Only allow children of certain type in WPF/XAML?

Question

In one of my applications I need to allow the user to customize the user interface in certain ways. To do so, I'm allowing the user to specify the user interface in XAML, which I load dynamically when the application is started. This works fine.

Now I want to provide custom control to the user, which is derived from Border (let's call it MyFrame ).

I need to make sure that MyFrame may not be child of another MyFrame , so the following should be prevented:

<ns:MyFrame>
    <Grid>
        <ns:MyFrame />
    </Grid>
</ns:MyFrame>

How should I implement this?

Answer 1

You won't be able to do this at compile time.

However, at runtime, you can check whenever a new instance of MyFrame is created, if it has a parent MyFrame, using this visual tree walker . If it does, raise an exception.

Answer 2

I would throw an exception with a clear message from the Loaded event handler on MyFrame: step through the ancestors of the new MyFrame instance and try to find another one; if you do find one, throw the exception.

Alternative: use an inheritable attached property that you set on all instances of MyFrame - and check that in the Loaded handler.

I can't imagine any way to enforce your rule in a static manner.