WPF: Binding a ContextMenu to an MVVM Command

Question

Let's say I have a Window with a property returning a Command (in fact, it's a UserControl with a Command in a ViewModel class, but let's keep things as simple as possible to reproduce the problem).

The following works:

<Window x:Class="Window1" ... x:Name="myWindow">
    <Menu>
        <MenuItem Command="{Binding MyCommand, ElementName=myWindow}" Header="Test" />
    </Menu>
</Window>

But the following does not work.

<Window x:Class="Window1" ... x:Name="myWindow">
    <Grid>
        <Grid.ContextMenu>
            <ContextMenu>
                <MenuItem Command="{Binding MyCommand, ElementName=myWindow}" Header="Test" />
            </ContextMenu>            
        </Grid.ContextMenu>
    </Grid>
</Window>

The error message I get is

System.Windows.Data Error: 4 : Cannot find source for binding with reference 'ElementName=myWindow'. BindingExpression:Path=MyCommand; DataItem=null; target element is 'MenuItem' (Name=''); target property is 'Command' (type 'ICommand')

Why? And how do I fix this? Using the DataContext is not an option, since this problem occurs way down the visual tree where the DataContext already contains the actual data being displayed. I already tried using {RelativeSource FindAncestor, ...} instead, but that yields a similar error message.

---

EDIT (Solution): Based on HCLs answer, this is what I ended up using:

<Window x:Class="Window1" ... x:Name="myWindow">
    ...
    <Grid Tag="{Binding ElementName=myWindow}">
        <Grid.ContextMenu>
            <ContextMenu>
                <MenuItem Command="{Binding Parent.PlacementTarget.Tag.MyCommand, 
                                            RelativeSource={RelativeSource Self}}"
                          Header="Test" />
            </ContextMenu>
        </Grid.ContextMenu>
    </Grid>
</Window>

Answer 1

See this article from Justin Taylor for a workaround.