collection dependency properties

Question

I have a custom control that has a DependencyProperty of type ObservableCollection that is bound to an observableCollection:

Answer 1

Below is a working example that may help.

In this example, the method OnChanged is called immediately, when the Add button is clicked "Changed" is written to the console.

The Control

public class MyControl : Control
{

    public ObservableCollection<string> ExtraColumns
    {
        get { return (ObservableCollection<string>)GetValue(ExtraColumnsProperty); }
        set { SetValue(ExtraColumnsProperty, value); }
    }

    public static readonly DependencyProperty ExtraColumnsProperty =
        DependencyProperty.Register("ExtraColumns", typeof(ObservableCollection<string>), typeof(MyControl),
                                    new PropertyMetadata(new ObservableCollection<string>(), OnChanged));

    static void OnChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        (sender as MyControl).OnChanged();

    }

    void OnChanged()
    {
        if ( ExtraColumns != null )
            ExtraColumns.CollectionChanged += new System.Collections.Specialized.NotifyCollectionChangedEventHandler(ExtraColumns_CollectionChanged);
    }

    void ExtraColumns_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        Console.WriteLine("Changed");    
    }
}

The Window

<Window x:Class="WpfApplication18.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WpfApplication18"
    Title="MainWindow" Height="350" Width="525">
  <StackPanel>
    <local:MyControl ExtraColumns="{Binding Extras}"/>
    <Button Click="Button_Click">Add</Button>
  </StackPanel>
</Window>

Window Code Behind

public partial class MainWindow : Window
{
    private ObservableCollection<string> _extras = new ObservableCollection<string>( );
    public ObservableCollection<string> Extras
    {
        get { return _extras; }
        set
        {
            if (value != _extras)
            {
                _extras = value;
            }
        }
    }


    public MainWindow()
    {
        InitializeComponent();
        DataContext = this;
    }

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

Answer 2

In addition to what grantz has answered, I would suggest to declare the property with type IEnumerable<string> and check at runtime if the collection object implements the INotifyCollectionChanged interface. This provides greater flexibility as to which concrete collection implementation may be used as property value. A user may then decide to have their own specialized implementation of an observable collection.

Note also that in the ColumnsPropertyChanged callback the CollectionChanged event handler is attached to the new collection, but also removed from the old one.

public static readonly DependencyProperty ColumnsProperty =
    DependencyProperty.Register(
        "Columns", typeof(IEnumerable<string>), typeof(MyControl),
        new PropertyMetadata(null, ColumnsPropertyChanged));

public IEnumerable<string> Columns
{
    get { return (IEnumerable<string>)GetValue(ColumnsProperty); }
    set { SetValue(ColumnsProperty, value); }
}

private static void ColumnsPropertyChanged(
    DependencyObject obj, DependencyPropertyChangedEventArgs e)
{
    var control= (MyControl)obj;
    var oldCollection = e.OldValue as INotifyCollectionChanged;
    var newCollection = e.NewValue as INotifyCollectionChanged;

    if (oldCollection != null)
    {
        oldCollection.CollectionChanged -= control.ColumnsCollectionChanged;
    }

    if (newCollection != null)
    {
        newCollection.CollectionChanged += control.ColumnsCollectionChanged;
    }

    control.UpdateColumns();
}

private void ColumnsCollectionChanged(
    object sender, NotifyCollectionChangedEventArgs e)
{
    // optionally take e.Action into account
    UpdateColumns();
}

private void UpdateColumns()
{
    ...
}