public CommandFactory(ListCommand.Factory listCommandFactory, DownloadCommand.Factory downloadCommandFactory)
        {
            if (listCommandFactory == null) throw new ArgumentNullException("listCommandFactory");
            if (downloadCommandFactory == null) throw new ArgumentNullException("downloadCommandFactory");

            _listCommandFactory = listCommandFactory;
            _downloadCommandFactory = downloadCommandFactory;
        }
        public CommandFactory(ListCommand.Factory listCommandFactory, DownloadCommand.Factory downloadCommandFactory)
        {
            if (listCommandFactory == null)
            {
                throw new ArgumentNullException("listCommandFactory");
            }
            if (downloadCommandFactory == null)
            {
                throw new ArgumentNullException("downloadCommandFactory");
            }

            _listCommandFactory     = listCommandFactory;
            _downloadCommandFactory = downloadCommandFactory;
        }