using System;
using System.Collections.Generic;
using System.Linq;
using System.Xml.Linq;

class Program
{
    private class ChangedSemaphore { }

    private static EventHandler<XObjectChangeEventArgs> ElementChanged =
        new EventHandler<XObjectChangeEventArgs>(Program.ElementChangedHandler);

    private static void ElementChangedHandler(object sender, XObjectChangeEventArgs e)
    {
        XObject xSender = (XObject)sender;
        XDocument xDocument = xSender.Document;

        // Sometimes while moving a node, we may receive an event for a node that has been
        // removed from its parent (and therefore its document).  This removed node will receive
        // an event, and its parent document will be null.  In this case, we don't care
        // about removing the event handler and adding an annotation.  The document will receive
        // another event for the parent of the removed node, which will cause the semaphore
        // annotation to be added to the parent XDocument.
        if (xDocument != null)
        {
            xDocument.Changing -= ElementChanged;
            xDocument.Changed -= ElementChanged;
            xDocument.AddAnnotation(new ChangedSemaphore());
        }
    }

    static void Main(string[] args)
    {
        // load a very large, complicated XDocument for which we want to track change status.
        XDocument doc = XDocument.Parse(
          @"<?xml version='1.0' encoding='utf-8' ?>
            <Root>
              <Child>1</Child>
              <Child>2</Child>
              <Child>3</Child>
            </Root>");

        // add event handlers to the document so that any changes will be noted
        doc.Changing += ElementChanged;
        doc.Changed += ElementChanged;

        //
        //
        //
        // execute a bunch of code that may or may not change the XDocument
        // (in this case, we'll not change the XDocument)
        //
        //
        //

        if (doc.Annotation<ChangedSemaphore>() != null)
            Console.WriteLine("XDocument has changed");
        else
            Console.WriteLine("XDocument has not changed");

        //
        //
        //
        // execute a bunch of code that may or may not change the XDocument
        // (in this case, we'll change the value of the first element)
        doc.Root.Element("Child").Value = "5";
        //
        //
        //

        if (doc.Annotation<ChangedSemaphore>() != null)
            Console.WriteLine("XDocument has changed");
        else
            Console.WriteLine("XDocument has not changed");
    }
}