Collection Properties

Hi, I've been able to use the PropertyGrid for simple properties, but can't get collections to work properly.  I've put together an isolated example of what I'm trying to show on screen:

XAML

<Window x:Class="PropertyGridTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525" xmlns:ms="http://namespaces.mindscape.co.nz/wpf"
        Loaded="Window_Loaded">
    <Grid>
        <ms:PropertyGrid Name="propertyGrid1" />
    </Grid>
</Window>

Code Behind:

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            this.propertyGrid1.SelectedObject = new ClassA();
        }

Class A Definition:

public class ClassA
    {
        public ClassA()
        {
            this.Items = new ClassBCollection();
        }

        public int IntegerProperty
        {
            get;
            set;
        }

        public ClassBCollection Items
        {
            get;
            set;
        }
    }

    public class ClassBCollection : Collection<ClassB>
    {
    }

    public class ClassB
    {
        public ClassB()
        {
            this.Items = new ClassCCollection();
        }
       
        public string Name
        {
            get;
            set;
        }

        public ClassCCollection Items
        {
            get;
            set;
        }
    }

    public class ClassCCollection : Collection<ClassC>
    {
    }

    public class ClassC
    {
        public DateTime CreatedDate
        {
            get;
            set;
        }
    }

What I see on screen:

http://www.forumimagecodes.com/images/coz0391nmbiitorkjn6.png

However, clicking the "+" button doesn't add an item into the list.

Can you tell me how to do that?  I'm able to send the example .sln through.

For anyone else having trouble, switching to ObservableCollections fixed it, e.g.:

public class ClassCCollection : ObservableCollection<ClassC>
{
}