Originally Posted by
Quantum1024
If by GUI in the same source file you mean the window procedure then no but you'll need to declare it's prototype in a header and include the header in the source file that calls CreateWindowEx so that it knows where to look for the function defination.
As for giving them values. There are messages like EM_SETTEXT/WM_SETTEXT for setting the tex on controls. Some controls like listboxes let you set an lParam value to each item when you send the appropriate message.