我想以压缩格式将 xml 文档写入磁盘。为此,我使用了net框架的方法XmlDictionaryWriter.CreateBinaryWriter(Stream stream,IXmlDictionary dictionary)
此方法写入一个自定义的紧凑二进制 xml 表示,以后可以由 XmlDictionaryWriter.CreateBinaryReader
读取。该方法接受一个可以包含常用字符串的 XmlDictionary
,这样就不必每次都在输出中打印这些字符串。字典索引将代替字符串打印在文件中。 CreateBinaryReader
稍后可以使用相同的字典来反转该过程。
但是我传递的词典显然没有被使用。考虑这段代码:
using System.IO;
using System.Xml;
using System.Xml.Linq;
class Program
{
public static void Main()
{
XmlDictionary dict = new XmlDictionary();
dict.Add("myLongRoot");
dict.Add("myLongAttribute");
dict.Add("myLongValue");
dict.Add("myLongChild");
dict.Add("myLongText");
XDocument xdoc = new XDocument();
xdoc.Add(new XElement("myLongRoot",
new XAttribute("myLongAttribute", "myLongValue"),
new XElement("myLongChild", "myLongText"),
new XElement("myLongChild", "myLongText"),
new XElement("myLongChild", "myLongText")
));
using (Stream stream = File.Create("binaryXml.txt"))
using (var writer = XmlDictionaryWriter.CreateBinaryWriter(stream, dict))
{
xdoc.WriteTo(writer);
}
}
}
生成的输出是这样的(二进制控制字符未显示)
@
myLongRootmyLongAttribute˜myLongValue@myLongChild™
myLongText@myLongChild™
myLongText@myLongChild™
myLongText
很明显 XmlDictionary 还没有被使用。所有字符串都完整地出现在输出中,甚至出现多次。
这不是仅限于 XDocument 的问题。在上面的最小示例中,我使用 XDocument 来演示问题,但最初我是在将 XmlDictionaryWriter 与 DataContractSerializer 结合使用时偶然发现的,因为它很常用。结果是一样的:
[Serializable]
public class myLongChild
{
public double myLongText = 0;
}
...
using (Stream stream = File.Create("binaryXml.txt"))
using (var writer = XmlDictionaryWriter.CreateBinaryWriter(stream, dict))
{
var dcs = new DataContractSerializer(typeof(myLongChild));
dcs.WriteObject(writer, new myLongChild());
}
生成的输出没有使用我的 XmlDictionary。
如何让 XmlDictionaryWriter 使用提供的 XmlDictionary?
还是我误解了它的工作原理?
使用 DataContractSerializer 方法,我尝试调试网络框架代码(visual studio/options/debugging/enable net.framework source stepping)。显然,Writer 确实尝试按预期在字典中查找上述每个字符串。但是在 line 356 of XmlbinaryWriter.cs 中查找失败,出于我不清楚的原因。
我考虑过的备选方案:
XmlDictionaryWriter.CreatebinaryWriter 有一个重载,它也接受 XmlBinaryWriterSession。然后编写器将它遇到的任何新字符串添加到 session 字典中。但是,我只想使用事先已知的静态字典进行读写。
我可以将整个内容包装到
GzipStream
中,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,总体上看起来是一个笨拙的解决方法。
最佳答案
是的,有一个误解。 XmlDictionaryWriter
主要用于对象的序列化,它是XmlWriter
的子类。 XDocument.WriteTo(XmlWriter something)
将 XmlWriter
作为参数。调用 XmlDictionaryWriter.CreateBinaryWriter
将在内部创建 System.Xml.XmlBinaryNodeWriter
的实例。此类具有“常规”写作的两种方法:
// 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 DataContractSerializer
序列化对象(注意其方法 WriteObject
接受 XmlDictionaryWriter
和 XmlWriter 的参数
类型),而 XDocument
只需要 XmlWriter
。
至于您的问题 - 如果我是您,我会制作自己的 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)
如果您确实使用了 DataContractSerializer
,那么您的代码中几乎没有错误。
1)POC类必须用[DataContract]
和[DataMember]
属性修饰,序列化的值应该是property而不是field;还将 namespace 设置为空值,否则您还必须处理字典中的 namespace 。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 session 实例;对于空 session ,字典编写器使用默认(XmlWriter
-like)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);
}
}
}
关于c# - 使用 XmlDictionaryWriter.CreateBinaryWriter 和 XmlDictionary 编写紧凑的 xml,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/35271458/