An application sends a CB_GETEDITSEL message to get the starting and ending character positions of the current selection in the edit control of a combo box.
CB_GETEDITSEL 
wParam = (WPARAM) (LPDWORD) lpdwStart; // receives starting position 
lParam = (LPARAM) (LPDWORD) lpdwEnd;   // receives ending position 
 
The return value is a zero-based 32-bit value with the starting position of the selection in the low-order word and with the ending position of the first character after the last selected character in the high-order word.
  Windows NT: Requires version 3.1 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Requires version 1.0 or later.
  Header: Declared in winuser.h.
Combo Boxes Overview, Combo Box Messages, CB_SETEDITSEL