An application sends an EM_GETLINECOUNT message to retrieve the number of lines in a multiline edit control.
EM_GETLINECOUNT 
wParam = 0; // not used; must be zero 
lParam = 0; // not used; must be zero 
 This message has no parameters.
The return value is an integer specifying the number of lines in the multiline edit control. If no text is in the edit control, the return value is 1.
  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.
Edit Controls Overview, Edit Control Messages, EM_GETLINE, EM_LINELENGTH