c# - 如果缺少元素，如何处理失败的 LINQ 表达式

Question

我是 LINQ to XML 的新手，我得到了这段代码(大部分时间):

private long processFile(StreamWriter oWriter, string inFileName)
    {
        XDocument xmlDoc = XDocument.Load(inFileName);
        List<DocMetaData> docList =
            (from d in xmlDoc.Descendants("DOCUMENT")
             select new DocMetaData
             {
                 Folder = d.Element("FOLDER").Attribute("name").Value
                 ,
                 File = d.Element("FILE").Attribute("filename").Value
                 ,
                 Comment = d.Elements("INDEX")
                    .Where(i => i.Attribute("name").Value == "Comment(idmComment)")
                    .First()
                    .Attribute("value").Value
                 ,
                 Title = d.Elements("INDEX")
                    .Where(i => i.Attribute("name").Value == "Title(idmName)")
                    .First()
                    .Attribute("value").Value
                 ,
                 DocClass = d.Elements("INDEX")
                    .Where(i => i.Attribute("name").Value == "Document Class(idmDocType)")
                    .First()
                    .Attribute("value").Value
             }
            ).ToList<DocMetaData>();
        OutputListToFile(oWriter, docList);
        return docList.LongCount();
    }

这在第 117 行(选择表达式)失败了:

    System.NullReferenceException: Object reference not set to an instance of an object.
   at CBMI.WinFormsUI.GridForm.<processFile>b__3(XElement d) in C:\ProjectsVS2010\CBMI.LatitudePostConverter\CBMI.LatitudePostConverter\CBMI.WinFormsUI\GridForm.cs:line 117
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at CBMI.WinFormsUI.GridForm.processFile(StreamWriter oWriter, String inFileName) in C:\ProjectsVS2010\CBMI.LatitudePostConverter\CBMI.LatitudePostConverter\CBMI.WinFormsUI\GridForm.cs:line 115
   at CBMI.WinFormsUI.GridForm.btnProcess_Click(Object sender, EventArgs e) in C:\ProjectsVS2010\CBMI.LatitudePostConverter\CBMI.LatitudePostConverter\CBMI.WinFormsUI\GridForm.cs:line 85

数据是格式正确的 XML。有很多<DOCUMENT>此给定 XML 文件中的节点和大部分但不是全部，<DOCUMENT>节点包含 <FOLDER>节点。我通过蛮力使用 VStudio 2010 打开 XML 文件并使用 Find 命令发现了这一点，该命令给出了匹配行的计数。

有没有一种方法可以改进 LINQ，使其在数据不完美时不会失败？并且，有没有一种方法可以查看 LINQ 表达式的哪一部分实际上失败了(我猜这是由于缺少 <FOLDER> 节点，但这可能是错误的，并且是一种丑陋的强力解决方法)。

这是一个 <DOCUMENT>确实包含正确的 <FOLDER>节点(在最底部):

    <?xml version="1.0" ?>
<DOCUMENTCOLLECTION>
<DOCUMENT>
<FILE filename="P:\LatitudeConsulting\LatConConverter-1.8.2\ConverterOutput\B0000002\3rd Party CON\D003694452.0001.tif" 
      outputpath="P:\LatitudeConsulting\LatConConverter-1.8.2\ConverterOutput\B0000002\3rd Party CON"/>
<ANNOTATION filename=""/>
<INDEX name="Access Level(idmAccessLevel)" value="Admin"/>
<INDEX name="Added By Group(idmAddedByGroup)" value="General Users"/>
<INDEX name="Added By User(idmDocOwner)" value="Import"/>
<INDEX name="Allow Secondary Version Lines?(idmDocVariants)" value="Yes"/>
<INDEX name="Application(idmVerApplication)" value=""/>
<INDEX name="Archive Category(idmDocDispCategory)" value="Archive"/>
<INDEX name="Archive Date(idmVerDispDate)" value=""/>
<INDEX name="Archive Repository(idmVerDispId)" value=""/>
<INDEX name="ArchivedDocument" value="NO"/>
<INDEX name="Availability Status(idmVerAvailStat)" value="Online"/>
<INDEX name="CAN(idmDocCustom4)" value=""/>
<INDEX name="Checked In By Group(idmVerCheckinGroup)" value="General Users"/>
<INDEX name="Checked In By User(idmVerCheckinUser)" value="Import"/>
<INDEX name="Checked Out?(idmVerCheckoutPending)" value="No"/>
<INDEX name="Checkin Date(idmVerCreateDate)" value="3/9/2001 9:20:38 AM"/>
<INDEX name="Child Count(idmVerCD)" value="0"/>
<INDEX name="Comment(idmComment)" value="1983\06_June_Meeting"/>
<INDEX name="Comment(idmVerComment)" value=""/>
<INDEX name="Content Search Repository(idmVerCsiId)" value=""/>
<INDEX name="Current Content Srch Repository(idmDocCurVerCsiId)" value=""/>
<INDEX name="Current Version Author(idmAddedByUser)" value="Import"/>
<INDEX name="Current Version Checked Out?(idmDocCurVerCheckedOut)" value="No"/>
<INDEX name="Current Version Date(idmDocCurVerDate)" value="3/9/2001 9:20:38 AM"/>
<INDEX name="Current Version ID(idmDocCurVerNum)" value="1"/>
<INDEX name="Current Version Index ID(idmDocCurVerCsiCid)" value=""/>
<INDEX name="Date Added(idmDateAdded)" value="3/9/2001 9:20:37 AM"/>
<INDEX name="Default Index Versions?(idmDocCsiDefault)" value="No"/>
<INDEX name="DiagnosticID(idmDocCustom5)" value="2-16.MDB-00015"/>
<INDEX name="Document Class(idmDocType)" value="3rd Party CON"/>
<INDEX name="Encrypted File Name(idmVerShelfFileId)" value="_276no__.__1"/>
<INDEX name="ExternalDocument" value="NO"/>
<INDEX name="File Name" value="51099.TIF"/>
<INDEX name="File Name(idmVerFileName)" value="51099.TIF"/>
<INDEX name="File Size(idmVerFileSize)" value="1166770"/>
<INDEX name="Has Annotations?(idmAnnotation)" value=""/>
<INDEX name="Index ID(idmVerCsiCid)" value=""/>
<INDEX name="Indexed Version Limit(idmDocCsiLimit)" value="1"/>
<INDEX name="Indexing Status(idmVerCsiStatus)" value="Not Indexed"/>
<INDEX name="Item ID(idmId)" value="003694452"/>
<INDEX name="Item ID(idmVerDocId)" value="003694452"/>
<INDEX name="Keyword(idmDocKeywords)" value=""/>
<INDEX name="Last Access Date(idmDateAccessed)" value="11/28/2003 3:05:30 PM"/>
<INDEX name="Last Access Date(idmDateModified)" value="8/24/2011 5:52:34 PM"/>
<INDEX name="Last Access Group(idmVerLastGroup)" value="Administrators"/>
<INDEX name="Last Access User(idmModifiedByUser)" value="Admin"/>
<INDEX name="Last Accessed Version(idmDocLastVerId)" value="1"/>
<INDEX name="Latest Version?(idmVerBranchCurVer)" value="Yes"/>
<INDEX name="Merge-Destination Version ID(idmVerMergeDst)" value="0"/>
<INDEX name="Merge-Source Version ID(idmVerMergeSrc)" value="0"/>
<INDEX name="MimeType" value="image/tiff"/>
<INDEX name="Min Item Delete Access Level(idmDocDeleteAccess)" value=""/>
<INDEX name="Modification Date(idmVerFileDate)" value="12/19/2000 11:12:30 AM"/>
<INDEX name="Number of Indexed Versions(idmDocCsiCount)" value="0"/>
<INDEX name="Offline Location(idmVerOfflineLocation)" value=""/>
<INDEX name="Online Disk Space(idmDocOnlineSize)" value="1166770"/>
<INDEX name="Online Limit(idmDocOnlineLimit)" value="5"/>
<INDEX name="Online Version Count(idmDocOnlineCount)" value="1"/>
<INDEX name="Origin ID(idmDocOriginID)" value=""/>
<INDEX name="Origin Library(idmDocOriginLibrary)" value=""/>
<INDEX name="Original File Name(idmDocOriginalFile)" value="51099.TIF"/>
<INDEX name="Permanent Index?(idmVerCsiPermanent)" value="No"/>
<INDEX name="Permanent Version?(idmVerPermanent)" value="No"/>
<INDEX name="Property ID(idmDocDynPropertyId)" value=""/>
<INDEX name="Protected?(idmDocProtected)" value="Yes"/>
<INDEX name="Publishing Status(idmPublish)" value=""/>
<INDEX name="Reclaim Pending?(idmVerReclaimPending)" value=""/>
<INDEX name="Reclaim Submitted Date(idmVerReclaimDate)" value=""/>
<INDEX name="Replica?(idmDocIsReplica)" value="No"/>
<INDEX name="ReplicatedDocument" value="NO"/>
<INDEX name="Secondary Version Line Count(idmVerBranchCount)" value="0"/>
<INDEX name="Source Version Checkout Date(idmVerPrevCheckoutDate)" value=""/>
<INDEX name="Storage Category(idmDocFileCategory)" value="Documents"/>
<INDEX name="Storage Repository(idmVerShelfId)" value="2"/>
<INDEX name="Title(idmName)" value="3rd Party CON Comments"/>
<INDEX name="Version ID(idmVerId)" value="1"/>
<FOLDER name="/NACAIE/1983/06_June_Meeting/NAPNSC"/>
</DOCUMENT>

编辑:解决方案如下(包含在 FOLDER 节点可能丢失时修复此问题的 LINQ；使用 First() 可能是危险的做法，正如其他人所指出的，但在这种情况下必须处理丢失的 FOLDER 节点):

namespace CBMI.Common
{
    public static class Extensions
    {
    public static string SafeGetAttributeValue(this XElement element, string attribute)
    {
        return (element != null) ?
          (element.Attribute(attribute) != null) ? 
              element.Attribute(attribute).Value : null : null;
    }
}
}
private long processFile(StreamWriter oWriter, string inFileName)
    {
        XDocument xmlDoc = XDocument.Load(inFileName);
        List<DocMetaData> docList =
            (from d in xmlDoc.Descendants("DOCUMENT")
             select new DocMetaData
             {
                 File = d.Element("FILE").Attribute("filename").Value
                 ,
                 ItemID = d.Elements("INDEX")
                    .Where(i => i.Attribute("name").Value == "Item ID(idmId)")
                    .First()
                    .Attribute("value").Value
                 ,
                 Comment = d.Elements("INDEX")
                    .Where(i => i.Attribute("name").Value == "Comment(idmComment)")
                    .First()
                    .Attribute("value").Value
                 ,
                 Title = d.Elements("INDEX")
                    .Where(i => i.Attribute("name").Value == "Title(idmName)")
                    .First()
                    .Attribute("value").Value
                 ,
                 DocClass = d.Elements("INDEX")
                    .Where(i => i.Attribute("name").Value == "Document Class(idmDocType)")
                    .First()
                    .Attribute("value").Value
                 ,
                 Folder = d.Element("FOLDER").SafeGetAttributeValue("name")
             }
            ).ToList<DocMetaData>();
        OutputListToFile(oWriter, docList);
        return docList.LongCount();
    }

Answer 1

您始终可以在尝试选择给定节点之前检查它:

Folder = (d.Element("FOLDER") != null) ? (d.Element("FOLDER").Attribute("name") != null)
                                          ? Attribute("name").Value : null
                                       : null

但我承认这会变得有点丑陋。在这种情况下，您可以创建一个执行此操作的 XElement 扩展方法:

public static class Extensions
{
   public static string SafeGetAttributeValue(this XElement element, string attribute)
   {
      return (element != null) ? (element.Attribute(attribute) != null)
                                              ? Attribute(attribute).Value : null
                                           : null
   }
}

你可以这样使用:

select new DocMetaData
{
   Folder = d.Element("FOLDER").SafeGetAttributeValue("name"),
   //the rest of your object creation
}