Multibinding: two columns to one column

Question

I have a table which has two columns ValueA and ValueB. I need to show the table in a grid, but I need to show either ValueA or ValueB, but not both. So if ValueA doesn't have a value, then ValueB is shown and vice versa. When user enters a value to the column, only ValueA should be updated. So ValueB in kind of a default value (it's update in another part of the program).

I've tried to resolve this problem using MultiBinding, but I've only managed to show the values. The value user enters to the grid always gets overwritten by the original value.

I have created a converter named ValueAValueBConverter that has a Convert method like this:

public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
    if (values[0] != DependencyProperty.UnsetValue && values[0] != null && !string.IsNullOrEmpty(values[0].ToString()))
    {
        Debug.WriteLine("values 0: " + values[0]);
        return values[0].ToString();
    }
    if (values[1] != DependencyProperty.UnsetValue && values[1] != null && !string.IsNullOrEmpty(values[1].ToString()))
    {
        Debug.WriteLine("values 1: " + values[1]);
        return values[1].ToString();
    }

    return null;
}

And a ConvertBack method (which never gets called):

public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
{
    return new object [] { value, value };
}

The XAML of the bound colums looks like this:

<toolkit:DataGridTextColumn Width="2*" Header="{x:Static localProperties:Resources.TValuesAB}">
    <toolkit:DataGridTextColumn.Binding>
        <MultiBinding Converter="{StaticResource myValueAValueBConverter}" Mode="TwoWay">
            <Binding Path="ValueA" UpdateSourceTrigger="PropertyChanged" />
            <Binding Path="ValueB" UpdateSourceTrigger="PropertyChanged" />
        </MultiBinding>
    </toolkit:DataGridTextColumn.Binding>
</toolkit:DataGridTextColumn>

Should I do this in the ViewModel (we're using MVVM pattern) or is there any other way to do this with XAML?

Answer 1

If I well understood, you just need one attribute more in your class, something like ValueAorB. With a getter which will do the work your Converter does and a setter a bit more complicated.

public object ValueAorB
{
    get
    {
        if (values[0] != DependencyProperty.UnsetValue && values[0] != null && !string.IsNullOrEmpty(values[0].ToString()))
        {
            Debug.WriteLine("values 0: " + values[0]);
            return values[0].ToString();
        }
        if (values[1] != DependencyProperty.UnsetValue && values[1] != null && !string.IsNullOrEmpty(values[1].ToString()))
        {
            Debug.WriteLine("values 1: " + values[1]);
            return values[1].ToString();
        }
    }
}

Don't forget to make it raises the PropertyChanged event.

And with that:

And a ConvertBack method (which never gets called):

May be you don't need a setter. Think about it. A two way will be more complicated and you don't even need it. In addition, I think you could do it with your own method if you were trying to make it one way.