An application sends an EM_LINELENGTH message to retrieve the length of a line, in characters, in an edit control.
EM_LINELENGTH wParam = (WPARAM) ich; // character index lParam = 0; // not used; must be zero
The return value is the length, in characters, of the line specified by the ich parameter when an EM_LINELENGTH message is sent to a multiline edit control. The return value is the length, in characters, of the text in the edit control when an EM_LINELENGTH message is sent to a single-line edit control.
Use the EM_LINEINDEX message to retrieve a character index for a given line number within a multiline edit control.