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When working with dynamic data the RadMenu container generator creates RadMenuItems as containers for its children. This is done in the GetContainerForItemOverride
method of the control. As each ItemsControl should specify in this method what type its item containers will be. You might create a custom RadMenu and override this method to return RadMenuGroupItems or RadMenuItems but never both.
That said, I suggest using a custom ItemContainerStyleSelectror
and style the data items that have SubItems in the desired way.
We have a great example demonstrating how to achieve that in our developer focused examples
Hope this helps.
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