Binding a collection to a custom control property

Question

I am having no luck trying to bind a collection of data to my custom control its property. I already have implemented the mechanism for a string property of this control (with a small help from here) and expected the collection type to be as easy. However I cannot make it work again.

Here is my custom control view

<UserControl x:Class="BadaniaOperacyjne.Controls.Matrix"
            mc:Ignorable="d" Name="CustomMatrix"
            d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <Grid.RowDefinitions>
            <!-- ... -->
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <!-- ... -->
        </Grid.ColumnDefinitions>
        <TextBlock Grid.Row="0" Grid.Column="1" Text="{Binding ElementName=CustomMatrix, Path=Title}"/>
        <Grid Grid.Row="2" Grid.Column="1" Name="contentGrid">
            <ListBox ItemsSource="{Binding ElementName=CustomMatrix, Path=ItemsList}">
                <ListBox.ItemTemplate>
                    <DataTemplate>
                        <TextBlock Text="{Binding}"/>
                    </DataTemplate>
                </ListBox.ItemTemplate>
            </ListBox>
        </Grid>
    </Grid>
</UserControl>

and its code-behind

#region ItemsList Property
public static readonly DependencyProperty ItemsListProperty =
    DependencyProperty.Register("ItemsList", typeof(ObservableCollection<object>), typeof(Matrix), new PropertyMetadata(new ObservableCollection<object>(), new PropertyChangedCallback(ItemsListChanged)));
public ObservableCollection<object> ItemsList
{
    get { return GetValue(ItemsListProperty) as ObservableCollection<object>; }
    set { SetValue(ItemsListProperty, value); }
}
private void ItemsListChanged(object value)
{
    System.Diagnostics.Debug.WriteLine("matrix: items list changed " + value);
    if (ItemsList != null)
    {
        ItemsList.CollectionChanged += ItemsList_CollectionChanged;
        System.Diagnostics.Debug.WriteLine("got " + string.Join(",", ItemsList.ToList()));
    }
    else
    {
        System.Diagnostics.Debug.WriteLine("got null");
    }
}

void ItemsList_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
{
    System.Diagnostics.Debug.WriteLine("matrix: current items list collection changed");
}
private static void ItemsListChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    // a breakpoint
    ((Matrix)d).ItemsListChanged(e.NewValue);
}
#endregion

// showing the Title property implementation just to state that
// it is done the same way as for ItemsList
#region Title Property
public static readonly DependencyProperty TitleProperty = 
    DependencyProperty.Register("Title", typeof(string), typeof(Matrix), new PropertyMetadata("", new PropertyChangedCallback(TitleChanged)));
public string Title
{
    get { return (string)GetValue(TitleProperty); }
    set { SetValue(TitleProperty, value); }
}
private void TitleChanged(string title)
{
    System.Diagnostics.Debug.WriteLine("matrix: title changed to: " + title);
}
private static void TitleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    ((Matrix)d).TitleChanged((string)e.NewValue);
}
#endregion

And here's how I am trying to bind to that control

<custom:Matrix x:Name="customMatrix" DockPanel.Dock="Top" Title="{Binding Title}" ItemsList="{Binding Items}"/>

and the code-behind for the main page is

//internal ObservableCollection<List<int>> ItemsList { get; set; }
public class ViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public ViewModel()
    {
        Items = new ObservableCollection<int> { 1, 2, 3, 4, 5, 6};
    }

    void Items_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        System.Diagnostics.Debug.WriteLine("problem manager: items list changed " + e.NewItems.Count);
    }

    public ObservableCollection<int> Items { get; set; }

    protected string title;
    public string Title
    {
        get { return title; }
        set
        {
            if (title != value)
            {
                title = value;
                NotifyPropertyChanged("Title");
            }
        }
    }

    protected void NotifyPropertyChanged(string name)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(name));
        }
    }
}

public ViewModel VM { get; private set; }

// this is the window constructor
private ProblemManager() 
{
    VM = new ViewModel();

    DataContext = VM;
    InitializeComponent();

    VM.Title = "title";
}

private int i = 0;
private void btnAddRow_Click(object sender, RoutedEventArgs e)
{
    // when doing either of these two lines below, 
    // the control breakpoint is never hit
    VM.Items.Add(++i);
    VM.Items = new ObservableCollection<int> { 2, 3 };

    // however, when directly assigning to the control's property, 
    // the event is raised and the breakpoint is hit and the UI is updated
    customMatrix.ItemsList = new ObservableCollection<object> { 1, 2, 3 };
    customMatrix.ItemsList.Add(66);
    // and this of course makes the control's title update
    VM.Title = (++i).ToString();
}

Both DependencyPropertys for the control's Title and ItemsList are, I believe, created the same way. Nonetheless, the binding is probably not working as the ItemsListChanged event is not raised by that binding. So, the problem is that I cannot bind my window's ViewModel.Items collection via XAML to the control's ItemsList collection. Is creating a DependencyProperty for a collection within a control any different from DependencyProperty for a simple string property?

Answer 1

Problem is in your DependencyProperty registration. Co-variance is not applicable for generic lists i.e. you cannot do this -

ObservableCollection<object> objects = new ObservableCollection<int>();

You have declared type of DP as ObservableCollection<object> but binding it with list of type ObservableCollection<int>.

You should change either type of DP to ObservableCollection<int> OR change binding collection type to ObservableCollection<object>.

public ViewModel()
{
    Items = new ObservableCollection<object> { 1, 2, 3, 4, 5, 6};
}

public ObservableCollection<object> Items { get; set; }