where to put SubClassDlgItem in a CFormView?
When the class wizard sets up the member variable for the subclassed control (the combo box in my case) the DDX statement, added by ClassWizard, already subclassed the control. There is no need to call SubClassDlgItem. The assertion is caused by SubClassDlgItem subclassing the control the second time (after the DDX statement).
This seems to be a common confusion amongst most books and documentation I've come across, including the msdn knowledge base.