xamarin.forms預設情況下，如果屬性是double?型別，繫結到Entry上，是無法實現雙向繫結的，

可以自定義Converter實現雙向繫結

    public class NullableConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return System.Convert.ChangeType(value, targetType);
        }

        
 
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value == null)
                return null;

            if( targetType.IsGenericType  )
            {
                Type valueType = targetType.GetGenericArguments()[0];
                
 
object result = System.Convert.ChangeType(value, valueType);
                return Activator.CreateInstance(targetType, result);
            }
            return System.Convert.ChangeType(value, targetType);
        }
    }

然後在app.xaml裡面，加入資源配置

    <Application.Resources>
        <ResourceDictionary>
            <local:NullableConverter x:Key="
 
NullableConverter"></local:NullableConverter>
        </ResourceDictionary>
    </Application.Resources>

然後，以後Entry繫結nullable型別時，可以這樣寫

Text="{Binding propertyName,Converter={StaticResource NullableConverter}}"