在以下的例子里，红色部分就是自己定义的，好象兰色部分不可以自己定义．具体的我也不是很明白，因为也是第一次遇到这样的问题．
using System;
using System.IO;
using System.Xml;

public class Sample
{
public static void Main()
{
// Create the XmlDocument.
XmlDocument doc = new XmlDocument();
doc.LoadXml("<!DOCTYPE sfsf [<!ENTITY h 'hardcover'>]>" +
"<book genre='novel' ISBN='1-861001-57-5'>" +
"<title>Pride And Prejudice</title>" +
"<style>&h;</style>" +
"</book>");

// Display information on the DocumentType node.
XmlDocumentType doctype = doc.DocumentType;
Console.WriteLine("Name of the document type: {0}", doctype.Name);
Console.WriteLine("The internal subset of the document type: {0}", doctype.InternalSubset);

}
}