WPF: Adding an element to a databound Collection (a Dependency Property)

Question

I have this DependencyProperty which holds an entity with a property that is a collection (ShoutBox.Entities):

public static readonly DependencyProperty ShoutBoxProperty = DependencyProperty.Register("ShoutBox",typeof (ShoutBox),typeof (ShoutBoxViewerControl));

public ShoutBox ShoutBox
{
    get { return (ShoutBox) GetValue(ShoutBoxProperty); }
    set { SetValue(ShoutBoxProperty, value); }
}

It is being binded in xaml like such:

<ItemsControl ItemsSource="{Binding ShoutBox.Entries}">
.
.
</ItemsControl>

When I bind it the first time, it works as expected but there are times when I need to add items to the collection (with a method that is in the same control), like such:

public void AddNewEntry(ShoutBoxEntry newEntry)
{
    Dispatcher.Invoke(new Action(() =>{
        ShoutBox.Entries.Add(newEntry); //Adding directly the the Dependency property
    }));
}

The problem is that when I add a new element with the above method, the item isn't being displayed in the ItemsControl.

---

My question is, why isn't the new element that I am adding isn't being displayed in the ItemsControl ?

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[Edit]

Entries (ShoutBox.Entries) is of type List<ShoutBoxEntry>

Answer 1

What is the type of Entries? It either needs to be ObservableCollection or implement ICollectionChanged. Otherwise the binding doesn't know that a new item has been added.

Answer 2

Changing the type of Entries should indeed solve the problem... If you want to avoid the explicit call to Dispatcher.Invoke, I wrote a collection that raises the CollectionChanged and PropertyChanged events on the thread that created the collection :

public class AsyncObservableCollection<T> : ObservableCollection<T>
{
    private SynchronizationContext _synchronizationContext = SynchronizationContext.Current;

    public AsyncObservableCollection()
    {
    }

    public AsyncObservableCollection(IEnumerable<T> list)
        : base(list)
    {
    }

    protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
        if (SynchronizationContext.Current == _synchronizationContext)
        {
            // Execute the CollectionChanged event on the current thread
            RaiseCollectionChanged(e);
        }
        else
        {
            // Post the CollectionChanged event on the creator thread
            _synchronizationContext.Post(RaiseCollectionChanged, e);
        }
    }

    private void RaiseCollectionChanged(object param)
    {
        // We are in the creator thread, call the base implementation directly
        base.OnCollectionChanged((NotifyCollectionChangedEventArgs)param);
    }

    protected override void OnPropertyChanged(PropertyChangedEventArgs e)
    {
        if (SynchronizationContext.Current == _synchronizationContext)
        {
            // Execute the PropertyChanged event on the current thread
            RaisePropertyChanged(e);
        }
        else
        {
            // Post the PropertyChanged event on the creator thread
            _synchronizationContext.Post(RaisePropertyChanged, e);
        }
    }

    private void RaisePropertyChanged(object param)
    {
        // We are in the creator thread, call the base implementation directly
        base.OnPropertyChanged((PropertyChangedEventArgs)param);
    }
}

More details can be found here : http://tomlev2.wordpress.com/2009/04/17/wpf-binding-to-an-asynchronous-collection/