Code:

procedure TfrmMain.Memo1Click(Sender: TObject);

var

Line: Integer;

begin

with (Sender as TMemo) do

begin

   Line      := Perform(EM_LINEFROMCHAR, SelStart, 0);

   SelStart  := Perform(EM_LINEINDEX, Line, 0);

   SelLength := Length(Lines[Line]);

end;

end;