How does FallbackValue work with a MultiBinding?

Question

I ask because it doesn't seem to work.

Assume we're binding to the following object:

public class HurrDurr
{
  public string Hurr {get{return null;}}
  public string Durr {get{return null;}}
}

Well, it would appear that if we used a MultiBinding against this the fallback value would be shown, right?

<TextBlock>
    <TextBlock.Text>                                
        <MultiBinding StringFormat="{}{0} to the {1}"
                        FallbackValue="Not set!  It works as expected!)">
            <Binding Path="Hurr"/>
            <Binding Path="Durr"/>
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

However the result is, in fact, " to the ". Even forcing the bindings to return DependencyProperty.UnsetValue doesn't work:

<TextBlock xmnlns:base="clr-namespace:System.Windows;assembly=WindowsBase">
    <TextBlock.Text>                                
        <MultiBinding StringFormat="{}{0} to the {1}"
            FallbackValue="Not set!  It works as expected!)">
            <Binding Path="Hurr"
                FallbackValue="{x:Static base:DependencyProperty.UnsetValue}" />
            <Binding Path="Durr"
                FallbackValue="{x:Static base:DependencyProperty.UnsetValue}" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

Tried the same with TargetNullValue, which was also a bust all the way around.

So it appears that MultiBinding will never ever use FallbackValue. Is this true, or am I missing something?

Answer 1

A little more messing around and I found that a converter can return the UnsetValue I need:

class MultiValueFailConverter : IMultiValueConverter
{
    public object Convert(
        object[] values, 
        Type targetType, 
        object parameter, 
        System.Globalization.CultureInfo culture)
    {
        if (values == null || 
            values.Length != 2 ||
            values.Any(x=>x == null))
            return System.Windows.DependencyProperty.UnsetValue;
        return values;
    }

    public object[] ConvertBack(
        object value, 
        Type[] targetTypes, 
        object parameter, 
        System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException("Too complex hurt brain.");
    }
}

However, this seems like a dirty filthy hack. I'd think a scenario like this would be accounted for in the framework. I can't find anything in Reflector, however.