Why is CompositeCollection not Freezable?

I just tried this tonight:

public class State{    public string Code { get; set; }    public string Name { get; set; }}public class MyWindowViewModel{    ObservableCollection<State> _states = new ObservableCollection<State>    {        new State { Code = "FL", Name = "Florida" },        new State { Code = "CA", Name = "California" },    };    public ObservableCollection<State> States    {        get        {            return _states;        }    }}

<Window x:Class="WpfApplication1.MyWindow"        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"        xmlns:app="clr-namespace:WpfApplication1"        Title="Window1"        Height="300"        Width="300">  <Window.Resources>    <app:ServiceLocator x:Key="Locator" />  </Window.Resources>  <StackPanel>    <ComboBox x:Name="TestCombo" SelectedIndex="0" DisplayMemberPath="Name" SelectedValuePath="Code">      <ComboBox.ItemsSource>        <CompositeCollection>          <app:State Code="" Name="Select a state..." />          <app:State Code="TX" Name="Texas" />          <CollectionContainer Collection="{Binding Source={StaticResource Locator}, Path=MyWindowViewModel.States}" />        </CompositeCollection>      </ComboBox.ItemsSource>    </ComboBox>  </StackPanel></Window>

The key here is to create an instance of your service locator as a static resource then go through it to get to your viewmodel. The service locator can wire up to instances of the ViewModel using Unity or whatever DI you want.

Edit:

Actually in my silverlight app I create the service locator as a static resoure in the App.xaml and then bind my other UserControls/Windows/Pages DataContext to a ViewModel property of the service locator. It should still work the same way for the combo boxes though even if the service locator is instantiated in the App.xaml's resources. I wish there was a silverlight version of CompositeCollection that I could use. This would work great for the app I'm working on. :(