Why doesn't TreeView automatically select DataTemplates for Childnodes after defining TreeView.ItemTemplate?

Question

I don't understand what wpf does here (using .NET 3.5 with C#):

in my Application Resources I defined several DataTemplates and HierarchicalDataTemplates for different Types of ViewModels. So far this works well, and the TreeView in my Window shows the Nodes like expected. I have to say that all Childelements in every ViewModel came as ICollectionView. Now I decided to use the specialities of ICollectionView, to group, sort and filter the "Rootnodes" of the TreeView. I made the TreeView look like this:

<TreeView ItemsSource="{Binding Path=Elements.Groups}">
  <TreeView.ItemTemplate>
    <HierarchicalDataTemplate ItemsSource="{Binding Path=Items}" ItemTemplate="{StaticResource SOMEITEMTEMPLATENAME}">
      <TextBlock Text="{Binding Path=Name}"/>
    </HierarchicalDataTemplate>
  </TreeView.ItemTemplate>
</TreeView>

The ViewModel looks like this:

class RootViewModel
  : INotifyPropertyChanged
{

  public ICollectionView Elements
  {
     get
     {
       ICollectionView view = CollectionViewSource.GetDefaultView(_elementsFromModel.Select(x => new FirstChildViewModel(x));
       view.GroupDescriptions.Add(new PropertyGroupDescription("Name"));
       view.Filter = delegate(Object x) { /*do some filter stuff here*/ };

       return view;
     }
  }
}

class FirstchildViewModel
  : INotifyPropertyChanged
{
  /*some other stuff here*/

  public ICollectionView Items
  {
    get;
  }
}

The Problem now is that the TreeView doesn't select autmatically the right Templates for the deeper childelements. So what is wrong with that?

Answer 1

You have defined the TreeView.ItemTemplate explicitly, so WPF will use only that ItemTemplate. You have to define your HierarchicalDataTemplate for the type, which correspond to the Groups element type.

But the straightforward way to define the group is to defint GroupStyle property. you can read more about ICollectionView features on msdn

Answer 2

The best solution, after some research would be to define the TreeView like this:

<TreeView ItemsSource="{Binding Path=Elements}">
  <TreeView.GroupStyle>
    <GroupStyle>
      <GroupStyle.ContainerStyle>
        <Style TargetType="{x:Type GroupItem}">
          <Setter Property="Template">
            <Setter.Value>
              <ControlTemplate TargetType="{x:Type GroupItem}">
                <TreeViewItem>
                  <TreeViewItem.Header>
                    <TextBlock Text="{Binding Path=Name}"/>
                  </TreeViewItem.Header>

                  <ItemsPresenter Margin="-20,0,0,0"/>
                </TreeViewItem>
              </ControlTemplate>
            </Setter.Value>
          </Setter>
        </Style>
      </GroupStyle.ContainerStyle>
    </GroupStyle>
  </TreeView.GroupStyle>
</TreeView>

now the Group nodes are able to expand and collapse an the different childnodes use different DataTemplates. Thanks to @stukselbax for pointing me to the right direction.