如何在某些情况下禁用转换器

当autosize为false时，如何禁用以下转换器。我应该将哪些代码放入无所事事区域。

[ValueConversion(typeof(Boolean), typeof(Double))]
public class ConvertAutoSize2HeightWidth : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        Boolean autosize = (Boolean)value;
        if (autosize)
            return Double.NaN;
        else
            //**do nothing**
    }
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}