从XML声明片段获取XML编码:部分内容解析不支持XmlDeclaration

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问题描述

我正在处理一些代码以读取包含XML声明的XML片段，例如<?xml version="1.0" encoding="utf-8"?>并解析编码.从 MSDN 中，我应该能够为此:

I'm working on some code to read an XML fragment which contains an XML declaration, e.g. <?xml version="1.0" encoding="utf-8"?> and parse the encoding. From MSDN, I should be able to do it like this:

var nt = new NameTable();
var mgr = new XmlNamespaceManager(nt);
var context = new XmlParserContext(null, mgr, null, XmlSpace.None);

var reader = new System.Xml.XmlTextReader(@"<?xml version=""1.0"" encoding=""UTF-8""?>",
    System.Xml.XmlNodeType.XmlDeclaration, context);

但是，在对System.Xml.XmlTextReader构造函数的调用中出现System.Xml.XmlException并出现错误消息:

However, I'm getting a System.Xml.XmlException on the call to the System.Xml.XmlTextReader constructor with an error message:

我已经用引号对这个错误进行了搜索-找到的结果恰好为零(现在有一个结果:该帖子)-并且没有引号，这没有任何用处.我还查看了 MSDN XmlNodeType ，并且没有说明它不受支持.

I've googled this error in quotes -- exactly zero results found (edit: now there's one result: this post) -- and without quotes, which yields nothing useful. I've also looked at MSDN for the XmlNodeType, and it doesn't say anything about it not being supported.

我在这里想念什么? 如何从XML声明片段中获取XmlTextReader实例?

What am I missing here? How can I get an XmlTextReader instance from an XML declaration fragment?

请注意，我的目标只是确定部分构建的XML文档的编码，在此我假设它至少包含一个声明节点.因此，我正在尝试获取reader.Encoding.如果还有其他方法可以做到，那么我很乐意.

Note, my goal here is just to determine the encoding of a partially-built XML document where I'm making the assumption that it at least contains a declaration node; thus, I'm trying to get reader.Encoding. If there's another way to do that, I'm open to that.

目前，我正在使用正则表达式手动解析声明，这不是最好的方法.

At present, I'm parsing the declaration manually using regex, which is not the best approach.

推荐答案

更新:从XML文档或XML片段获取编码:

这是一种使用 XmlReader.Create .

private static string GetXmlEncoding(string xmlString)
{
    if (string.IsNullOrWhiteSpace(xmlString)) throw new ArgumentException("The provided string value is null or empty.");

    using (var stringReader = new StringReader(xmlString))
    {
        var settings = new XmlReaderSettings { ConformanceLevel = ConformanceLevel.Fragment };

        using (var xmlReader = XmlReader.Create(stringReader, settings))
        {
            if (!xmlReader.Read()) throw new ArgumentException(
                "The provided XML string does not contain enough data to be valid XML (see https://msdn.microsoft.com/en-us/library/system.xml.xmlreader.read)");

            var result = xmlReader.GetAttribute("encoding");
            return result;
        }
    }
}

以下是输出，其中包含完整的XML片段:

Here's the output, with a full and fragment XML:

如果要使用System.Text.Encoding，可以将代码修改为如下所示:

If you want to have System.Text.Encoding, you can modify the code to look like this:

    private static Encoding GetXmlEncoding(string xmlString)
    {
        using (StringReader stringReader = new StringReader(xmlString))
        {
            var settings = new XmlReaderSettings { ConformanceLevel = ConformanceLevel.Fragment };

            var reader = XmlReader.Create(stringReader, settings);
            reader.Read();

            var encoding = reader.GetAttribute("encoding");

            var result = Encoding.GetEncoding(encoding);
            return result;
        }
    }

旧答案:

如您所述， XmlTextReader的Encoding 属性包含编码.

As you mentioned, XmlTextReader's Encoding-property contains the encoding.

以下是完整的控制台应用程序的源代码，希望对您有用:

Here's a full Console app's source code which hopefully is useful:

class Program
{
    static void Main(string[] args)
    {
        var asciiXML = @"<?xml version=""1.0"" encoding=""ASCII""?><note><to>Tove</to><from>Jani</from><heading>Reminder</heading><body>Don't forget me this weekend!</body></note>";
        var utf8XML = @"<?xml version=""1.0"" encoding=""UTF-8""?><note><to>Tove</to><from>Jani</from><heading>Reminder</heading><body>Don't forget me this weekend!</body></note>";

        var asciiResult = GetXmlEncoding(asciiXML);
        var utfResult = GetXmlEncoding(utf8XML);

        Console.WriteLine(asciiResult);
        Console.WriteLine(utfResult);

        Console.ReadLine();
    }
    private static Encoding GetXmlEncoding(string s)
    {
        var stream = new MemoryStream(Encoding.UTF8.GetBytes(s));

        using (var xmlreader = new XmlTextReader(stream))
        {
            xmlreader.MoveToContent();
            var encoding = xmlreader.Encoding;

            return encoding;
        }
    }
}

这是程序的输出:

如果您知道XML仅包含声明，也许您可以添加一个空的根?例如:

If you know that the XML only contains the declaration, maybe you can add an empty root? So for example:

        var fragmentResult = GetXmlEncoding(xmlFragment + "<root/>");

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