Hi

Add it at main.prg

Here's the sample code that will center forms when size of the main VFP Window changes.

_Screen.AddObject("oResizer", "Resizer")
DEFINE CLASS Resizer AS Custom
  oScreen = _Screen
  PROCEDURE oScreen.Resize
    IF _screen.WindowState <> 1  && Main VFP Window isn't minimized
      FOR i=1 TO _screen.FormCount
         IF _screen.Forms(i).AutoCenter
            _screen.Forms(i).AutoCenter = .F.        
            _screen.Forms(i).AutoCenter = .T.        
         ENDIF
      ENDFOR 
    ENDIF
  ENDPROC
ENDDEFINE