如何序列化XDocument对象？

using System;
using System.IO;
using System.Xml.Linq;
using System.Xml.Serialization;

public class Program
{
    public static void Main()
    {
        XDocument xdoc = new XDocument(
            new XElement("Root",
                new XElement("Child", "Content")
            )
        );

        XmlSerializer serializer = new XmlSerializer(typeof(XDocument));
        using StringWriter sw = new StringWriter();
        serializer.Serialize(sw, xdoc);
        string xmlString = sw.ToString();

        Console.WriteLine(xmlString);
    }
}

<?xml version="1.0" encoding="utf-16"?>
<Root>
 <Child>Content</Child>
</Root>

using System;
using System.IO;
using System.Runtime.Serialization;
using System.Xml.Linq;

public class Program
{
    public static void Main()
    {
        XDocument xdoc = new XDocument(
            new XElement("Root",
                new XElement("Child", "Content")
            )
        );

        DataContractSerializer serializer = new DataContractSerializer(typeof(XDocument));
        using MemoryStream ms = new MemoryStream();
        serializer.WriteObject(ms, xdoc);
        ms.Position = 0;

        using StreamReader sr = new StreamReader(ms);
        string xmlString = sr.ReadToEnd();

        Console.WriteLine(xmlString);
    }
}

<XDocument xmlns="http://schemas.datacontract.org/2004/07/System.Xml.Linq"><Declaration>
 <Version>1.0</Version>
 <Encoding>utf-8</Encoding>
 <Standalone>yes</Standalone>
</Declaration><Root>
 <Child>Content</Child>
</Root></XDocument>