>ThisForm.SetAll("visible", !.t., "textbox" + "combo") OR

This would set the visible for all "textboxcombo" controls in the container. Unless you've defined one, there is no such control

>ThisForm.SetAll("visible", !.t., "textbox" AND "combo")
>

AND performs a logical (boolean) comparison. You are trying to perform a boolean operation on character arguements. This will result in a "Function arguement, type, or count is invalid" error.



From VFP8 help (it might have changed in VFP9)
Container.SetAll(cProperty, Value [, cClass])
Use the SetAll method to set a property for all, or a certain class of, controls in a Container

There is no option to have a list of controls.

>ThisForm.SetAll("visible", !.t., "textbox" , "combo")

gives an error about the number of arguements

>ThisForm.SetAll("visible", !.t., "textbox,combo")

does nothing since there is no "textboax,combo" control
>

You can either have ALL controls made invisible

>ThisForm.SetAll("visible", !.t.)

or use a separate line for each class of controls

ThisForm.SetAll("visible", !.t., "textbox") 
ThisForm.SetAll("visible", !.t., "combo")

I would also recommend using .F. for false rather than !.T. I think it's easier to read and understand, but that's a personal preference unrelated to your real questions here