How to set the RelativeSource in a DataTemplate that is nested in a HierarchicalDataTemplate?

Answer 1

Í don't have access to an IDE to try it out (holdays...), but I'd try using the FindAncestor mode, like this:

RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type PrivateMessengerUI:GroupContainer}}

This should go up the logical tree to find the group container.

Answer 2

The correct answer is:

<Binding RelativeSource="{RelativeSource FindAncestor,
                                         AncestorType={x:Type TreeViewItem},
                                         AncestorLevel=2}"
                           Path="DataContext.Users"/>

The ancestorlevel is crucial and deceptive: when you omit it, a level of 1 is assumed and that actually points to the container of the DataTemplate (which is a childless TreeViewItem too!), not the container of the HierarchicalDataTemplate.