dsLinq.Count() 引发了“System.NullReferenceException”类型的异常

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