How to refresh whole ribbon (not just control)?

I have an VSTO Office/Outlook add-in and ribbon that I want to reload at some point. Unfortunatelly ribbon.invalidate is not something that suits my case, because it just re-fires getLabel etc. methods, while I need to rebuild whole ribbon/group.

I need to force executing method GetCustomUI().

Do you know how can I do it?

There is no way to force Office applications invoke the GetCustomUI callback. If you need to keep your custom ribbon UI hidden at startup by default you can use the getVisible callback for the ribbon controls, including tabs, and when required you may call the IRibbonUI.Invalidate to get your controls callback invoked, so you could return an appropriate value and get your controls appeared on the ribbon.

For example, if an add-in writer implements the getVisible callback procedure for a button, the function is called once, the state loads, and then if the state needs to be updated, the cached state is used instead of recalling the procedure. This process remains in place until the add-in signals that the cached values are invalid by using the Invalidate method, at which time, the callback procedure is again called and the return response is cached. The add-in can then force an immediate update of the UI by calling the Refresh method.

No way to do that - by design, Ribbon is static, even though some controls can be hidden initially and be shown later when their getVisible callback is invoked.

Note that control contents can be populated dynamically (sounds like that is what you actually want), that applies to the dropDown control. In that case, you need to specify getItemCount/getItemLabel/getItemImage/getSelectedItemIndex callbacks.

See the dropDown control description on MSDN: https://learn.microsoft.com/en-us/openspecs/office_standards/ms-customui/700e4451-8706-40c5-8d7b-896e4ae21b69