c# - Property Changed in DependencyProperty

Question

In a previous post I asked how to register a property as DependencyProperty. I got an answer and it works fine.

But now I want to add some Items to this DependencyProperty on a Click. This doesn't work. My code to register the DependencyProperty is:

public static readonly DependencyProperty ChartEntriesProperty = DependencyProperty.Register(
        "ChartEntries", typeof(ObservableCollection<ChartEntry>), typeof(ChartView),
        new FrameworkPropertyMetadata(OnChartEntriesChanged));

    private static void OnChartEntriesChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {

    }

The OnChartEntriesChanged-Event is called at the moment I do the Binding from my XAML to my c#-code. But if I add a ChartEntry afterwards (on button-click) the event is not fired.

Does anyone know why?

Answer 1

When you add an item to the ChartEntries collection, you do not actually change that property, so the PropertyChangedCallback isn't called. In order to get notified about changes in the collection, you need to register an additional CollectionChanged event handler:

private static void OnChartEntriesChanged(
    DependencyObject obj, DependencyPropertyChangedEventArgs e)
{
    var chartView = (ChartView)obj;
    var oldCollection = e.OldValue as INotifyCollectionChanged;
    var newCollection = e.NewValue as INotifyCollectionChanged;

    if (oldCollection != null)
    {
        oldCollection.CollectionChanged -= chartView.OnChartEntriesCollectionChanged;
    }

    if (newCollection != null)
    {
        newCollection.CollectionChanged += chartView.OnChartEntriesCollectionChanged;
    }
}

private void OnChartEntriesCollectionChanged(
    object sender, NotifyCollectionChangedEventArgs e)
{
    ...
}

---

It would also make sense not to use ObservableCollection<ChartEntry> for the property type, but simply ICollection or IEnumerable instead. This would allow for other implementations of INotifyCollectionChanged in the concrete collection type. See here and here for more information.