The custom user interface XML markup that you return in the IRibbonExtensibility.GetCustomUI call typically contains callbacks that run when the corresponding Microsoft Office Fluent user interface (UI) that you are customizing (for example, explorer, inspector, or menu) is about to be displayed. You must add each callback in your XML markup to the add-in class that implements IRibbonExtensibility. In addition, you must declare the callbacks as public procedures. If for some reason you omit a callback or use an incorrect callback signature, your UI customization will fail silently unless you turn on error reporting when you debug your solution.
Note that if any portion of the XML markup specified by an add-in and returned by GetCustomUI does not adhere to the Office Fluent UI XML schema, none of the UI customization specified by that add-in is applied. For example, if you have a problem with one control that you have added to the ribbon, your customizations for that ribbon are not displayed.
To view any errors that your XML markup generates when it is loaded, follow these steps:
Click the File tab, and then click Options.
Under Developers, select Show add-in user interface errors.
Click OK to save your changes.