Why isn't my dependency property set when binding to it?

Question

I have two collections displayed in my WPF application, and I'd like to have elements from one of them disabled in the other. Doing this I'm creating a custom control FilteringListBox that inherits ListBox, and I want to add some handling inside it to disable elements that are set in a collection set through a property on the FilteringListBox. Now, my problem is that the dependency property taking the ObservableCollection that I want to filter elements from isn't set - even if I bind to it in the xaml.

I've created a simplified app where I reproduce the problem. Here is my Xaml:

<StackPanel>
    <StackPanel Orientation="Horizontal">
        <StackPanel Orientation="Vertical">
            <TextBlock>Included</TextBlock>
            <ListBox x:Name="IncludedFooList" ItemsSource="{Binding IncludedFoos}"></ListBox>
        </StackPanel>
        <Button Margin="10" Click="Button_Click">Add selected</Button>
        <StackPanel Orientation="Vertical">
            <TextBlock>Available</TextBlock>
            <Listbox:FilteringListBox x:Name="AvailableFooList" ItemsSource="{Binding AvailableFoos}" FilteringCollection="{Binding IncludedFoos}"></Listbox:FilteringListBox>
        </StackPanel>                
    </StackPanel>            
</StackPanel>

This is my custom component - currently only holding the Dependency Property:

public class FilteringListBox : ListBox
{
    public static readonly DependencyProperty FilteringCollectionProperty =
        DependencyProperty.Register("FilteringCollection", typeof(ObservableCollection<Foo>), typeof(FilteringListBox));                                                 

    public ObservableCollection<Foo> FilteringCollection
    {
        get
        {
            return (ObservableCollection<Foo>)GetValue(FilteringCollectionProperty);
        }
        set
        {
            SetValue(FilteringCollectionProperty, value);
        }
    }
}

And for the complete code the code behind and class definitions are here:

public partial class MainWindow : Window
{
    private MainViewModel _vm;

    public MainWindow()
    {
        InitializeComponent();
        _vm = new MainViewModel();
        DataContext = _vm;
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        if (AvailableFooList.SelectedItem == null)
            return;
        var selectedFoo = AvailableFooList.SelectedItem as Foo;
        _vm.IncludedFoos.Add(selectedFoo);
    }
}

public class MainViewModel
{
    public MainViewModel()
    {
        IncludedFoos = new ObservableCollection<Foo>();
        AvailableFoos = new ObservableCollection<Foo>();
        GenerateAvailableFoos(); 
    }

    private void GenerateAvailableFoos()
    {
        AvailableFoos.Add(new Foo { Text = "Number1" });
        AvailableFoos.Add(new Foo { Text = "Number2" });
        AvailableFoos.Add(new Foo { Text = "Number3" });
        AvailableFoos.Add(new Foo { Text = "Number4" });
    }

    public ObservableCollection<Foo> IncludedFoos { get; set; }
    public ObservableCollection<Foo> AvailableFoos { get; set; }
}

public class Foo
{
    public string Text { get; set; }
    public override string ToString()
    {
        return Text;
    }
}

I add breakpoints to the setter and getter of the DependencyProperty FilteringCollection in the FilteringListBox, but it is never triggered. Why? How can I fix it?

Answer 1

The binding system bypasses set and get accessors for dependency properties. If you want to execute code when a dependency property changes, you should add a PropertyChangedCallback to the DependencyProperty definition.

Answer 2

MSDN has a section about Dependency Property Callbacks and Validation, you need to register a PropertyChangedCallback

Example from msdn

public static readonly DependencyProperty AquariumGraphicProperty 
= DependencyProperty.Register(
  "AquariumGraphic",
  typeof(Uri),
  typeof(AquariumObject),
  new FrameworkPropertyMetadata(null,
      FrameworkPropertyMetadataOptions.AffectsRender, 
      new PropertyChangedCallback(OnUriChanged)
  )
);

private static void OnUriChanged(DependencyObject d, 
                                 DependencyPropertyChangedEventArgs e) {
  Shape sh = (Shape) d;
  sh.Fill = new ImageBrush(new BitmapImage((Uri)e.NewValue));
}

Answer 3

The get and set properties are never used directly by the WPF framework. You only provide them as a convenience for yourself. Instead you need to add a callback to your dependency property registration. The callback will be called when a value is bound to the dependency property. Hence your code for FilteredListBox should be changed to something similar to the following:

public partial class FilteringListBox : ListBox
{
    public static readonly DependencyProperty FilteringCollectionProperty =
        DependencyProperty.Register("FilteringCollection", typeof(ObservableCollection<Foo>), typeof(FilteringListBox), 
        new PropertyMetadata(null, FilteringCollectionPropertyCallback));

    static void FilteringCollectionPropertyCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        FilteringListBox listbox = d as FilteringListBox;
        // Do some work here
    }

    public ObservableCollection<Foo> FilteringCollection
    {
        get
        {
            return (ObservableCollection<Foo>) GetValue(FilteringCollectionProperty);
        }
        set
        {
            SetValue(FilteringCollectionProperty, value);
        }
    }
}