Combobox-Yeni item ekleme

procedure TFormMain.BitBtn1Click(Sender: TObject);
var
Str : String;
begin
Str :=Edit1.Text;
if Str='' then exit else
if ComboBox1.Items.IndexOf(Str) = -1 then
begin
ComboBox1.Items.Add(Str);
ComboBox1.Items.SaveToFile('Deneme.Txt');
end;
end;