如何加载内联DTD以用于XDocument？

using System;
using System.Xml.Linq;

class Program
{
    static void Main(string[] args)
    {
        string xml = @"<?xml version=""1.0""?>
                        <!DOCTYPE root [
                            <!ELEMENT root (child)>
                            <!ELEMENT child (#PCDATA)>
                        ]>
                        <root>
                           <child>Hello, World!</child>
                        </root>";

        XDocument xdoc = XDocument.Parse(xml);
        XElement root = xdoc.Root;
        foreach (XElement child in root.Elements())
        {
            Console.WriteLine(child.Value);
        }
    }
}