Question

I have a ComboBox with its Items property bound to a collection of objects. I also have SelectedItem property bound to the entire collection, with a ValueConverter designed to examine the elements in the collection and return the 1 item to be selected. This part works.

What doesn’t work is when the user makes a selection change on the ComboBox, the ConvertBack(...) method of the ValueConverter is not being called. I need ConvertBack(...) called because I need to take the user’s selection, re-examine the collection, and edit the old selected item and newly selected item appropriately.

I know this approach is a awkward, but it’s the way it is. Here’s the relevant code:

ComboBox:

<ComboBox ItemsSource="{Binding}" SelectedItem="{Binding Path=., Converter={StaticResource ResourceKey=DataInputAssetChoiceSelectedItemConverter}}" />

ValueConverter:

public class DataInputAssetChoiceSelectedItemConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value != null)
        {
            foreach (CustomObject Choice in (Collection<CustomObject>)value)
            {
                if (Choice.IsSelected)
                {
                    return Choice;
                }
            }
            return ((Collection<CustomObject>)value).First();
        }
        return null;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {   //breakpoint...execution never gets here!
        return null;
    }
}

So why doesn’t ConvertBack(...) ever get called? Is it just something I’m misunderstanding about ComboBox? I’ve tried this approach using SelectedItem, SelectedValue, SelectedIndex, and have tried messing with UpdateSourceTrigger, various binding modes, DataTriggers, and can never seem to get ConvertBack(...) to be called. Is using the SelectionChanged event the only option? If so, why?

Answer 1

You are not binding to a property, so the Binding can’t set anything. You are binding directly to the DataContext object, and the Binding won’t update that.

If you had {Binding Path=SomeProperty, Converter=...} then the ConvertBack would be called. As it stands though, it won’t be called.