XML serializing with XmlWriter via StringBuilder is utf-16 while via Stream is utf-8?

When you create an XmlWriter around a TextWriter, the XmlWriter always uses the encoding of the underlying TextWriter. The encoding of a StringWriter is always UTF-16, since that's how .NET strings are encoded internally.

When you create an XmlWriter around a Stream, there is no encoding defined for the Stream, so it uses the encoding specified in the XmlWriterSettings.

The most elegant solution for me is to write to a memorystream and then using encoding to encode the stream to whatever encoding is required.like so

        using (MemoryStream memS = new MemoryStream())        {            //set up the xml settings            XmlWriterSettings settings = new XmlWriterSettings();            settings.OmitXmlDeclaration = OmitXmlHeader;            using (XmlWriter writer = XmlTextWriter.Create(memS, settings))            {                //write the XML to a stream                xmlSerializer.Serialize(writer, objectToSerialize);                writer.Close();            }            //encode the memory stream to xml            retString.AppendFormat("{0}", encoding.GetString(memS.ToArray()));            memS.Close();        }

where the encoding takes place at ....encoding.GetString(memS.ToArray())...

Where possible, the XmlWriter uses the encoding of the underlying stream. It it wrote UTF-8 data to a stream it knew was UTF-16, you'd end up with a mess. Writing UTF-16 data to a UTF-8 stream also causes problems, especially for environments that use null terminated strings (like C/C++).

The StringBuilder/StringWriter presents a UTF-16 stream to the XmlWriter, so the XmlWriter ignores your requested setting and uses that.

In practise I usually don't emit the header, that way I can use a StringBuilder underneath and save a few lines of code messing about with switching encodings.