Why do I get a DependencyProperty.UnsetValue when converting a value in a MultiBinding?

Question

I have an extremely simple IMultiValueConverter that simply OR's two values. In the example below, I want to invert the first value using an equally simple boolean inverter.

<MultiBinding Converter="{StaticResource multiBoolToVis}">     <Binding Path="ConditionA" Converter="{StaticResource boolInverter}"/>     <Binding Path="ConditionB"/> </MultiBinding> 

and the inverter:

public class BoolInverterConverter : IValueConverter {     #region IValueConverter Members     public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)     {         if (value is bool)         {             return !((bool)value);         }         return null;     }     public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)     {         throw new NotImplementedException();     }     #endregion } 

When I include the boolInverter, the first value in the MultiValueConverter becomes a "DependencyProperty.UnsetValue". There are no problems when I do not use the converter (other than not the logic I am aiming for, of course).

Am I missing something? Stepping through the debugger shows that the InverseBoolConverter is properly inverting the value I pass it, but that value is then not being 'sent' to the MultiValueConverter.

Answer 1

From MSDN:

UnsetValue is a sentinel value that is used for scenarios where the WPF property system is unable to determine a requested DependencyProperty value. UnsetValue is used rather than null reference (Nothing in Visual Basic), because null reference could be a valid property value, as well as a valid (and frequently used) DefaultValue.

Which means one of the following things:

• You use a template (ControlTemplate or DataTemplate), and the value does not have a DataSource set at the time of being Loaded. So it will hit your converter twice, first with the UnsetValue, second with the boolean value; so nothing to worry about;
• Your Binding is incorrect, meaning the Binding cannot determine a value, thus resulting in the UnsetValue.. You should propbably see a warning..

Also, you cannot combine Converters like you do.. So its probably that.

Remove the Converter in the inner Binding, and it should be fixed! :)

Hope this helps!