Voici un peu plus de détails sur mon code et mes test.
J'ai tout mis dans le même fichier pour simplifier.

<Window x:Class="Binding.ListBox.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:ListBox="clr-namespace:Binding.ListBox">
		
    <StackPanel>
        <ListBox ItemsSource="{Binding Path=ListUserFull, Mode=OneWay}" DisplayMemberPath="Lastname" ListBox:ListBoxSelectedItems.SelectedItems="{Binding RelativeSource={RelativeSource AncestorType={x:Type Window}}, Path=TheGroup.Users, Mode=TwoWay}" SelectionMode="Extended"/>
        <Button Content="Test" Click="Button_Click" />
    </StackPanel>
</Window>

namespace Binding.ListBox
{
    //GUI
    public partial class MainWindow : Window
    {
        public Group TheGroup { get; set; }
        public struct StructContext
        {
            public List<User> ListUserFull { get; set; }
        }

        public MainWindow()
        {
            InitializeComponent();
            var context = new StructContext() { ListUserFull = new Business().UserGetAll() };
            
            TheGroup = new Business().GetOneGroup();
            TheGroup.Users = TheGroup.Users.Select(oneUser => context.ListUserFull.FirstOrDefault(u => u.Id == oneUser.Id)).ToList();
            DataContext = context;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            if(TheGroup!=null && TheGroup.Users!=null)
                Console.WriteLine("Il y a "+TheGroup.Users.Count+" utilisateurs selectionnés");
            else
                Console.WriteLine("Il n'y a aucun utilisateur selectionné");
        }
    }

    //BLL
    public class Business
    {
        public List<User> UserGetAll()
        {
            return new List<User>{
                           new User(1, "DUPOND", "Martin"),
                           new User(2, "DURAND", "François"),
                           new User(3, "MACHIN", "Marc"),
                           new User(4, "GILLES", "Franck")
                       };
        }

        public Group GetOneGroup()
        {
            return new Group{
                           Users = new List<User>{
                                     new User(1, "DUPOND", "Martin"),
                                     new User(2, "DURAND", "François")
                                 }
                       };
        }
    }

    //ENTITY
    public class Group
    {
        public List<User> Users { get; set; }
    }

    public class User
    {
        public User(long id, string lastname, string firstname)
        {
            Id = id;
            Lastname = lastname;
            Firstname = firstname;
        }

        public long Id { get; set; }
        public string Lastname { get; set; }
        public string Firstname { get; set; }
    }

    //BEHAVIOR
    public static class ListBoxSelectedItems
    {
        public static void SetSelectedItems(System.Windows.Controls.ListBox listbox, IList list) { listbox.SetValue(SelectedItemsProperty, list); }
        public static IList GetSelectedItems(System.Windows.Controls.ListBox listbox) { return listbox.GetValue(SelectedItemsProperty) as IList; }
        public static DependencyProperty SelectedItemsProperty = DependencyProperty.RegisterAttached("SelectedItems", typeof(IList), typeof(ListBoxSelectedItems), new PropertyMetadata(null, new PropertyChangedCallback(SelectedItemsProperty_Changed)));

        private static void SelectedItemsProperty_Changed(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (!(d is System.Windows.Controls.ListBox) || !(e.NewValue is IList)) return;

            ((System.Windows.Controls.ListBox)d).SelectedItems.Clear();
            foreach (var oneValue in ((IList)e.NewValue))
                ((System.Windows.Controls.ListBox)d).SelectedItems.Add(oneValue);

            ((System.Windows.Controls.ListBox)d).SelectionChanged += ListBox_SelectionChanged;
        }

        private static void ListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            if (!(sender is System.Windows.Controls.ListBox)) return;
        }
    }
}

A ce niveau là:
1) Lorsque je charge ma page j'ai bien les deux premiers items de selectionnés
2) Lorsque je clique sur le bouton, il y a bien deux items.
3) Si je sélectionne 3 items et que je clique sur le bouton il ne connait que les deux premiers.


Je modifie donc la méthode SelectionChanged

private static void ListBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            if (!(sender is System.Windows.Controls.ListBox)) return;
            var theListbox = sender as System.Windows.Controls.ListBox;
            SetSelectedItems(theListbox, theListbox.SelectedItems);
        }

A ce niveau là:
1) IDEM
2) IDEM
3) Si je sélectionne 3 items et que je clique sur le bouton, j'obtiens la valeur null.


Je ne comprend pas pourquoi. Quelqu’un a t'il une idée?

Essaie ca :

ListBox theListbox = sender as ListBox;
MessageBox.Show(theListbox.SelectedItems.Count.ToString());