Question

I am attempting to expand a Child Node when its parent is expanded.

Otherwise stated: (Child.IsExpanded == Parent.IsExpanded)

This appears right, but does not seem to work:

<TreeView ItemsSource="{Binding}">
    <TreeView.ItemContainerStyle>
        <Style TargetType="TreeViewItem">
            <Style.Triggers>
                <DataTrigger Value="True" 
                             Binding="{Binding Path=IsExpanded, 
                             RelativeSource={RelativeSource 
                             Mode=FindAncestor, 
                             AncestorType={x:Type TreeViewItem}, 
                             AncestorLevel=2}}">
                    <Setter Property="IsExpanded" Value="True" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TreeView.ItemContainerStyle>
</TreeView>

Neither does this:

<TreeView ItemsSource="{Binding}">
    <TreeView.ItemContainerStyle>
        <Style TargetType="TreeViewItem">
            <Setter Property="IsExpanded"
                    Value="{Binding Path=IsExpanded, 
                             RelativeSource={RelativeSource 
                             Mode=FindAncestor, 
                             AncestorType={x:Type TreeViewItem}, 
                             AncestorLevel=2}}" />
        </Style>
    </TreeView.ItemContainerStyle>
</TreeView>

What’s missing here?

Thanks in advance.

Answer 1

Both work for me. If you tested it with static TreeViewItems make sure to apply the style via resources, the ItemContainerStyle is only relevant for dynamically created containers. Also note that user-interaction may set a local value, overriding those styles.