c# - 如何防止 .NET XML 解析器扩展 XML 中的参数实体？

Question

当我尝试解析下面的 xml(使用下面的代码)时，我不断收到 <sgml>&question;&signature;</sgml>

扩展为

<sgml>Why couldn’t I publish my books directly in standard SGML? — William Shakespeare.</sgml>

或

<sgml></sgml>

因为我正在研究 XML 3 向合并算法，所以我想检索未扩展的 <sgml>&question;&signature;</sgml>

我试过:

• 正常解析 xml(这导致扩展的 sgml 标记)
• 从 xml 开头删除 Doctype 这会导致空 sgml 标记)
• 各种 XmlReader DTD 设置

我有以下 XML 文件:

<!DOCTYPE sgml [
  <!ELEMENT sgml ANY>
  <!ENTITY  std       "standard SGML">
  <!ENTITY  signature " &#x2014; &author;.">
  <!ENTITY  question  "Why couldn&#x2019;t I publish my books directly in &std;?">
  <!ENTITY  author    "William Shakespeare">
]>
<sgml>&question;&signature;</sgml>

这是我尝试过的代码(多次尝试):

using System.IO;
using System.Xml;
using System.Xml.Linq;
using System.Reflection;

class Program
{
    static void Main(string[] args)
    {
        string xml = @"C:\src\Apps\Wit\MergingAlgorithmTest\MergingAlgorithmTest\Tests\XMLMerge-DocTypeExpansion\DocTypeExpansion.0.xml";
        var xmlSettingsIgnore = new XmlReaderSettings 
            {
                CheckCharacters = false,
                DtdProcessing = DtdProcessing.Ignore
            };

        var xmlSettingsParse = new XmlReaderSettings
        {
            CheckCharacters = false,
            DtdProcessing = DtdProcessing.Parse
        };

        using (var fs = File.Open(xml, FileMode.Open, FileAccess.Read))
        {
            using (var xmkReaderIgnore = XmlReader.Create(fs, xmlSettingsIgnore))
            {
                // Prevents Exception "Reference to undeclared entity 'question'"
                PropertyInfo propertyInfo = xmkReaderIgnore.GetType().GetProperty("DisableUndeclaredEntityCheck", BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);
                propertyInfo.SetValue(xmkReaderIgnore, true, null);

                var doc = XDocument.Load(xmkReaderIgnore);

                Console.WriteLine(doc.Root.ToString()); // outputs <sgml></sgml> not <sgml>&question;&signature;</sgml>
            }// using xml ignore

            fs.Position = 0;
            using (var xmkReaderIgnore = XmlReader.Create(fs, xmlSettingsParse))
            {
                var doc = XDocument.Load(xmkReaderIgnore);
                Console.WriteLine(doc.Root.ToString()); // outputs <sgml>Why couldn't I publish my books directly in standard SGML? - William Shakespeare.</sgml> not <sgml>&question;&signature;</sgml>
            }

            fs.Position = 0;
            string parseXmlString = String.Empty;
            using (StreamReader sr = new StreamReader(fs))
            {
                for (int i = 0; i < 7; ++i) // Skip DocType
                    sr.ReadLine();

                parseXmlString = sr.ReadLine();
            }

            using (XmlReader xmlReaderSkip = XmlReader.Create(new StringReader(parseXmlString),xmlSettingsParse))
            {
                // Prevents Exception "Reference to undeclared entity 'question'"
                PropertyInfo propertyInfo = xmlReaderSkip.GetType().GetProperty("DisableUndeclaredEntityCheck", BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);
                propertyInfo.SetValue(xmlReaderSkip, true, null);

                var doc2 = XDocument.Load(xmlReaderSkip); // Empty sgml tag

            }
        }//using FileStream
    }
}

Answer 1

Linq-to-XML 不支持实体引用的建模——它们会自动扩展为它们的值(source 1、source 2)。根本就没有 XObject 的子类为一般实体引用定义。

但是，假设您的 XML 有效(即实体引用存在于 DTD 中，它们在您的示例中就是这样做的)，您可以使用 旧的 XML Document Object Model 解析您的 XML 并插入 XmlEntityReference节点到您的 XML DOM 树中，而不是将实体引用扩展为纯文本:

        using (var sr = new StreamReader(xml))
        using (var xtr = new XmlTextReader(sr))
        {
            xtr.EntityHandling = EntityHandling.ExpandCharEntities; // Expands character entities and returns general entities as System.Xml.XmlNodeType.EntityReference
            var oldDoc = new XmlDocument();
            oldDoc.Load(xtr);
            Debug.WriteLine(oldDoc.DocumentElement.OuterXml); // Outputs <sgml>&question;&signature;</sgml>
            Debug.Assert(oldDoc.DocumentElement.OuterXml.Contains("&question;")); // Verify that the entity references are still there - no assert
            Debug.Assert(oldDoc.DocumentElement.OuterXml.Contains("&signature;")); // Verify that the entity references are still there - no assert
        }

ChildNodes每个 XmlEntityReference 都将具有一般实体的文本值。如果一个通用实体引用其他通用实体，就像您的情况一样，相应的内部 XmlEntityReference 将嵌套在外部的 ChildNodes 中。然后，您可以使用旧的 XmlDocument API 比较旧的和新的 XML。

请注意，您还需要使用旧的 XmlTextReader并设置 EntityHandling = EntityHandling.ExpandCharEntities .