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从我的早期帖子扩展到我自己的问题

如果您注意到我正在使用let关键字加入我的元素

有时我的 XML 可能没有所有这些元素。

那么是否可以在退出时加入此元素？如果不存在，则为该字段提供null 或空值

例如，如果没有Pharmacy Element。

那么这 let Pharmacy = MD.Element(NameSpace + "Pharmacy") 应该是可能的，这

phStoreName = Pharmacy.Element(NameSpace + "StoreName").Value,

应该给我一个空值或空值phStoreName 

var MedicationDispensed =
  (from MD in xdoc.Descendants(NameSpace + "MedicationDispensed")
   let DrugCoded = MD.Element(NameSpace + "DrugCoded")
   let Quantity = MD.Element(NameSpace + "Quantity")
   let Pharmacy = MD.Element(NameSpace + "Pharmacy")
   let phIdentification = Pharmacy.Element(NameSpace + "Identification")
   let phAddress = Pharmacy.Element(NameSpace + "Address")
   let phPhoneNumbers = Pharmacy.Element(NameSpace + "PhoneNumbers")
   let phPhone = phPhoneNumbers.Element(NameSpace + "Phone")
   let Prescriber = MD.Element(NameSpace + "Prescriber")
   let prIdentification = Prescriber.Element(NameSpace + "Identification")
   let prName = Prescriber.Element(NameSpace + "Name")
   let prAddress = Prescriber.Element(NameSpace + "Address")
   select new
   {
       DrugDescription = MD.Element(NameSpace + "DrugDescription").Value,
       ProductCode = DrugCoded.Element(NameSpace + "ProductCode").Value,
       ProductCodeQualifier = DrugCoded.Element(NameSpace + "ProductCodeQualifier").Value,
       Qualifier = Quantity.Element(NameSpace + "Qualifier").Value,
       Value = Quantity.Element(NameSpace + "Value").Value,
       CodeListQualifier = Quantity.Element(NameSpace + "CodeListQualifier").Value,
       DaysSupply = MD.Element(NameSpace + "DaysSupply").Value,
       LastFillDate = MD.Element(NameSpace + "LastFillDate").Value,
       phStoreName = Pharmacy.Element(NameSpace + "StoreName").Value,
       phNCPDPID = phIdentification.Element(NameSpace + "NCPDPID").Value,
       phAddress1 = phAddress.Element(NameSpace + "AddressLine1").Value,
       phCity = phAddress.Element(NameSpace + "City").Value,
       phState = phAddress.Element(NameSpace + "State").Value,
       phZipcode = phAddress.Element(NameSpace + "ZipCode").Value,
       phPhoneNumber = phPhone.Element(NameSpace + "Number").Value,
       phQualifier = phPhone.Element(NameSpace + "Qualifier").Value,
       prDEANumber = prIdentification.Element(NameSpace + "DEANumber").Value,
       prLastName = prName.Element(NameSpace + "LastName").Value,
       prFirstName = prName.Element(NameSpace + "FirstName").Value,
       prMiddleName = prName.Element(NameSpace + "MiddleName").Value,
       prAddress1 = prAddress.Element(NameSpace + "AddressLine1").Value,
       prCity = prAddress.Element(NameSpace + "City").Value,
       prState = prAddress.Element(NameSpace + "State").Value,
       prZipCode = prAddress.Element(NameSpace + "ZipCode").Value
   }).ToList();
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2 回答 2

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而不是这样做：

Pharmacy.Element(NameSpace + "StoreName").Value

像这样使用显式转换运算符：

(string)Pharmacy.Element(NameSpace + "StoreName")

如果元素不为 null，它将返回 .Value，如果元素不存在，则返回空字符串。在这里您可以找到有关 XElement 的显式转换运算符的更多详细信息：http: //msdn.microsoft.com/en-us/library/system.xml.linq.xelement.op_explicit.aspx

于 2012-04-26T04:21:17.297 回答
0

终于找到了解决办法 参考这个链接

    What if "let" returns null?

If you declare an object with "let" such as

let myPayment = bill.Element("payment")

select new Payment

  {

     Amount = (decimal) myPayment.Element("Amount")

  }

If myPayment is null, this blows up with a null reference exception.  The alternative would be

  Amount = myPayment != null ? (decimal) myPayment.Element("Amount")

but think how cumbersome that is for a bunch of fields.

What's the solution?
于 2012-05-02T17:24:59.683 回答