public Stock GetStockBySymbol(string symbol)
        {
            if (string.IsNullOrWhiteSpace(symbol))
            {
                throw new ArgumentNullException("smybol");
            }

            using (var context = ContainerHelper.Instance.Resolve<IRepositoryContext>())
            {
                try
                {
                    var repository = new StockRepository(context);
                    return repository.GetStockBySymbol(symbol);
                }
                catch
                {
                    //这里可能会因为Repository<Stock>.Single()找不到记录抛出异常
                    return null;
                }
            }
        }