Serializing an object as UTF-8 XML in .NET

Proper object disposal removed for brevity but I'm shocked if this is the simplest way to encode an object as UTF-8 in memory. There has to be an easier way doesn't there?

var serializer = new XmlSerializer(typeof(SomeSerializableObject));


var memoryStream = new MemoryStream();
var streamWriter = new StreamWriter(memoryStream, System.Text.Encoding.UTF8);


serializer.Serialize(streamWriter, entry);


memoryStream.Seek(0, SeekOrigin.Begin);
var streamReader = new StreamReader(memoryStream, System.Text.Encoding.UTF8);
var utf8EncodedXml = streamReader.ReadToEnd();
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No, you can use a StringWriter to get rid of the intermediate MemoryStream. However, to force it into XML you need to use a StringWriter which overrides the Encoding property:

public class Utf8StringWriter : StringWriter
{
public override Encoding Encoding => Encoding.UTF8;
}

Or if you're not using C# 6 yet:

public class Utf8StringWriter : StringWriter
{
public override Encoding Encoding { get { return Encoding.UTF8; } }
}

Then:

var serializer = new XmlSerializer(typeof(SomeSerializableObject));
string utf8;
using (StringWriter writer = new Utf8StringWriter())
{
serializer.Serialize(writer, entry);
utf8 = writer.ToString();
}

Obviously you can make Utf8StringWriter into a more general class which accepts any encoding in its constructor - but in my experience UTF-8 is by far the most commonly required "custom" encoding for a StringWriter :)

Now as Jon Hanna says, this will still be UTF-16 internally, but presumably you're going to pass it to something else at some point, to convert it into binary data... at that point you can use the above string, convert it into UTF-8 bytes, and all will be well - because the XML declaration will specify "utf-8" as the encoding.

EDIT: A short but complete example to show this working:

using System;
using System.Text;
using System.IO;
using System.Xml.Serialization;


public class Test
{
public int X { get; set; }


static void Main()
{
Test t = new Test();
var serializer = new XmlSerializer(typeof(Test));
string utf8;
using (StringWriter writer = new Utf8StringWriter())
{
serializer.Serialize(writer, t);
utf8 = writer.ToString();
}
Console.WriteLine(utf8);
}




public class Utf8StringWriter : StringWriter
{
public override Encoding Encoding => Encoding.UTF8;
}
}

Result:

<?xml version="1.0" encoding="utf-8"?>
<Test xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<X>0</X>
</Test>

Note the declared encoding of "utf-8" which is what we wanted, I believe.

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最佳答案

Your code doesn't get the UTF-8 into memory as you read it back into a string again, so its no longer in UTF-8, but back in UTF-16 (though ideally its best to consider strings at a higher level than any encoding, except when forced to do so).

To get the actual UTF-8 octets you could use:

var serializer = new XmlSerializer(typeof(SomeSerializableObject));


var memoryStream = new MemoryStream();
var streamWriter = new StreamWriter(memoryStream, System.Text.Encoding.UTF8);


serializer.Serialize(streamWriter, entry);


byte[] utf8EncodedXml = memoryStream.ToArray();

I've left out the same disposal you've left. I slightly favour the following (with normal disposal left in):

var serializer = new XmlSerializer(typeof(SomeSerializableObject));
using(var memStm = new MemoryStream())
using(var  xw = XmlWriter.Create(memStm))
{
serializer.Serialize(xw, entry);
var utf8 = memStm.ToArray();
}

Which is much the same amount of complexity, but does show that at every stage there is a reasonable choice to do something else, the most pressing of which is to serialise to somewhere other than to memory, such as to a file, TCP/IP stream, database, etc. All in all, it's not really that verbose.

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I found this blog post which explains the problem very well, and defines a few different solutions:

(dead link removed)

I've settled for the idea that the best way to do it is to completely omit the XML declaration when in memory. It actually is UTF-16 at that point anyway, but the XML declaration doesn't seem meaningful until it has been written to a file with a particular encoding; and even then the declaration is not required. It doesn't seem to break deserialization, at least.

As @Jon Hanna mentions, this can be done with an XmlWriter created like this:

XmlWriter writer = XmlWriter.Create (output, new XmlWriterSettings() { OmitXmlDeclaration = true });
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Very good answer using inheritance, just remember to override the initializer

public class Utf8StringWriter : StringWriter
{
public Utf8StringWriter(StringBuilder sb) : base (sb)
{
}
public override Encoding Encoding { get { return Encoding.UTF8; } }
}

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