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

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!

Just in addition to all other answers, I usually add these lines to the beginning of Convert method:

    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)    {        if (values.Any(x => x == DependencyProperty.UnsetValue))            return DependencyProperty.UnsetValue;        ...    }

to make sure that none of the values is unset (that usually happens with DataGrid with CanUserAddRows="True").

If occuring in a datagrid try setting CanUserAddRows="False"