When developing an Outlook add-in using VSTO 4, if you are implementing the IRibbonExtensibility
interface (for customizing a Ribbon, for example) you might encounter cases where your Ribbon or menu items don’t appear in Outlook’s UI.
In those cases, you should check the following things first:
- Your XML is valid – invalid Ribbon XML will prevent your controls from appearing
- Your class should be marks as ComVisible:
[ComVisible(true)] public class Ribbon : IRibbonExtensibility { //... }