var
  ConnectionService := TConnService;

procedure TfrmMain.FormCreate(Sender: TObject);
begin
  ConnectionService := TConnService.Create(Application);
end;

destructor TfrmMain.Destroy;
begin
  ConnectionService.Free//执行这里时出错，原因使用Application当Owner
//，在这之前Application自动把ConnectionService释放掉了(把Owner为Application的都释放掉了)
  inherited;
end;

这样会出错，frmMain为Application的主窗体