你可能需要一个转换：

    public class EmptyStringDoubleConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return value?.ToString();
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            var str = value as String;

            if (string.IsNullOrWhiteSpace(str))
            {
                return null;
            }

            return double.Parse(str);
        }
    }