XmlDocument.IsReadOnly Property

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 book [<!ENTITY h 'hardcover'>]>" +
                "<book genre='novel' ISBN='1-861001-57-5'>" +
                "<title>Pride And Prejudice</title>" +
                "<style>&h;</style>" +
                "</book>");

    //Determine whether the node is read-only.
    if (doc.DocumentElement.LastChild.FirstChild.IsReadOnly)
       Console.WriteLine("Entity reference nodes are always read-only");

  }
}