Hello James.

Does that mean this should work?

If This.Valid()
    ThisForm.AddQuery()
EndIf

No it does not. Calling the Control's Valid() in code does not cause the event to fire. It merely runs any code in the method. If you want the event to fire, you have to do something like this:

This.SetFocus()

Also, is this only the case for comboboxes? (The textbox controls seem to work how I expect them)

No. The same is true for text boxes as well. They do not update their controlSources from their values until the Valid() fires.