//引入命名空间
using System;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using System.Xml;
using System.Xml.Linq;
public class MainClass {
    public static void Main() {
        XElement firstParticipant;
        XDocument xDocument =  new XDocument(
         new XDeclaration("1.0", "UTF-8", "yes"),
         new XDocumentType("Books", null, "Books.dtd", null),
         new XProcessingInstruction("Book", "out-of-print"),
       new XElement("Books", firstParticipant =
         new XElement("Book",
         new XComment("This is a new author."),
         new XProcessingInstruction("AuthorHandler", "new"),
         new XAttribute("type", "Author"),
         new XElement("FirstName", "J"),
         new XElement("LastName", "R")),
         new XElement("Book",
         new XAttribute("type", "Author"),
         new XElement("FirstName", "E"),
         new XElement("LastName", "B"))));
        foreach (XNode node in firstParticipant.Nodes().OfType<XComment>()) {
            Console.WriteLine(node);
        }
    }
}