.net - Why Does This Code Not Bind SelectedItem To A Property

I'untangle a particularly malodorous collection of spaghetti code. Anyway, I have the following XAML code:

<UserControl.Resources>
<CollectionViewSource x:Key="XMLObjectGroups" Source="{Binding Path=XMLObjectList}">
<CollectionViewSource.GroupDescriptions>
<PropertyGroupDescription PropertyName="IsDateType"/>

[code].....

On the second line XMLObjectList is a readonly property of a ViewModel class, returning a Collections.ObjectModel.ReadOnlyObservableCollection(Of MyOrder).On the final line SelectedOrder is a property of the same ViewModel class, which allows setting and getting of a MyOrder object.

I have confirmed that XMLObjectList is being correctly referenced by renaming the property, mistyping the string, breakpoints, etc. XMLObjectList definitely references the XMLObjectList property of this particular ViewModel class.The SelectedOrder property, however, is never accessed at runtime, meaning that it isn't properly hooked up to the SelectedItem of this ListView.