WPF ListBox with SelectedIndices property

MultiIndexedListBox.cs

    using System.Collections.Specialized;
    using System.Windows;
    using System.Windows.Controls;
    using System.Windows.Media;

    public class MultiIndexedListBox : ListBox
    {
        public IList<int> SelectedIndices { get; private set; }
        
        
        protected override void OnSelectionChanged(SelectionChangedEventArgs e)
        {
            UpdateSelectedIndices();
            
            base.OnSelectionChanged(e);
        }


        protected override void OnItemsChanged(NotifyCollectionChangedEventArgs e)
        {
            UpdateSelectedIndices();
            
            base.OnItemsChanged(e);
        }


        private void UpdateSelectedIndices()
        {
            SelectedIndices = FindVisualChildren<ListBoxItem>(this).Select((item, index) => new {index, item.IsSelected})
                                                       .Where(item => item.IsSelected)
                                                       .Select(a => a.index).ToList();
        }


        private static IEnumerable<T> FindVisualChildren<T>(DependencyObject depObj) where T : DependencyObject
        {
            if (depObj != null)
            {
                for (int i = 0; i < VisualTreeHelper.GetChildrenCount(depObj); i++)
                {
                    DependencyObject child = VisualTreeHelper.GetChild(depObj, i);
                    if (child != null && child is T)
                    {
                        yield return (T)child;
                    }

                    foreach (T childOfChild in FindVisualChildren<T>(child))
                    {
                        yield return childOfChild;
                    }
                }
            }
        }
    }