WM_NOTIFY 
    idCtrl = (int) wParam; 
    pnmh = (LPNMHDR) lParam; 

Parameters
idCtrl 
Identifier of the common control sending the message.
 This identifier is not guaranteed to be unique. An application should use the hwndFrom or idFrom member 
of the NMHDR structure (passed as the lParam parameter) to identify the control. 
pnmh 
Pointer to an NMHDR structure that contains the notification code and additional information. 
For some notification messages, this parameter points to a larger structure that has the
 NMHDR structure as its first member. 

case WM_NOTIFY:
	if (((LPNMHDR)(lParam))->code == HDN_DIVIDERDBLCLICK) 
		MessageBox( 0 ,  "HDN_DIVIDERDBLCLICK!" ,  0 , MB_OK);
	break;