WPF TreeView context menu is disabled if no items exist

Question

I am trying to show a ContextMenu in a TreeView. Some entries must be available whether an item was selected or not, but all commands are disabled until I populate the TreeView with at least one item:

<TreeView Name="myTreeView" Width="200px">
    <TreeView.ContextMenu>
        <ContextMenu>
            <MenuItem Command="New" IsEnabled="True" />
        </ContextMenu>
    </TreeView.ContextMenu>
    <TreeView.ItemTemplate>
        <HierarchicalDataTemplate>
            <TextBlock Text="{Binding Path=Title}" />
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
</TreeView>

However, the menu item is still disabled:

The very same command is enabled in the File menu in the menu bar and there is no CanExecute attribute.

How can I enable the context menu entry even if no item exists?

Joe · Accepted Answer · 2017-01-15T11:02:02.690

The issue is that the DataContext of the ContextMenu (i.e. where it's looking to bind the New command) is the tree-view node, not the tree view itself. Great if you've got commands related to the node - editing, moving, changing settings.

Not so good for the few that are pan-node like adding and deleting.

As it's looking in the node's DataContext (and no nodes exit) it can't find the command (and it doesn't make sense for it to be there anyway, as the object that manages the TreeView should be creating new items, not the items themselves).

The solution is to bind to a New command that's not in the DataContext of the item, but the TreeView. There's the frustration of dealing with data-binding with ContextMenu... as it's not in the same visual tree as the rest of the window it's often frustrating to deal with.

A solution is to reference the PlacementTarget of the context menu like this:

    <TreeView Name="myTreeView" Width="200px">
    <TreeView.ContextMenu>
        <ContextMenu>
            <MenuItem Header="Edit (This command exists in the Node's ViewModel)" Command="{Binding Edit}"/>
            <MenuItem Header="New (This command exists in the Window's ViewModel)" Command="{Binding PlacementTarget.DataContext.New, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ContextMenu}}}"/>
        </ContextMenu>
    </TreeView.ContextMenu>
    <TreeView.ItemTemplate>
        <HierarchicalDataTemplate>
            <TextBlock Text="{Binding Path=Title}" />
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
</TreeView>

Further questions

An example of adding a command as a static resource (Change Window to UserControl if you're in a view that's a UserControl):

<Window.Resources>
    <local:MyCommand x:Key="MyCommand"/>
</Window.Resources>

Then referenced with:

<MenuItem Header="MyCommand" Command="{StaticResource MyCommand}"/>

Binding to your commands in the ViewModel (i.e. DataContext) is done like in the first example. In the same way you bind the Title, you can bind to any property, such as an ICommand.

So for a view:

<MenuItem Header="New" Command="{Binding New}"/>

The View Model has a property NewCommand named New:

public NewCommand New { get; private set; }

People often use this because they have a generic ICommand that takes a delegate so they can configure all the actions that relate to that ViewModel. For example:

public class MyCommand : ICommand
{
    public event EventHandler CanExecuteChanged;
    public Action<object> Action { get; set; }

    public MyCommand(Action<object> action)
    {
        Action = Action;
    }

    public bool CanExecute(object parameter)
    {
        return true;
    }

    public void Execute(object parameter)
    {
        Action(parameter);
    }
}

Then in the ViewModel, instead of having loads of ICommand classes all implemented, we can just re-use this and get it to do different things:

public MyCommand New { get; private set; }
public MyCommand Delete { get; private set; }
public MyCommand ClearAll { get; private set; }

public MyViewModelConstructor()
{
    New = new MyCommand((parameter) =>
    {
        //Add new object
    });
    Delete = new MyCommand((parameter) =>
    {
        //Delete object
    });
    ClearAll = new MyCommand((parameter) =>
    {
        //Clear all objects
    });
}

Thank you for your answer! How would I use this with a custom command, e.g. `local:MyCommands.NewItem`? — just.me, Jan 15 '17 at 10:12
Never used commands not on a view model, but you could probably define it in resources and reference it as a static resource — Joe, Jan 15 '17 at 10:40
So far, I only defined commands by adding them to ``. How do you add commands to the ViewModel? — just.me, Jan 15 '17 at 10:46
I think I will try to follow [these instructions](http://stackoverflow.com/questions/12422945/how-to-bind-wpf-button-to-a-command-in-viewmodelbase). Will accept your answer as soon as I get it to work. — just.me, Jan 15 '17 at 10:50

WPF TreeView context menu is disabled if no items exist

1 Answers1