为什么 XmlWriter 总是输出 utf-16 编码？

Question

我有这个扩展方法

    public static string SerializeObject<T>(this T value)
    {
        var serializer = new XmlSerializer(typeof(T));           
        var settings = new XmlWriterSettings
                       {
                        Encoding = new UTF8Encoding(true), 
                        Indent = false, 
                        OmitXmlDeclaration = false,
                        NewLineHandling = NewLineHandling.None
                       };

        using(var stringWriter = new StringWriter()) 
        {
            using(var xmlWriter = XmlWriter.Create(stringWriter, settings)) 
            {
                serializer.Serialize(xmlWriter, value);
            }

            return stringWriter.ToString();
        }
    }

但每当我调用它时，它都会指定utf-16 的编码，即<?xml version="1.0" encoding="utf-16"?>。我做错了什么？

Answer 1

字符串是 UTF-16，因此写入 StringWriter 将始终使用 UTF-16。如果这不是你想要的，那么使用其他一些TextWriter 派生类，使用你喜欢的编码。

Answer 2

据我所知，StringWriter 类在序列化为字符串时将始终使用 UTF 16 编码。您可以编写自己的覆盖类来接受不同的编码：

public class StringWriterWithEncoding : StringWriter
{
    private readonly Encoding _encoding;

    public StringWriterWithEncoding()
    {
    }

    public StringWriterWithEncoding(IFormatProvider formatProvider)
        : base(formatProvider)
    {
    }

    public StringWriterWithEncoding(StringBuilder sb)
        : base(sb)
    {
    }

    public StringWriterWithEncoding(StringBuilder sb, IFormatProvider formatProvider)
        : base(sb, formatProvider)
    {
    }


    public StringWriterWithEncoding(Encoding encoding)
    {
        _encoding = encoding;
    }

    public StringWriterWithEncoding(IFormatProvider formatProvider, Encoding encoding)
        : base(formatProvider)
    {
        _encoding = encoding;
    }

    public StringWriterWithEncoding(StringBuilder sb, Encoding encoding)
        : base(sb)
    {
        _encoding = encoding;
    }

    public StringWriterWithEncoding(StringBuilder sb, IFormatProvider formatProvider, Encoding encoding)
        : base(sb, formatProvider)
    {
        _encoding = encoding;
    }

    public override Encoding Encoding
    {
        get { return (null == _encoding) ? base.Encoding : _encoding; }
    }
}

所以你可以改用这个：

using(var stringWriter = new StringWriterWithEncoding( Encoding.UTF8))
{
   ...
}

Answer 3

正如@john-saunders 在他的回答中提到的那样：

StringWriter 将始终使用 UTF-16

所以我为此使用了 MemoryStream。

在我的情况下，我使用 windows-1251 编码。

var xmlSstring = "";
using (var ms = new MemoryStream())
{
    var encoding = Encoding.GetEncoding(1251);
    var settings = new XmlWriterSettings
    {
        Indent = true,
        Encoding = encoding
    };

    using (var xmlTextWriter = XmlWriter.Create(ms, settings))
    {
        doc.Save(xmlTextWriter);
        xmlString = encoding.GetString(ms.ToArray());
    }
}

Answer 4

您应该从 StringWriter 派生一个新类，该类具有覆盖的编码属性。

Answer 5

正如公认的答案所说，StringWriter 默认和设计是 UTF-16 (Unicode)。如果你想通过最后得到一个 UTF-8 字符串来做到这一点，我可以给你两种方法来完成它：

解决方案 #1（不是很有效，不好的做法，但可以完成工作）： 将其转储到文本文件并读回，删除文件 （可能只适合对于小文件，如果您甚至想这样做 - 只是想表明它可以做到！）

public static string SerializeObject<T>(this T value)
{
    var serializer = new XmlSerializer(typeof(T));           
    var settings = new XmlWriterSettings
                   {
                    Encoding = new UTF8Encoding(true), 
                    Indent = false, 
                    OmitXmlDeclaration = false,
                    NewLineHandling = NewLineHandling.None
                   };


    using(var xmlWriter = XmlWriter.Create("MyFile.xml", settings)) 
    {
        serializer.Serialize(xmlWriter, value);
    }

    XmlDocument xml = new XmlDocument();
    xml.Load("MyFile.xml");
    byte[] bytes = Encoding.UTF8.GetBytes(xml.OuterXml);        
    File.Delete("MyFile.xml");

    return Encoding.UTF8.GetString(bytes);

}

解决方案 #2（更好、更简单、更优雅的解决方案！）： 使用StringWriter，但使用其Encoding 属性将其设置为UTF-8 ：

public static string SerializeObject<T>(this T value)
{
    var serializer = new XmlSerializer(typeof(T));           
    var settings = new XmlWriterSettings
                   {
                    Encoding = new UTF8Encoding(true), 
                    Indent = false, 
                    OmitXmlDeclaration = false,
                    NewLineHandling = NewLineHandling.None
                   };

    using(var stringWriter = new UTF8StringWriter())
    {
        using(var xmlWriter = XmlWriter.Create(stringWriter, settings)) 
        {
            serializer.Serialize(xmlWriter, value);
        }

        return stringWriter.ToString();
    }
}

public class UTF8StringWriter : StringWriter
{
    public override Encoding Encoding
    {
        get
        {
            return Encoding.UTF8;
        }
    }
}

Answer 6

如果您不想使用派生自 StringWriter 的类，那么在您的情况下，您可以简单地将 OmitXmlDeclaration 设置为 false 并声明您自己的，就像我在下面做的那样：

 public static string Serialize<T>(this T value, string xmlDeclaration = "<?xml version=\"1.0\"?>") where T : class, new()
        {
            if (value == null) return string.Empty;

            using (var stringWriter = new StringWriter())
            {
                var settings = new XmlWriterSettings
                {
                    Indent = true,
                    OmitXmlDeclaration = xmlDeclaration != null,
                };

                using (var xmlWriter = XmlWriter.Create(stringWriter, settings))
                {
                    var xmlSerializer = new XmlSerializer(typeof(T));

                    xmlSerializer.Serialize(xmlWriter, value);

                    var sb = new StringBuilder($"{Environment.NewLine}{stringWriter}");

                    sb.Insert(0, xmlDeclaration);

                    return sb.ToString();
                }
            }