Expandir dinámicamente la vista de árbol

I am using a class as follows:

Class DirectoryViewItem
    Property Namee As String
    Property Iconn As BitmapImage
    Property Path As String
    Property SubNodes As New List(Of DirectoryViewItem)
End Class

and the xaml I used is:

<TreeView Name="DirectoryTreeView"
          TreeViewItem.Expanded="DirectoryTreeView_Expanded"
          Grid.Row="0">
    <TreeView.ItemTemplate >
        <HierarchicalDataTemplate ItemsSource="{Binding SubNodes}">
            <StackPanel Orientation="Horizontal">
                <Image Source="{Binding Iconn}"
                       Width="32" Height="32"
                       VerticalAlignment="Center" HorizontalAlignment="Left" />
                <TextBlock Text="{Binding Namee}"
                           VerticalAlignment="Center" HorizontalAlignment="Left" />
            </StackPanel>
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
</TreeView>

The code is working fine, now I want to expand the 3 or some x node through code, and I found the solution to use something like this:

CType(DirectoryTreeView.Items(3), TreeViewItem).ExpandSubtree()

Encontré que el CType here is not TreeViewItem but it is my DirectoryViewItem type, ... how can this be done?