doubt about binding a ListBox

// -----------------------------------------------------------------------
// <copyright file="Extensions.cs" company="">
// Source: http://darkorbit.net/articles/flatten-a-c-hierarchy.html
// </copyright>
// -----------------------------------------------------------------------
using System.Collections.Generic;
using System;

namespace HierarchicalData.Extensions
{
    public static class IEnumerableExtensions
    {
        /// <summary>
        /// Flattens an object hierarchy.
        /// </summary>
        /// <param name="rootLevel">The root level in the hierarchy.</param>
        /// <param name="nextLevel">A function that returns the next level below a given item.</param>
        /// <returns><![CDATA[An IEnumerable<T> containing every item from every level in the hierarchy.]]></returns>
        public static IEnumerable<T> Flatten<T>(this IEnumerable<T> rootLevel, Func<T, IEnumerable<T>> nextLevel)
        {
            List<T> accumulation = new List<T>();
            accumulation.AddRange(rootLevel);
            flattenLevel<T>(accumulation, rootLevel, nextLevel);
            return accumulation;
        }

        /// <summary>
        /// Recursive helper method that traverses a hierarchy, accumulating items along the way.
        /// </summary>
        /// <param name="accumulation">A collection in which to accumulate items.</param>
        /// <param name="currentLevel">The current level we are traversing.</param>
        /// <param name="nextLevel">A function that returns the next level below a given item.</param>
        private static void flattenLevel<T>(List<T> accumulation, IEnumerable<T> currentLevel, Func<T, IEnumerable<T>> nextLevel)
        {
            foreach (T item in currentLevel)
            {
                accumulation.AddRange(currentLevel);
                flattenLevel<T>(accumulation, nextLevel(item), nextLevel);
            }
        }
    }
}

<Window x:Class="HierarchicalData.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" 
        Height="600" 
        Width="640">
    
    <StackPanel x:Name="LayoutRoot" Background="White">
        <StackPanel.Resources>
            <HierarchicalDataTemplate x:Key="ChildTemplate">
                <TextBlock FontStyle="Italic" Text="{Binding Path=Field1}" />
            </HierarchicalDataTemplate>
            <HierarchicalDataTemplate x:Key="NameTemplate" 
            ItemsSource="{Binding Path=subitems}" 
            ItemTemplate="{StaticResource ChildTemplate}">
                <TextBlock Text="{Binding Path=Field1}" FontWeight="Bold" />
            </HierarchicalDataTemplate>
        </StackPanel.Resources>
        <StackPanel Orientation="Vertical">
            <TreeView Width="400"  
                          Height="300" 
                          ItemsSource="{Binding}" 
                          ItemTemplate="{StaticResource NameTemplate}" 
                          x:Name="myTreeView"/>
            <ListBox Width="400"  
                          Height="300" 
                          ItemsSource="{Binding}" 
                          ItemTemplate="{StaticResource NameTemplate}" 
                          x:Name="myListBox"/>
        </StackPanel>
    </StackPanel>
</Window>

using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Windows;
using HierarchicalData.Extensions;

namespace HierarchicalData
{
    public partial class MainWindow : Window
    {
        static public ObservableCollection<item> Topics = new ObservableCollection<item>();
        static public List<item> FlatTopics = new List<item>();

        public MainWindow()
        {
            InitializeComponent();

            // Initialise some dummy data
            Topics.Add(new item("Orange", "Fruit"));
            Topics.Add(new item("Banana", "Fruit"));
            item DataGridTopic = new item("Gift Set", "Toiletries");
            DataGridTopic.subitems.Add(new item("Shampoo", "Toiletries"));
            DataGridTopic.subitems.Add(new item("Bath Bomb", "Toiletries"));
            DataGridTopic.subitems.Add(new item("Scented Candle", "Household"));
            Topics.Add(DataGridTopic);

            // Add the data to the controls
            myTreeView.DataContext = Topics;
            myListBox.DataContext = Topics.Flatten(n => n.subitems).Distinct().ToList();
            
        }
    }

    public class item
    {
        public string Field1 { get; set; }
        public string Field2 { get; set; }
        private ObservableCollection<item> subitemsValue = new ObservableCollection<item>();
        public ObservableCollection<item> subitems
        {
            get
            {
                return subitemsValue;
            }
            set
            {
                subitemsValue = value;
            }
        }
        public item() { }
        public item(string field1, string field2)
        {
            Field1 = field1;
            Field2 = field2;
        }
    }
}

• MSDN
• Dark Orbit