Binding to a single element inside a CompositeCollection

Question

I am trying to produce a list of servers for browsing on a network such that it produces a tree view which looks like this:

-Local Server
 - Endpoint 1
 - Endpoint 2
-Remote
 - <Double-click to add a server...>
 - Remote Server 1
   - Endpoint 1
   - Endpoint 2
 - Remote Server 2
   - Endpoint 1
   - Endpoint 2

My ViewModel looks like this:

...
public Server LocalServer;
public ObservableCollection<Server> RemoteServers;
...

So, how does one go about constructing the list in xaml with a binding to a single object and a list of objects? I might be thinking about it completely the wrong way, but what my brain really wants to be able to do is something like this:

<CompositeCollection>
  <SingleElement Content="{Binding LocalServer}"> 
  <!-- ^^ something along the lines of a ContentPresenter -->
  <TreeViewItem Header="Remote">
    <TreeViewItem.ItemsSource>
      <CompositeCollection>
        <TreeViewItem Header="&lt;Click to add...&gt;" />
        <CollectionContainer Collection="{Binding RemoteServers}" />
      </CompositeCollection>
    </TreeViewItem.ItemsSource>
  </TreeViewItem>
</CompositeCollection>

I feel like there must be a fundamental element I'm missing which keeps me from being able to specify what I want here. That single item has children. I did try using a ContentPresenter, but for whatever reason, it was not expandable even though it picked up the HierarchicalDataTemplate to display the title correctly.

---

Update

So for now, I've exposed a property on the view model that wraps the single element in a collection so that a CollectionContainer may bind to it. I would really like to hear folks' ideas on how to do this, though. It seems awfully fundamental.

Answer 1

I posted a question very similar to yours regarding CompositeCollections: Why is CompositeCollection not Freezable?

This is apparently a bug in WPF, believe it or not. Here's a post by an MS employee admitting as much: http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/b15cbd9d-95aa-47c6-8068-7ae9f7dca88a

The CompositeCollection is not freezable, but should be. This makes it difficult to combine nonstatic elements into one collection. It's a common scenario for a lot of things. For example, a "Select One" element at the top of a combobox filled with other databound objects would be nice, but you can't do it declaratively.

Anyway, I'm sorry this is not an answer, but hopefully it helps you see why this isn't working how you thought it should.

Answer 2

Can't you just expose a new collection from your ViewModel that the tree can bind to?

Something like:

public Server LocalServer;
public ObservableCollection<Server> RemoteServers;

public IEnumerable ServerTree { return new[] { LocalServer, RemoteServers } }

After all your ViewModel is a ViewModel. It should be exposing exactly what is needed by the view.