BindingList<T> INotifyPropertyChanged unexpected behavior

Question

Suppose, I have objects:

public interface ITest
{
    string Data { get; set; }
}
public class Test1 : ITest, INotifyPropertyChanged
{
    private string _data;
    public string Data
    {
        get { return _data; }
        set
        {
            if (_data == value) return;
            _data = value;
            OnPropertyChanged("Data");
        }
    }
    protected void OnPropertyChanged(string propertyName)
    {
        var h = PropertyChanged;
        if (null != h) h(this, new PropertyChangedEventArgs(propertyName));
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

and its holder:

    private BindingList<ITest> _listTest1;
    public BindingList<ITest> ListTest1 { get { return _listTest1 ?? (_listTest1 = new BindingList<ITest>() { RaiseListChangedEvents = true }); }
    }

Also, I subscribe to ListChangedEvent

    public MainWindow()
    {
        InitializeComponent();            
        ListTest1.ListChanged += new ListChangedEventHandler(ListTest1_ListChanged);
    }
    void ListTest1_ListChanged(object sender, ListChangedEventArgs e)
    {
        MessageBox.Show("ListChanged1: " + e.ListChangedType);
    }

And 2 test handlers: For adding object

    private void AddITestHandler(object sender, RoutedEventArgs e)
    {
        ListTest1.Add(new Test1 { Data = Guid.NewGuid().ToString() });
    }

and for changing

    private void ChangeITestHandler(object sender, RoutedEventArgs e)
    {
        if (ListTest1.Count == 0) return;
        ListTest1[0].Data = Guid.NewGuid().ToString();
        //if (ListTest1[0] is INotifyPropertyChanged)
        //    MessageBox.Show("really pch");
    }

ItemAdded occurs, but ItemChanged not. Inside seeting proprty "Data" I found that no subscribers for my event PropertyChanged:

    protected void OnPropertyChanged(string propertyName)
    {
        var h = PropertyChanged; // h is null! why??
        if (null != h) h(this, new PropertyChangedEventArgs(propertyName));
    }

    public event PropertyChangedEventHandler PropertyChanged;

Digging deeper i took reflector and discover BindingList:

    protected override void InsertItem(int index, T item)
    {
        this.EndNew(this.addNewPos);
        base.InsertItem(index, item);
        if (this.raiseItemChangedEvents)
        {
            this.HookPropertyChanged(item);
        }
        this.FireListChanged(ListChangedType.ItemAdded, index);
    }
private void HookPropertyChanged(T item)
    {
        INotifyPropertyChanged changed = item as INotifyPropertyChanged;
        if (changed != null) // Its seems like null reference! really??
        {
            if (this.propertyChangedEventHandler == null)
            {
                this.propertyChangedEventHandler = new PropertyChangedEventHandler(this.Child_PropertyChanged);
            }
            changed.PropertyChanged += this.propertyChangedEventHandler;
        }
    }

Where am I wrong? Or this is known bug and i need to find some workaround? Thanks!

Answer 1

BindingList<T> doesn't check if each particular item implements INotifyPropertyChanged. Instead, it checks it once for the Generic Type Parameter. So if your BindingList<T> is declared as follows:

private BindingList<ITest> _listTest1;

Then ITest should be inherited fromINotifyPropertyChanged in order to get BindingList raise ItemChanged events.