CCmdUI

I have a dirt simple Dialog based application that has uses a menu. I want to check or uncheck menu items based on the user's selection. The problen I am experiencing is that in the UPDATE_COMMAND_UI handler, the CCmdUI* does not contain complete information. The only information available is the menu ID that was selected. Why?

I have a workable solution that gets the CMenu associated with the CWnd, but I am trying to understand why MFC behaves this way.

Thanks for your help!
Jim