How do I update an IValueConverter on CollectionChanged?

Question

Here's a basic example to explain my problem. Let's say I have

ObservableCollection<int> Numbers {get; set;}

and an IValueConverter that returns the sum of Numbers.

Normally what I'd do is changed the IValueConverter into an IMultiValueConverter and bind a second value to Numbers.Count like this

<MultiBinding Converter="{StaticResource SumTheIntegersConverter}">
    <Binding Path="Numbers"     />
    <Binding Path="Numbers.Count"   />
</MultiBinding>

However I'm unable to use this method to solve my actual problem. It seems like there should be a better way to update the binding when the collection changes that I'm just not thinking of. What's the best way to get the value converter to run when items are added and removed to Numbers?

Answer 1

I ended up doing something like this which seems to work. It's far from an optimal solution and I'd still be interested in something better but it seems to work for my purposes.

class CollectionChangedHandlingValueConverter : IValueConverter
{
    DependencyObject myTarget;
    DependencyProperty myTargetProperty;

    //If this ever needs to be called from XAML you can make it a MarkupExtension and use ProvideValue to set up the Target and TargetProperty
    public CollectionChangedHandlingValueConverter(DependencyObject target, DependencyProperty dp)
    {
        myTarget = target;
        myTargetProperty = dp;
    }

    #region IValueConverter Members

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        INotifyCollectionChanged collection = value as INotifyCollectionChanged;
        if (collection != null)
        {
            //It notifies of collection changed, try again when it changes
            collection.CollectionChanged += DataCollectionChanged;
        }

        //Do whatever conversions here
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }

    #endregion

    void DataCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        if ((myTarget != null) && (myTargetProperty != null))
        {
            BindingOperations.GetBindingExpressionBase(myTarget, myTargetProperty).UpdateTarget();
        }
    }
}

Answer 2

This is actually surprisingly very difficult. An IValueConverter doesn't update, so this does not work as you'd hope.

I wrote a sample on the Microsoft Expression Gallery called Collection Aggregator that shows a working, if convoluted, approach to making this work via a Behavior that does the aggregation (Count, in your case, although I also support Sum, Average, etc) for you, instead of a converter.

Answer 3

And I ended up synchronizing collection (original with converter), take a look at the buttom of my post for example:

http://alexburtsev.wordpress.com/2011/03/05/mvvm-pattern-in-silverlight-and-wpf/

Answer 4

In your model, subscribe to CollectionChanged and raise PropertyChanged:

Numbers.CollectionChanged += (o,e) => 
  OnPropertyChanged(new PropertyChangedEventArgs(nameof(Numbers)));

And, as thomasgalliker mentioned, you should unsubscribe from the event when the model containing the connection is no longer used.