我需要使用XSLT转换一个HTML网页的内容。因此,我使用了SgmlReader并编写了下面显示的代码片段(最终我认为它也是一个XmlReader...)
XmlReader xslr = XmlReader.Create(new StringReader(
"<?xml version=\"1.0\" encoding=\"UTF-8\"?>" +
"<xsl:stylesheet xmlns:xsl=\"http://www.w3.org/1999/XSL/Transform\" version=\"1.0\">" +
"<xsl:output method=\"xml\" encoding=\"UTF-8\" version=\"1.0\" />" +
"<xsl:template match=\"/\">" +
"<XXX xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\"><xsl:value-of select=\"count(//br)\" /></XXX>" +
"</xsl:template>" +
"</xsl:stylesheet>"));
XslCompiledTransform xslt = new XslCompiledTransform();
xslt.Load(xslr);
using (SgmlReader html = new SgmlReader())
{
StringBuilder sb = new StringBuilder();
using (TextWriter sw = new StringWriter(sb))
using (XmlWriter xw = new XmlTextWriter(sw))
{
html.InputStream = new StringReader(Resources.html_orig);
html.DocType = "HTML";
try
{
xslt.Transform(html, xw);
string output = sb.ToString();
System.Console.WriteLine(output);
}
catch (Exception exc)
{
System.Console.WriteLine("{0} : {1}", exc.GetType().Name, exc.Message);
System.Console.WriteLine(exc.StackTrace);
}
}
}
然而,我收到了这个错误信息。
NullReferenceException : Object reference not set to an instance of an object.
at MS.Internal.Xml.Cache.XPathDocumentBuilder.Initialize(XPathDocument doc, IXmlLineInfo lineInfo, String baseUri, LoadFlags flags)
at MS.Internal.Xml.Cache.XPathDocumentBuilder..ctor(XPathDocument doc, IXmlLineInfo lineInfo, String baseUri, LoadFlags flags)
at System.Xml.XPath.XPathDocument.LoadFromReader(XmlReader reader, XmlSpace space)
at System.Xml.XPath.XPathDocument..ctor(XmlReader reader, XmlSpace space)
at System.Xml.Xsl.Runtime.XmlQueryContext.ConstructDocument(Object dataSource, String uriRelative, Uri uriResolved)
at System.Xml.Xsl.Runtime.XmlQueryContext..ctor(XmlQueryRuntime runtime, Object defaultDataSource, XmlResolver dataSources, XsltArgumentList argList, WhitespaceRuleLookup wsRules)
at System.Xml.Xsl.Runtime.XmlQueryRuntime..ctor(XmlQueryStaticData data, Object defaultDataSource, XmlResolver dataSources, XsltArgumentList argList, XmlSequenceWriter seqWrt)
at System.Xml.Xsl.XmlILCommand.Execute(Object defaultDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlSequenceWriter results)
at System.Xml.Xsl.XmlILCommand.Execute(Object defaultDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlWriter writer, Boolean closeWriter)
at System.Xml.Xsl.XmlILCommand.Execute(XmlReader contextDocument, XmlResolver dataSources, XsltArgumentList argumentList, XmlWriter results)
at System.Xml.Xsl.XslCompiledTransform.Transform(XmlReader input, XmlWriter results)
我发现一种解决方法是将 HTML 转换为 XML,然后应用变换,但这是一种低效的解决方案,因为:
- 中间的 XHTML 输出会进入缓冲区,因此需要额外的内存。
- 转换过程需要额外的 CPU 处理,并且相同的层次结构被遍历两次(在理论上是不必要的)。