c# - 我在这里做错了什么，使用 C# 的 XSLT 有问题

Question

请看下面的代码，因为我在这一行得到了错误:

xslt.Transform(mydoc.CreateReader(), writer);

错误:

步入:跨过非用户代码'System.Xml.Linq.XNode.CreateReader'System.Data.SqlXml.dll 中发生类型为“System.NullReferenceException”的第一次机会异常

((System.NullReferenceException)(ex))

PromotionsDataContext db = new PromotionsDataContext();
//XmlDocument myxml;

XElement Categories =
    new XElement("Promotions",
        from b in db.GetPromotions()
        select new XElement("Promotion",
            new XElement("Category", b.CategoryName),
               new XElement("Client", b.ClientName),
               new XElement("Title", b.Title)));



XDocument mydoc = new XDocument();
mydoc.Add(Categories);


try
{

    XDocument newTree = new XDocument();


    XmlWriterSettings settings = new XmlWriterSettings();
    settings.OmitXmlDeclaration = true;
    settings.ConformanceLevel = ConformanceLevel.Fragment;
    settings.CloseOutput = false;

    //using (XmlWriter writer = newTree.CreateWriter())
    using (XmlWriter writer = XmlWriter.Create(newTree.CreateWriter(), settings))
    {

        // Load the style sheet.
        XslCompiledTransform xslt = new XslCompiledTransform();


        xslt.Load(XmlReader.Create(new FileStream(@"C:\1\TransList.xslt", System.IO.FileMode.Open)));

        // Execute the transform and output the results to a writer.

        xslt.Transform(mydoc.CreateReader(), writer);
    }

    Console.WriteLine(newTree);
}
catch (Exception ex)
{
    Console.Write(ex);

}

这是 XSLT:

<xsl:stylesheet version='1.0' xmlns:xsl='http://www.w3.org/1999/XSL/Transform'>
  <xsl:output method='xml' />
  <xsl:key name='categories' match='Category' use='.' />
  <xsl:template match='/'>
    <xsl:for-each select="/Promotions/Promotion/Category[ 
        generate-id(.) = generate-id(key('categories', .)[1]) 
      ]">
      <xsl:variable name='cname' select='.' />
      <Category title='{.}'>
        <xsl:for-each select='/Promotions/Promotion[Category=$cname]'>
          <Title>
            <xsl:value-of select='Title' />
          </Title>
        </xsl:for-each>
      </Category>
    </xsl:for-each>
  </xsl:template>
</xsl:stylesheet>

错误:

System.NullReferenceException:对象引用未设置到对象的实例。 在 System.Xml.Xsl.Runtime.XmlMergeSequenceWriter.StartTree(XPathNodeType rootType，IXmlNamespaceResolver nsResolver，XmlNameTable nameTable) 在 System.Xml.Xsl.Runtime.XmlQueryOutput.StartTree(XPathNodeType 根类型) 在 System.Xml.Xsl.Runtime.XmlQueryOutput.WriteStartRoot() 在根目录(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime) 在执行(XmlQueryRuntime {urn:schemas-microsoft-com:xslt-debug}runtime) 在 System.Xml.Xsl.XmlILCommand.Execute(对象 defaultDocument、XmlResolver dataSources、XsltArgumentList argumentList、XmlWriter writer、Boolean closeWriter) 在 System.Xml.Xsl.XmlILCommand.Execute(XmlReader contextDocument、XmlResolver 数据源、XsltArgumentList argumentList、XmlWriter 结果) 在 System.Xml.Xsl.XslCompiledTransform.Transform(XmlReader 输入、XsltArgumentList 参数、XmlWriter 结果) 在 c:\1\promo.ascx.cs:line 144 中的 Promo.Page_Load(Object sender, EventArgs e)

现在，如果我这样做，它就会起作用:

StringWriter sw = new StringWriter(); 
xslt.Transform(mydoc.CreateReader(),null, sw);

我对 XmlWriter 做错了什么？

崩溃前 xdoc 的值:

<Promotions>
  <Promotion>
    <Category>Arts &amp; Entertainment</Category>
    <Client>Client1</Client>
    <Title>Get your Free 2</Title>
  </Promotion>
  <Promotion>
    <Category>Arts &amp; Entertainment</Category>
    <Client>Client1</Client>
    <Title>Get your Free 4</Title>
  </Promotion>
  <Promotion>
    <Category>Arts &amp; Entertainment</Category>
    <Client>client1</Client>
    <Title>Get your Free 5</Title>
  </Promotion>
  <Promotion>
    <Category>Community &amp; Neighborhood</Category>
    <Client>Client2</Client>
    <Title>Get your Free 1</Title>
  </Promotion>
  <Promotion>
    <Category>Education</Category>
    <Client>Client3</Client>
    <Title>Get Your Free 3</Title>
  </Promotion>
</Promotions>

Answer 1

我认为问题确实是您似乎想要使用样式表和 XmlWriter 创建 XML 片段，而 LINQ to XML 对象模型(即 System.Xml.Linq.XDocument/XNode)没有任何表示的类碎片。

如果我接受您的 XML 输入(例如

<Promotions>
  <Promotion>
    <Category>Arts &amp; Entertainment</Category>
    <Client>Client1</Client>
    <Title>Get your Free 2</Title>
  </Promotion>
  <Promotion>
    <Category>Arts &amp; Entertainment</Category>
    <Client>Client1</Client>
    <Title>Get your Free 4</Title>
  </Promotion>
  <Promotion>
    <Category>Arts &amp; Entertainment</Category>
    <Client>client1</Client>
    <Title>Get your Free 5</Title>
  </Promotion>
  <Promotion>
    <Category>Community &amp; Neighborhood</Category>
    <Client>Client2</Client>
    <Title>Get your Free 1</Title>
  </Promotion>
  <Promotion>
    <Category>Education</Category>
    <Client>Client3</Client>
    <Title>Get Your Free 3</Title>
  </Promotion>
</Promotions>

) 和发布的第二个样式表(例如

<xsl:stylesheet version='1.0' xmlns:xsl='http://www.w3.org/1999/XSL/Transform'>
  <xsl:output method='xml' />
  <xsl:key name='categories' match='Category' use='.' />
  <xsl:template match='/'>
    <Categories>
      <!-- Added a root element here -->
      <xsl:for-each select="/Promotions/Promotion/Category[ 
            generate-id(.) = generate-id(key('categories', .)[1]) 
          ]">
        <xsl:variable name='cname' select='.' />
        <Category title='{.}'>
          <xsl:for-each select='/Promotions/Promotion[Category=$cname]'>
            <Title>
              <xsl:value-of select='Title' />
            </Title>
          </xsl:for-each>
        </Category>
      </xsl:for-each>
    </Categories>
  </xsl:template>
</xsl:stylesheet>

)，然后使用以下 C# 代码:

    XDocument mydoc = XDocument.Load(@"..\..\XMLFile1.xml");

    XDocument newTree = new XDocument();

    using (XmlWriter writer = newTree.CreateWriter())
    {
        XslCompiledTransform xslt = new XslCompiledTransform();

        xslt.Load(@"..\..\XSLTFile2.xslt");

        xslt.Transform(mydoc.CreateReader(), writer);
        writer.Close();
    }

    Console.WriteLine(newTree);

当我使用原始样式表时(例如

<xsl:stylesheet version='1.0' xmlns:xsl='http://www.w3.org/1999/XSL/Transform'>
  <xsl:output method='xml' />
  <xsl:key name='categories' match='Category' use='.' />
  <xsl:template match='/'>
    <xsl:for-each select="/Promotions/Promotion/Category[ 
        generate-id(.) = generate-id(key('categories', .)[1]) 
      ]">
      <xsl:variable name='cname' select='.' />
      <Category title='{.}'>
        <xsl:for-each select='/Promotions/Promotion[Category=$cname]'>
          <Title>
            <xsl:value-of select='Title' />
          </Title>
        </xsl:for-each>
      </Category>
    </xsl:for-each>
  </xsl:template>
</xsl:stylesheet>

) 我得到一个 InvalidOperationException 说“状态 EndRootElement 中的 token StartElement 会导致无效的 XML 文档。如果要编写 XML 片段，请确保将 ConformanceLevel 设置设置为 ConformanceLevel.Fragment 或 ConformanceLevel.Auto。”。我怀疑您试图通过使用 ConformanceLevel.Fragment 将您的 XmlWriter 包装到另一个 XmlWriter 中来解决这个问题，就像您在 C# 代码中所做的那样，但我认为这不起作用，它只会导致不同的异常。

在我看来应该起作用的是在 XElement 实例上使用 CreateWriter()，因为它应该可以将片段添加到 XElement。然而，我的测试仍然抛出异常，因此我已经提交了一个错误，请参阅 https://connect.microsoft.com/VisualStudio/feedback/details/530052/xslt-stylesheet-writing-an-xml-fragment-to-an-xmlwriter-created-with-xelementinstance-createwriter-causes-nullreferenceexception