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

Answer 1

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").