Breakpoint (and extra code) not being hit in set block of data-bound property, though property value is being updated - wpf

Question

I am using a custom control derived from a listbox, but with added features. One of the key features is the addition of a bindable SelectedItems property on the control, so the view model can keep track of the multiple selections made in the control. The binding does work - when you select items in the control, the view model's property is updated. However, I would like to add INotifyDataErrorInfo validation to the view model, so I implemented the interface and added a call to my validation method in the set block of the data-bound property in the viewmodel. For some reason that set block is never being called, even though I am updating the control in the view, and am verifying that the view model's property value is actually being changed correctly to match the control.
I know that when I use binding with standard WPF controls, such as a TextBox, the set block of the source (view model) property is called when the target (view) property changes. Is there a reason it wouldn't be called here?
The custom control I am using is found here. This is my property on the viewmodel (I have the console output there just to ensure the code isn't being called):
private ObservableCollection<Car> _testListSelections;
public ObservableCollection<Car> testListSelections
{
get
{
return _testListSelections;
}
set
{
Console.WriteLine("Value changed.");
_testListSelections = value;
OnPropertyChanged("testListSelections");
Validate();
}
}
This is my XAML (note that I didn't need to use Mode=TwoWay here as I am using an ObservableCollection, but I did try specifying Mode=TwoWay and the set block still didn't get hit):
<src:MultiComboBox SelectionMode="Multiple"
VerticalAlignment="Center"
ItemsSource="{Binding testList}"
SelectedItems="{Binding testListSelections, ValidatesOnNotifyDataErrors=True}"/>
This is the SelectedItems property on the custom control (the author overrode the base read-only SelectedItems property in order to allow binding):
/// <summary>
/// The SelectedItems dependency property. Access to the values of the items that are
/// selected in the selectedItems box. If SelectionMode is Single, this property returns an array
/// of length one.
/// </summary>
public static new readonly DependencyProperty SelectedItemsProperty =
DependencyProperty.Register("SelectedItems", typeof(IList), typeof(BindableListBox),
new FrameworkPropertyMetadata(
(d, e) =>
{
// When the property changes, update the selected values in the selectedItems box.
(d as BindableListBox).SetSelectedItemsNew(e.NewValue as IList);
}));
/// <summary>
/// Get or set the selected items.
/// </summary>
public new IList SelectedItems
{
get
{
return GetValue(SelectedItemsProperty) as IList;
}
set { SetValue(SelectedItemsProperty, value); }
}

Answer 1

You should perform the validation in the OnCollectionChanged event of the list.
The SelectedItems list should be set only once, and then changes are made to the same list.
You can then check if the operation is Add, Remove or Reset, and perform validation accordingly.