MySyndicationFeed feed = new MySyndicationFeed("Test Feed", "This is a test feed", new Uri("http://Contoso/testfeed"), "TestFeedID", DateTime.Now);
SyndicationItem item = new SyndicationItem("Test Item", "This is the content for Test Item", new Uri("http://localhost/ItemOne"), "TestItemID", DateTime.Now);

item.Links.Add(new SyndicationLink(new Uri("http://someuri"), "alternate", "MyItemLink", "text/html", 100));
item.PublishDate = new DateTime(1968, 2, 23);
item.LastUpdatedTime = DateTime.Today;
item.SourceFeed = feed;
item.Summary = new TextSyndicationContent("This the item summary");

List<SyndicationItem> items = new List<SyndicationItem>();
items.Add(item);
feed.Items = items;

XmlWriter atomWriter = XmlWriter.Create("Atom.xml");
Atom10FeedFormatter<MySyndicationFeed> atomFormatter = new Atom10FeedFormatter<MySyndicationFeed>(feed);
atomFormatter.WriteTo(atomWriter);
atomWriter.Close();