Question

I was learning about ObservableCollections in WPF and something is not clear to me. I understand that if I bind a control to an ObservableCollection and the collection changes, the control will reflect the changes. My questions are:

• ObservableCollection implements INotifyCollectionChanged, which is just an event, CollectionChanged. That event should be fired whenever the collection changes but who subscribes to that event? Is it done automatically when you create a binding to the collection?

• I was looking at ObservableCollection using Reflector and was trying to see when the CollectionChanged event gets fired. However I couldn’t find where it is done. For example, I wanted to see when it was fired when I added a new item to the collection. Add(…) is implemented in ObservableCollection’s base class, Collection, but Collection does not implement INotifyCollectionChanged so I don’t understand how bound controls are notified of the change.

I am guessing that a lot of this is taken care of under the covers but any info is greatly appreciated.

Answer 1

To answer your first question: it all starts inside the “Items” property of the ItemContainerGenerator class (which, all ItemsControl objects have an instance of). If you look at the setter for the said “Items” property, you’ll see that it has special logic that checks if the given IList is of type INotifyCollectionChanged, it will attach an event listener.

ItemContainerGenerator.Items property:

internal IList Items
{
    get
    {
        return this._items;
    }
    set
    {
        if (this._items != value)
        {
            INotifyCollectionChanged source = this._items as INotifyCollectionChanged;
            if ((this._items != this.Host.View) && (source != null))
            {
                CollectionChangedEventManager.RemoveListener(source, this);
            }
            this._items = value;
            source = this._items as INotifyCollectionChanged;
            if ((this._items != this.Host.View) && (source != null))
            {
                CollectionChangedEventManager.AddListener(source, this);
            }
        }
    }
}

It really has nothing to do with Bindings. See, if you have the following code that doesn’t use Bindings at all, the collection changed notification will still work:

<Window x:Class="DynamicObjectTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <StackPanel>
            <Button Click="Button_Click">click</Button>
            <ListBox x:Name="listBox"/>
        </StackPanel>
    </Grid>
</Window>

public partial class MainWindow : Window
    {
        ObservableCollection items = new ObservableCollection();

        public MainWindow()
        {
            InitializeComponent();

            this.listBox.ItemsSource = items;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            items.Add("A");
        }
    }