DataTable dt = PurchaseArriveInfoBus.GetPurchaseArriveInfo(companyCD, txtArriveNo, txtTitle, txtProvider);        XElement dsXML = ConvertDataTableToXML(dt);        //linq排序        var dsLinq =            (order == "ascending") ?            (from x in dsXML.Descendants("Data")             orderby x.Element(orderBy).Value ascending             select new DataSourceModel()             {                 ID = x.Element("ID").Value,                 ArriveNo = x.Element("ArriveNo").Value,                 Title = x.Element("Title").Value,                 ProviderID = x.Element("ProviderName").Value,                 Purchaser = x.Element("Purchaser").Value,                 ProductNo = x.Element("ProductNo").Value,                 ProductName = x.Element("ProductName").Value,                 ColorName = x.Element("ColorName").Value,                                  TotalPrice = x.Element("TotalPrice").Value,                 ProductCount = x.Element("ProductCount").Value,                 InCount = x.Element("InCount").Value,                 DetailID = x.Element("DetailID").Value,                 UnitName = x.Element("UnitName").Value,                 JiBenCount = x.Element("JiBenCount").Value,                 UnitPrice = IsDisplayPrice ? x.Element("UnitPrice").Value : "0.000000"             })                      :            (from x in dsXML.Descendants("Data")             orderby x.Element(orderBy).Value descending             select new DataSourceModel()             {                 ID = x.Element("ID").Value,                 ArriveNo = x.Element("ArriveNo").Value,                 Title = x.Element("Title").Value,                 ProviderID = x.Element("ProviderName").Value,                 Purchaser = x.Element("Purchaser").Value,                 ProductNo = x.Element("ProductNo").Value,                 ProductName = x.Element("ProductName").Value,                 ColorName = x.Element("ColorName").Value,                                  TotalPrice = x.Element("TotalPrice").Value,                 ProductCount = x.Element("ProductCount").Value,                 InCount = x.Element("InCount").Value,                 DetailID = x.Element("DetailID").Value,                 UnitName = x.Element("UnitName").Value,                 JiBenCount = x.Element("JiBenCount").Value,                 UnitPrice = IsDisplayPrice ? x.Element("UnitPrice").Value : "0.000000"             });        int totalCount = dsLinq.Count();

　　dsLinq.Count() 引发了“System.NullReferenceException”类型的异常,其中任何字段都不能为null