<?xml version="1.0"?>
<feed xmlns:media="http://search.yahoo.com/mrss/" 
      xmlns:gr="http://www.google.com/schemas/reader/atom/" 
   xmlns:idx="urn:atom-extension:indexing" 
   xmlns="http://www.w3.org/2005/Atom" 
   idx:index="no" 
   gr:dir="ltr">

 ...
   
  <entry gr:crawl-timestamp-msec="1316627782108">
    ...
    <gr:annotation>
      <content type="html">text-text-text</content>
      <author>
        <name>Vahid</name>
      </author>
    </gr:annotation>
 ...
  </entry>
  
    ...
  
</feed>

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

namespace Linq2Rss
{
    public class RssEntry
    {
        public string Title { set; get; }
        public string Description { set; get; }
        public string Link { set; get; }
        public DateTime PublicationDate { set; get; }
        public string Author { set; get; }
        public string BlogName { set; get; }
        public string BlogAddress { set; get; }
        public string Annotation { set; get; }
    }

    public static class AtomReader
    {
        private static string getAtomAnnotation(this SyndicationElementExtensionCollection items)
        {
            if (!items.Any()) return string.Empty;
            var item = items.Where(x => x.OuterName.ToLowerInvariant() == "annotation").FirstOrDefault();
            if (item == null) return string.Empty;

            var element = item.GetObject<XElement>();
            var content = element.Element("{http://www.w3.org/2005/Atom}content");
            return content == null ? string.Empty : content.Value;
        }

        public static IList<RssEntry> GetEntries(string feedUrl)
        {
            using (var reader = XmlReader.Create(feedUrl))
            {
                var feed = SyndicationFeed.Load(reader);
                if (feed == null) return null;

                return feed.Items.Select(x =>
                    new RssEntry
                    {
                        Title = x.Title.Text,
                        Author = x.Authors.Any() ? x.Authors.First().Name : string.Empty,
                        Description = x.Content == null ? string.Empty : ((TextSyndicationContent)x.Content).Text,
                        Link = x.Links.Any() ? x.Links.First().Uri.AbsoluteUri : string.Empty,
                        PublicationDate = x.PublishDate.UtcDateTime,
                        BlogName = x.SourceFeed.Title.Text,
                        BlogAddress = x.SourceFeed.Links.Any() ? x.SourceFeed.Links.First().Uri.AbsoluteUri : string.Empty,
                        Annotation = x.ElementExtensions.getAtomAnnotation()

                    }).ToList();
            }
        }
    }
}