How to Databind to a SelectedItems property in WPF

Many moons ago, I asked on the WPF forums if anybody had a way of data-binding the SelectedItems property of a ListBox. Standard data binding doesn’t work, because the SelectedItems property is read-only, and understandably so: how would you like it if I injected an arbitrary collection into your ListBox and expected you to keep it up to date as the selection changed?

As no one gave me an answer, I was forced to use my gray-matter and invent a solution of my own. Since a similar question came up again recently on Stack Overflow, I thought I’d share what I came up with.

In my solution, I define an attached property that you attach to a ListBox (or DataGrid, or anything that inherits from MultiSelector) and allows you to specify a collection (via data binding, of course) that you want to be kept in sync with the SelectedItems collection of the target. To work properly, the collection you give should implement INotifyCollectionChanged – using ObservableCollection<T> should do the trick. When you set the property, I instantiate another class of my invention, a TwoListSynchronizer. This listens to CollectionChanged events in both collections, and when either changes, it updates the other.

To help me demonstrate, I’ve knocked up a toy ViewModel: it has on it an AvailableNames property that will be the data source for the ListBox, a SelectedNames property which is the reason we’re going through the whole exercise, and a Summary property which displays the number of selected items to prove that the data binding is working correctly.  I won’t bore you with the code – see the download link below.

The XAML looks like this:

<Window x:Class="SelectedItemsBindingDemo.Window1"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:d="clr-namespace:SelectedItemsBindingDemo"
  xmlns:ff="clr-namespace:FunctionalFun.UI.Behaviours"
  Title="SelectedItems Binding Demo" Height="300" Width="300"
      >
<Window.DataContext>
  <d:ViewModel/>
</Window.DataContext>
  <Grid Margin="5">
    <Grid.RowDefinitions>
      <RowDefinition Height="*"/>
      <RowDefinition Height="Auto"/>
    </Grid.RowDefinitions>
    <TextBlock Grid.Row="1" HorizontalAlignment="Left" VerticalAlignment="Center"
               Text="{Binding Summary}" FontWeight="Bold" Margin ="0,5,5,5"/>
    <Button Content="Select All" Command="{Binding SelectAll}" Grid.Row="1"
            HorizontalAlignment="Right" VerticalAlignment="Center"/>
    <ListBox Grid.Row="0"
             ItemsSource="{Binding AvailableNames}"
             ff:MultiSelectorBehaviours.SynchronizedSelectedItems="{Binding SelectedNames}"
             SelectionMode="Extended" />
  </Grid>
</Window>

All the magic is initiated by line 22 where I set the attached property MultiSelectorBehaviours.SynchronizedSelectedItems using a data binding to my SelectedNames property. With that in place the SelectedNames collection in my ViewModel is updated whenever SelectedItems on the ListBox changes. And to prove it works the other way to, I’ve created a SelectAll command on the ViewModel that puts all available names into the SelectedNames collection. When you press the button you’ll see that the ListBox obediently updates to show all items selected.

The TwoListSynchronizer code does not make especially thrilling reading, so I won’t show it here. If you should peruse it however, there are a couple of things you might wonder about:

• I’ve made it implement IWeakEventListener so that it can subscribe to CollectionChanged events using weak events. This means I don’t have to worry about creating managed memory leaks if the source SelectedItems collection has a longer lifetime than the ListBox.
• The code occasionally refers to something called a IListItemConverter. I’m not make use of this in this application of the TwoListSynchronizer, but it might have uses elsewhere. The scenario I had in mind was where there is some kind of mapping between the items in the two collections that need to be synchronized.

Update (25/07/2013): All the code is now available on Github (under a Public Domain License). Download a zip file directly here.