如何处理缺少元素时失败的 LINQ 表达式

Question

我是 LINQ to XML 的新手，我已经得到了这段可以工作的代码（大多数时候）：

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）上失败了：

    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

数据是格式良好的 XML。此给定 XML 文件中有许多 节点，并且大多数（但不是全部） 节点都包含 > 节点。我通过使用 VStudio 2010 强力打开 XML 文件并使用 Find 命令来发现这一点，该命令给出了匹配行的计数。

有没有办法改进 LINQ，使其在数据不完美时不会失败？并且，有没有办法查看 LINQ 表达式的哪一部分实际上失败了（我猜测这是由于缺少 节点，但这可能是错误的并且是一个丑陋的暴力解决问题的方法）。

这是一个确实包含正确的 节点（在最底部）的 ：

    <?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（包含解决此问题的 LINQ） FOLDER 节点可能丢失；正如其他人指出的那样，使用 First() 可能是危险的做法，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

在尝试选择给定节点之前，您始终可以检查它：

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

但我承认这可能会变得有点难看。在这种情况下，您可以创建一个 XElement 扩展方法来执行以下操作：

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
   }
}

您可以使用以下方法：

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

Answer 2

我无法测试这是否是您的确切问题，但如果查询中没有任何值，First() 会引发异常。您可以通过使用 FirstOrDefault() 来避免这种情况，然后检查结果是否为 null。

Answer 3

我的经验法则是永远使用 First()、FirstOrDefault()、Single() 或 < code>SingleOrDefault() 方法位于查询中间。它应该始终是最后调用的方法。如果需要从所选项目中选择属性，请先进行投影，然后调用该方法。它将导致代码看起来更简洁，使这种情况更容易处理。

例如，

// Change this:
d.Elements("INDEX")
 .Where(i => i.Attribute("name").Value == "Comment(idmComment)")
 .First()
 .Attribute("value").Value

// to this:
d.Elements("INDEX")
 .Where(i => i.Attribute("name").Value == "Comment(idmComment)")
 .Select(i => i.Attribute("value").Value)
 .First()

我将其重写为：

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

Answer 4

在您的 linq 查询中，在每个可能为空的字段上，因此如果您对它们进行赋值，则会抛出异常，您可以：

MyDone = PossibleNullNode != null ? PossibleNullNode.Value : WhateverFloatsYourBoat