Getting 'ConvertBack(...)' to fire on WPF ComboBox selection change - wpf

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.

Answer 2

You're right, it is awkward, but only because you're trying to add some management to the collection in a value converter instead of on the collection itself. I think it would help if your collection was more aware that its items have an IsSelected property:
public CustomCollection : Collection<CustomObject> {
CustomObject _current;
public CustomObject CurrentSelection {
get { return _current; }
set {
if (_current == value)
return;
if (_current != null)
_current.IsSelected = false;
if (value != null)
value.IsSelected = true;
_current = value;
}
}
}
Just add a little extra to make sure that _current is at least the first element in the collection.
<ComboBox ItemsSource="{Binding}" SelectedItem="{Binding CurrentSelection}">
Now you shouldn't need the converter anymore. There are some considerations missing, however. You may want to use ObservableCollection<T> instead and raise the PropertyChanged event when CurrentSelection is changed, so that if anything else is bound to that property or it changed in code, all bindings will update appropriately.
Edit: Wrapping the model
One easy way to wrap the collection instead of making a custom collection like above:
public class CollectionWrapper : INotifyPropertyChanged {
public event PropertyChangedEventHandler PropertyChanged = (o,e)=>{};
// never have to check for null
public CollectionWrapper(Collection<CustomObject> collection) {
Items = collection;
}
// unlikely to change, so let's prevent it for now
public Collection<CustomObject> Items {
get;
private set;
}
CustomObject _current;
public CustomObject CurrentSelection {
get { return _current; }
set {
if (_current == value)
return;
if (_current != null)
_current.IsSelected = false;
if (value != null)
value.IsSelected = true;
_current = value;
PropertyChanged(this, new PropertyChangedEventArgs("CurrentSelection"));
}
}
}
Then this object becomes your data context and the ComboBox bindings change to this:
<ComboBox ItemsSource="{Binding Items}" SelectedItem="{Binding CurrentSelection}">