CommandParameters in ContextMenu in WPF

Question

I have a scenario where I have a WPF TreeView control that has an HierarchicalDataTemplate for its items. Now inside the HierarchicalDataTemplate, I have a Label and the Label has a ContextMenu with a menuitem for Delete. The Delete menuitem is binded with a Command called DeleteCommand which is a part of the class that has been set as the DataType of the HierarchicalDataTemplate.

Now, I want to pass the TreeView control in the CommandParameters of the ContextMenu's Delete menuitem's DeleteCommand so that I can handle the selection of the TreeViewItems on the deletion of the currently selected item.

But if I bind the CommandParameters as the {Binding ElementName=TreeViewName} or whatever for that matter, it is always null unless the binded element is a property in the DataContext.

Can anyone help me with a solution because I think, I have tried all the possible things such as RelativeSource and AncestorType etc but its always null. To me, it looks like either a limitation or a bug in the framework.

Answer 1

Take a look at WPF CommandParameter Binding Problem. Maybe it can provide some pointers as to what's going on.

Answer 2

The problem is that the ContextMenu is at the root of its own visual tree, so any RelativeSource.FindAncestor bindings won't go past the ContextMenu.

One solution is to use the PlacementTarget property to set up a two-stage binding from your Label:

<Label Tag="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type TreeView}}}">
    <Label.ContextMenu>
        <ContextMenu>
            <MenuItem Header="Delete" Command="{x:Static local:Commands.DeleteCommand}"
                      CommandParameter="{Binding PlacementTarget.Tag,
                                                 RelativeSource={RelativeSource FindAncestor,
                                                                                AncestorType={x:Type ContextMenu}}}"/>
        </ContextMenu>
    </Label.ContextMenu>
</Label>

This is quite hacky, however. You're better off setting the CommandTarget property of your MenuItem to the ContextMenu's PlacementTarget and having the command handler on your TreeView. This means you won't have to pass the TreeView around.

Answer 3

Thank you, very useful!!!