Recently in a Silverlight 4 project our team needed to have a listbox that allowed the user to select multiple items.  Setting a listbox to handle multiple item selections should be as simple as setting the SelectionMode property; unfortunately there is the little problem of getting these selected items into the ViewModel. The problem boils down to the fact that the SelectedItems property is not a dependency property. This means you cannot do what you normally do in MVVM and use a binding. (How nice of Microsoft to give us the option to do multiple selections on listboxes, but not give a way to easily get this into the ViewModel.)

Well the first thing I do is look to see if anyone has solved this little dilemma, by looking on Google. What should I find; but a bunch of solutions for WPF.   WPF is so similar to Silverlight you would think the solutions would work, but no they do not. After the disappointment of finding non-functioning solutions, I finally came across the Telerik blog. While this article was specifically about the RadGridView, it was so close to what I needed, that only a few minor tweaks made it usable for the ListBox control. Without further ado, here is the code: 

 C# | 
 copy code |
?
 
public class MultiSelectBehavior : Behavior<ListBox>
    {
        public INotifyCollectionChanged SelectedItems
        {
            get { return (INotifyCollectionChanged)GetValue(SelectedItemsProperty); }
            set { SetValue(SelectedItemsProperty, value); }
        }

        // Using a DependencyProperty as the backing store for SelectedItemsProperty.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty SelectedItemsProperty =
            DependencyProperty.Register("SelectedItems", typeof(INotifyCollectionChanged), typeof(MultiSelectBehavior), new PropertyMetadata(OnSelectedItemsPropertyChanged));


        private static void OnSelectedItemsPropertyChanged(DependencyObject target, DependencyPropertyChangedEventArgs args)
        {
            var collection = args.NewValue as INotifyCollectionChanged;
            if (collection != null)
            {
                collection.CollectionChanged += ((MultiSelectBehavior)target).ContextSelectedItems_CollectionChanged;
            }
        }

        protected override void OnAttached() {
            base.OnAttached();

            AssociatedObject.SelectionChanged += AssociatedObject_SelectionChanged;
        }

        void AssociatedObject_SelectionChanged(object sender, SelectionChangedEventArgs e) {
            //Need to unsubscribe from the events so we don't override the transfer
            UnsubscribeFromEvents();

            //Move items from the list box selection to the selected items list
            Transfer(AssociatedObject.SelectedItems, SelectedItems as IList);

            //subscribe to the events again so we know when changes are made
            SubscribeToEvents();
        }

        void ContextSelectedItems_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e) {
            //Need to unsubscribe from the events so we don't override the transfer
            UnsubscribeFromEvents();

            //Move items from the selected items list to the list box selection
            Transfer(SelectedItems as IList, AssociatedObject.SelectedItems);

            //subscribe to the events again so we know when changes are made
            SubscribeToEvents();
        }
		
        private void SubscribeToEvents() {
            AssociatedObject.SelectionChanged += AssociatedObject_SelectionChanged;

            if (SelectedItems != null)
            {
                SelectedItems.CollectionChanged += ContextSelectedItems_CollectionChanged;
            }
        }

        private void UnsubscribeFromEvents() {
            AssociatedObject.SelectionChanged -= AssociatedObject_SelectionChanged;

            if (SelectedItems != null)
            {
                SelectedItems.CollectionChanged -= ContextSelectedItems_CollectionChanged;
            }
        }

        public static void Transfer(IList source, IList target)
        {
            if (source == null || target == null)
                return;

            target.Clear();

            foreach (var o in source)
            {
                target.Add(o);
            }
        }
    }

Now that you have the behavior you just need to use it. Here is a sample:

 HTML | 
 copy code |
?
 
<ListBox ItemsSource="{Binding ListOfItems}" DisplayMemberPath="ID" SelectionMode="Extended">
    <i:Interaction.Behaviors>
        <local:MultiSelectBehavior SelectedItems="{Binding SelectedListOfItems}" />
    </i:Interaction.Behaviors>
</ListBox>

That’s it! Hope this helps someone.