CompositeCollection + CollectionContainer: Bind CollectionContainer.Collection to property of ViewModel that is used as DataTemplates DataType

Question

I do not get the correct Binding syntax to access the Cats and Dogs properties of MyViewModel within a DateTemplate that

Answer 1

Try giving your ListBox a name and refer to its DataContext in the bindings:

<ListBox x:Name="myList" ItemsSource="{DynamicResource MyColl}">
   <ListBox.Resources>
      <CompositeCollection x:Key="MyColl">
         <CollectionContainer Collection="{Binding DataContext.Dogs, Source={x:Reference myList}}"/>
         <CollectionContainer Collection="{Binding DataContext.Cats, Source={x:Reference myList}}"/>
      </CompositeCollection>
   </ListBox.Resources>
</ListBox>

Answer 2

Due to the issue with data binding on CollectionContainer as described http://social.msdn.microsoft.com/Forums/vstudio/en-US/b15cbd9d-95aa-47c6-8068-7ae9f7dca88a/collectioncontainer-does-not-support-relativesource?forum=wpf I now use the following approach:

<ListBox>
  <ListBox.Resources>
    <CollectionViewSource x:Key="DogCollection" Source="{Binding Dogs}"/>
    <CollectionViewSource x:Key="CatCollection" Source="{Binding Cats}"/>
  </ListBox.Resources>
  <ListBox.ItemsSource>
    <CompositeCollection>
      <CollectionContainer Collection="{Binding Source={StaticResource DogCollection}}"/>
      <CollectionContainer Collection="{Binding Source={StaticResource CatCollection}}"/>
    </CompositeCollection>
  </ListBox.ItemsSource>
  <!-- ... -->
</ListBox>

Edit: The CompositeCollection class does not derive from FrameworkElement and thus does not have a DataContext property to support data binding. It will only work if you use Binding providing a Source. Have a look here https://stackoverflow.com/a/6446923/1254795 for more information.