How to bind a Localized string in the TargetNullValue attribute?

Question

I have a Textblock which Text attribute is binding to a DateTime? type data, and I want to show something when the DateTime? data is null.
The code below works great.

  < TextBlock Text="{Binding DueDate, TargetNullValue='wow,It's null'}"/>

But what about if I want to bind a Localizedstring to the TargetNullValue?
The Code below not work :(
How to?

  < TextBlock Text="{Binding DueDate, TargetNullValue={Binding LocalStrings.bt_help_Title1, Source={StaticResource LocalizedResources}} }"/>

Answer 1

I don't see any way to do that with TargetNullValue. As a workaround, you can try using a converter:

public class NullValueConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value != null)
        {
            return value;
        }

        var resourceName = (string)parameter;

        return AppResources.ResourceManager.GetString(resourceName);
    }

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

Then add it to the resources of your page:

<phone:PhoneApplicationPage.Resources>
    <local:NullValueConverter x:Key="NullValueConverter" />
</phone:PhoneApplicationPage.Resources>

Finally, use it instead of TargetNullValue:

<TextBlock Text="{Binding DueDate, Converter={StaticResource NullValueConverter}, ConverterParameter=bt_help_Title1}" />

Answer 2

Since you can't have a binding inside another binding you will need to use a multi-binding.

Something like:

<Window.Resources>
    <local:NullConverter x:Key="NullConverter" />
</Window.Resources>

<TextBlock>
    <TextBlock.Text>
        <MultiBinding Converter="{StaticResource NullConverter}">
            <Binding Path="DueDate"/>
            <!-- using a windows resx file for this demo -->
            <Binding Source="{x:Static local:LocalisedResources.ItsNull}" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

public class NullConverter : IMultiValueConverter
{
    #region Implementation of IMultiValueConverter

    public object Convert(object[] values, Type targetType, 
                          object parameter, CultureInfo culture)
    {
        if (values == null || values.Length != 2)
        {
            return string.Empty;
        }

        return (values[0] ?? values[1]).ToString();
    }

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

    #endregion
}