object value)
    {
        if (value is Enum)
        {
            return (Visibility)value;
        }
        return Visibility.Visible;
    }
}

我的问题是,wtf和Visual Studio设计器有问题吗?还有,有没有更好的方法可以将这些对象强制转换到相应的枚举中,这样设计人员就不会抛出一个fit?