Binding a ObservableCollection<int> to IEnumerable<object> of a custom control

Question

With some help, I recently made binding collections in my custom control work. However, to my surprise I was told that to make the custom control property more flexible (that is possible to be bound with other parametrized collections), I needed to make the custom control's property to be of type IEnumerable<object> because of covariance and contravariance. However, this seems not to work for me

This is the control's view

<UserControl x:Class="BadaniaOperacyjne.Controls.Matrix"
            mc:Ignorable="d" Name="CustomMatrix"
            d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
            <!-- ... -->
        <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 is here

#region ItemsList Property
public static readonly DependencyProperty ItemsListProperty =
    DependencyProperty.Register("ItemsList", typeof(IEnumerable<object>), typeof(Matrix), new PropertyMetadata(new PropertyChangedCallback(ItemsListChanged)));
public IEnumerable<object> ItemsList
{
    get { return GetValue(ItemsListProperty) as IEnumerable<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)
{
    ((Matrix)d).ItemsListChanged(e.NewValue);
}
#endregion

and the Window that consumes the control is the following

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

with the code-behind like

public class ViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

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

        Items.CollectionChanged += Items_CollectionChanged;
    }

    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; private 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; }

// the window's constructor
private ProblemManager()
{
    VM = new ViewModel();

    DataContext = VM;
    InitializeComponent();

    VM.Title = "title";
}

private int i = 0;
private void btnAddRow_Click(object sender, RoutedEventArgs e)
{
    //VM.Items.Add(++i);
    VM.Items[2] = 112;
    //customMatrix.ItemsList = new ObservableCollection<object> { 1, 2, 3 };
    //customMatrix.ItemsList.Add(66);
    //////
    VM.Title = (++i).ToString();
}

When I change the DependencyProperty of the ItemsList control to ObservableCollection<int> or at least ObservableCollection<object>, it works fine. Is it really possible? If so, then is the mistake I made?

Answer 1

Co-variance is allowed for IEnumerable but i just checked its only allowed for reference types and not for value types (e.g. int).

Your version will work if you bind with ObservableCollection<string> since string is reference type.

So what you can do is use IEnumerable (non-generic version) as return type of your DP like this so that it will work for value types as well:

public static readonly DependencyProperty ItemsListProperty =
    DependencyProperty.Register("ItemsList", typeof(IEnumerable), typeof(Matrix),
        new PropertyMetadata(new PropertyChangedCallback(ItemsListChanged)));

public IEnumerable ItemsList
{
    get { return (IEnumerable)GetValue(ItemsListProperty); }
    set { SetValue(ItemsListProperty, value); }
}