How to implement commands to use ancestor methods in WPF?

Question

I have this context menu resource:

Answer 1

I'm afraid MatthiasG beat me to it. My solution is similar:

Here the Help command is handled by the tab item's view model. It would be simple to pass a reference to the TestViewModel to each of the TestItemViewModel and have ShowHelp call back into TestViewModel if required.

public class TestViewModel
{
    public TestViewModel()
    {
        Items = new List{ 
                    new TestItemViewModel(), new TestItemViewModel() };
    }

    public ICommand HelpCommand { get; private set; }

    public IList Items { get; private set; }
}

public class TestItemViewModel
{
    public TestItemViewModel()
    {
        // Expression Blend ActionCommand
        HelpCommand = new ActionCommand(ShowHelp);
        Header = "header";
    }

    public ICommand HelpCommand { get; private set; }

    public string Header { get; private set; }

    private void ShowHelp()
    {
        Debug.WriteLine("Help item");
    }
}

The xaml

Alternative view models so that the help command is directed to the root view model

public class TestViewModel
{
    public TestViewModel()
    {
        var command = new ActionCommand(ShowHelp);

        Items = new List
                    {
                        new TestItemViewModel(command), 
                        new TestItemViewModel(command)
                    };
    }

    public IList Items { get; private set; }

    private void ShowHelp()
    {
        Debug.WriteLine("Help root");
    }
}

public class TestItemViewModel
{
    public TestItemViewModel(ICommand helpCommand)
    {
        HelpCommand = helpCommand;
        Header = "header";
    }

    public ICommand HelpCommand { get; private set; }

    public string Header { get; private set; }
}

A very simple implementation of ActionCommand

public class ActionCommand : ICommand
{
    private readonly Action _action;

    public ActionCommand(Action action)
    {
        if (action == null)
        {
            throw new ArgumentNullException("action");
        }

        _action = action;
    }

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

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

    // not used
    public event EventHandler CanExecuteChanged;
}