I'm struggling to understand what is going on with the ContextMenu. I know it is rendered as a separate window, with a separate visual tree, so we can't use relative binding to bind a command exposed as a property of the user control. e.g. the following does not work:
<MenuItem Command="{Binding RelativeSource={RelativeSource AncestorType=UserControl}, Path=TestCommand}" Header="Test" />
But, if you set the data context of the user control to a view model that exposes a command as a property, the following will work:
<MenuItem Command="{Binding TestCommand}" Header="Test" />
What I don't understand is, how is the ContextMenu inheriting the value of the DataContext if it is not part of the visual tree. I would expect both of these examples to behave the same (i.e. both work or both fail).
The second binding works because of so called "inheritance context". You can read about it here: http://blogs.msdn.com/b/nickkramer/archive/2006/08/18/705116.aspx. Basically this is a kind of special case where some properties inherit data context of the owner object. Thus, for example, the inheritance context always works on properties of type Freezable (another interesting article about Freezables: http://drwpf.com/blog/category/freezables/).
Actually the article says that the inheritance context doesn't work on ContextMenu, but in version 4 they added it, so it actually works now as you've shown it in your example.
来源：https://stackoverflow.com/questions/4921686/binding-wpf-contextmenu-menuitem-to-usercontrol-property-vs-viewmodel-property