Binding ItemsSource to a "proxy" collection. How to get DataContext?

Question

We've got a Silverlight application with several listboxes and comboboxes that display data sorted incorrectly, which I need to fix. Most of their ItemSource properties are set through XAML. Their DataContext may not be set directly on the control, and instead were set on a parent. So I can't easily slap an "OrderBy" on the ItemSource or DataContext assignment in the code behind, since that assignment may not explicitly exist.

So I had the idea to create a "proxy" collection. The proxy collection would get the original ItemSource and expose a sorted version. I'd then be able to convert this:

<ListBox ItemsSource="{Binding}"/>

into this:

<ListBox>
    <ListBox.ItemsSource>
        <my:ProxyCollection Source="{Binding}" SortBy="Name"/>
    </ListBox.ItemsSource>
</ListBox>

Not too shabby! But, since the ProxyCollection isn't a child of the ListBox, the ListBox's DataContext isn't propagated to it, and the binding doesn't magically work. If I manually set the ProxyCollection collection's DataContext it works great. But if I have to set the DataContext manually anyways I may as well just remove the proxy collection and manually set the Listbox's DataContext, adding an "OrderBy".

So any ideas on how I can automatically get the ListBox's DataContext set on the proxy collection? Or any other genius ideas?

Unknown Unknown · Accepted Answer · 2009-07-29T11:47:51

I've stumbled across a very similar problem. I wanted to cascade parameters from one combobox into a subsequent combobox with the same design goal of no code....

My solution is hardly elegant, but it works of a fashion :)

Basically I have a SharedDataContext control, this has a "Value" DP i bind to the datacontext directly.

I then use an attached property on the shared data context to allow for registration of other elements.

This makes the XAML look like this

<my:SharedDataContext Value="{Binding}" />
<ComboBox>
    <ComboBox.ItemsSource>
       <my:ProxyCollection my:SharedDataContext.Register="1" />    
    </ComboBox.ItemsSource>
</ComboBox>

Inside the SharedDataContext I have a static collection of framework elements.

When the Register attached property is "changed" I catch the PropertyChangedCallback and add the sender (in this case the ProxyCollection instance) to the collection.

When the databinding changes (and hence the Value property of the SharedDataContext) I enumerate through the collection and set the databindings.

This is a bit of a work in progress and I'm not 100% happy with it yet (things like cleaning up the collection are concerning me slightly), but I hope it gives you some ideas.

HTH

Simon

Binding ItemsSource to a "proxy" collection. How to get DataContext?

4 Answers