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Why does the MultiBinding not work for CornerRadius
<Border Background="Red" Width="100" Height="100"
        CornerRadius="{Binding Converter={vc:SingleAndMultiConverter}}" />
<Border Background="Red" Width="100" Height="100"
        CornerRadius="{MultiBinding Converter={vc:SingleAndMultiConverter}}" />

public class SingleAndMultiConverter : MarkupExtension, IValueConverter, IMultiValueConverter
{
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return this;
    }

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return Convert();
    }
    public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return Convert();
    }
    private object Convert()
    {
        return 15;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotSupportedException();
    }
    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotSupportedException();
    }
}

[ValueConversion(typeof(object[]),typeof(CornerRadius))]
public class Multi : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        return new CornerRadius(Double.Parse("15"));
    }

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

}