ListBoxの選択したアイテムを上下に移動させる

ListBoxChage.pas

procedure TForm1.Button1Click(Sender: TObject);
begin
  //選択したアイテムを上に移動
  ListItemChange(-1);
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  //選択したアイテムを下に移動
  ListItemChange(1);
end;

procedure TForm1.ListItemChange(sw: integer);
var
  i: integer;
  MySelectItemNum: integer;
  FileName:TStringList;
  FileCount: integer;
  DummyStr: string;
begin
  FileCount := ListBox1.Items.Count - 1;
  MySelectItemNum := ListBox1.ItemIndex;
  if (MySelectItemNum+sw >= 0) and (MySelectItemNum+sw <= FileCount) then
  begin
    FileName := TStringList.Create;

    for i := 0 to FileCount do
      begin
        FileName.Add(ListBox1.Items[i]);
      end;

    ListBox1.Items.Clear;

    DummyStr := FileName[MySelectItemNum];
    FileName[MySelectItemNum] := FileName[MySelectItemNum+sw];
    FileName[MySelectItemNum+sw] := DummyStr;

    for i := 0 to FileCount do
      begin
        ListBox1.Items.Add(FileName[i]);
      end;

    ListBox1.ItemIndex := MySelectItemNum+sw;
    FileName.Free;
  end;
end;