使用 XmlDictionaryWriter.CreateBinaryWriter 和 XmlDictionary 编写紧凑的 xml

Question

我想以紧凑格式将 xml 文档写入磁盘。为此，我使用.net框架的方法XmlDictionaryWriter.CreateBinaryWriter(Stream stream,IXmlDictionary dictionary)

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此方法编写一个自定义的紧凑二进制 xml 表示形式，稍后可以由XmlDictionaryWriter.CreateBinaryReader. 该方法接受XmlDictionary可以包含常见字符串的 ，因此不必每次都在输出中打印这些字符串。文件中将打印字典索引而不是字符串。CreateBinaryReader稍后可以使用相同的字典来反转该过程。

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然而我传递的字典显然没有被使用。考虑这段代码：

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using System.IO;\nusing System.Xml;\nusing System.Xml.Linq;\n\nclass Program\n{\n    public static void Main()\n    {\n        XmlDictionary dict = new XmlDictionary();\n        dict.Add("myLongRoot");\n        dict.Add("myLongAttribute");\n        dict.Add("myLongValue");\n        dict.Add("myLongChild");\n        dict.Add("myLongText");\n\n        XDocument xdoc = new XDocument();\n        xdoc.Add(new XElement("myLongRoot",\n                                new XAttribute("myLongAttribute", "myLongValue"),\n                                new XElement("myLongChild", "myLongText"),\n                                new XElement("myLongChild", "myLongText"),\n                                new XElement("myLongChild", "myLongText")\n                                ));\n\n        using (Stream stream = File.Create("binaryXml.txt"))\n        using (var writer = XmlDictionaryWriter.CreateBinaryWriter(stream, dict))\n        {\n            xdoc.WriteTo(writer);\n        }\n    }\n}\n

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产生的输出是这样的（二进制控制字符未显示）

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@\nmyLongRootmyLongAttribute\xcb\x9cmyLongValue@myLongChild\xe2\x84\xa2\nmyLongText@myLongChild\xe2\x84\xa2\nmyLongText@myLongChild\xe2\x84\xa2\nmyLongText\n

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显然 XmlDictionary 尚未被使用。所有字符串都会完整地出现在输出中，甚至多次出现。

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这不只是 XDocument 的问题。在上面的最小示例中，我使用 XDocument 来演示该问题，但最初我在将 XmlDictionaryWriter 与 DataContractSerializer 结合使用时偶然发现了这一点，因为它是常用的。结果是一样的：

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[Serializable]\npublic class myLongChild\n{\n    public double myLongText = 0;\n}\n...\nusing (Stream stream = File.Create("binaryXml.txt"))\nusing (var writer = XmlDictionaryWriter.CreateBinaryWriter(stream, dict))\n{\n    var dcs = new DataContractSerializer(typeof(myLongChild));\n    dcs.WriteObject(writer, new myLongChild());\n}\n

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生成的输出没有使用我的 XmlDictionary。

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如何让 XmlDictionaryWriter 使用提供的 XmlDictionary？

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或者我误解了它是如何工作的？

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使用 DataContractSerializer 方法，我尝试调试网络框架代码（Visual Studio/options/debugging/enable net.framework 源代码步进）。显然，作者确实按照预期尝试在字典中查找上述每个字符串。然而，查找在XmlbinaryWriter.cs 的第 356 行失败，原因我不清楚。

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我考虑过的替代方案：

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• XmlDictionaryWriter.CreatebinaryWriter 有一个重载，它也接受 XmlBinaryWriterSession。然后，编写器将遇到的任何新字符串添加到会话字典中。但是，我只想使用静态字典进行读写，这是事先知道的。

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• 我可以将整个内容包装成 aGzipStream并让压缩处理字符串的多个实例。但是，这不会压缩每个字符串的第一个实例，并且总体而言似乎是一个笨拙的解决方法。

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Answer 1

是的，有一个误会。XmlDictionaryWriter主要用于对象的序列化，它是 的子类XmlWriter。XDocument.WriteTo(XmlWriter something)作为XmlWriter参数。该调用XmlDictionaryWriter.CreateBinaryWriter将在内部创建一个实例System.Xml.XmlBinaryNodeWriter。这个类有两种“常规”编写方法：

// override of XmlWriter
public override void WriteStartElement(string prefix, string localName)
{
  // plain old "xml" for me please
}

对于基于字典的方法：

// override of XmlDictionaryWriter
public override void WriteStartElement(string prefix, XmlDictionaryString localName)
{
  // I will use dictionary to hash element names to get shorter output
}

后者主要用于通过序列化对象DataContractSerializer（注意它的方法WriteObject接受XmlDictionaryWriter和XmlWriter类型的参数），而XDocument仅接受XmlWriter.

至于你的问题 - 如果我是你，我会自己解决XmlWriter：

class CustomXmlWriter : XmlWriter
{
  private readonly XmlDictionaryWriter _writer;
  public CustomXmlWriter(XmlDictionaryWriter writer)
  {
    _writer = writer;
  }
  // override XmlWriter methods to use the dictionary-based approach instead
}

更新（根据您的评论）

如果你确实使用DataContractSerializer你的代码中几乎不会有错误。

1）POC类必须用属性修饰[DataContract]，[DataMember]序列化值应该是属性而不是字段；还将命名空间设置为空值，否则您还必须处理字典中的命名空间。喜欢：

namespace  XmlStuff {
  [DataContract(Namespace = "")]
  public class myLongChild
  {
    [DataMember]
    public double myLongText { get; set; }
  }

  [DataContract(Namespace = "")]
  public class myLongRoot
  {
    [DataMember]
    public IList<myLongChild> Items { get; set; }
  }
}

2）同时提供会话实例；对于空会话，字典编写器使用默认（XmlWriter类似）实现：

// order matters - add new items only at the bottom
static readonly string[] s_Terms = new string[]
{
    "myLongRoot", "myLongChild", "myLongText", 
    "http://www.w3.org/2001/XMLSchema-instance", "Items"
};

public class CustomXmlBinaryWriterSession : XmlBinaryWriterSession
{
  private bool m_Lock;
  public void Lock() { m_Lock = true; }

  public override bool TryAdd(XmlDictionaryString value, out int key)
  {
    if (m_Lock)
    {
      key = -1;
      return false;
    }

    return base.TryAdd(value, out key);
  }
}

static void InitializeWriter(out XmlDictionary dict, out XmlBinaryWriterSession session)
{
  dict = new XmlDictionary();
  var result = new CustomXmlBinaryWriterSession();
  var key = 0;
  foreach(var term in s_Terms)
  {
    result.TryAdd(dict.Add(term), out key);
  }
  result.Lock();
  session = result;
}

static void InitializeReader(out XmlDictionary dict, out XmlBinaryReaderSession session)
{
  dict = new XmlDictionary();
  var result = new XmlBinaryReaderSession();
  for (var i = 0; i < s_Terms.Length; i++)
  {
    result.Add(i, s_Terms[i]);
  }
  session = result;
}

static void Main(string[] args)
{
  XmlDictionary dict;
  XmlBinaryWriterSession session;
  InitializeWriter(out dict, out session);

  var root = new myLongRoot { Items = new List<myLongChild>() };
  root.Items.Add(new myLongChild { myLongText = 24 });
  root.Items.Add(new myLongChild { myLongText = 25 });
  root.Items.Add(new myLongChild { myLongText = 27 });

  byte[] buffer;
  using (var stream = new MemoryStream())
  {
    using (var writer = XmlDictionaryWriter.CreateBinaryWriter(stream, dict, session))
    {
      var dcs = new DataContractSerializer(typeof(myLongRoot));
      dcs.WriteObject(writer, root);
    }
    buffer = stream.ToArray();
  }


  XmlBinaryReaderSession readerSession;
  InitializeReader(out dict, out readerSession);
  using (var stream = new MemoryStream(buffer, false))
  {
    using (var reader = XmlDictionaryReader.CreateBinaryReader(stream, dict, new XmlDictionaryReaderQuotas(), readerSession))
    {
      var dcs = new DataContractSerializer(typeof(myLongRoot));
      var rootCopy = dcs.ReadObject(reader);
    }
  }
}