public class RssResult : ActionResult
{
    private readonly SyndicationFeed feed;
 
    public RssResult(SyndicationFeed feed)
    {
        this.feed = feed;   
    }
 
    public override void ExecuteResult(ControllerContext context)
    {
        context.HttpContext.Response.ContentType = "application/rss+xml";
 
        var formatter = new Rss20FeedFormatter(feed);
 
        using(var writer = new XmlTextWriter(context.HttpContext.Response.OutputStream, Encoding.UTF8))
        {
            formatter.WriteTo(writer);
        }
    }
}

....
public ActionResult Syndicate()
{
    var feed = new SyndicationFeed("Мой RSS канал", "MySiteName", new Uri("http://mysite.ru"))
    {
        Items = from p in DbData
                select new SyndicationItem(p.Title, p.Content, new Uri(String.Format("/posts/{0}", p.Canonical), UriKind.Relative))
                {
                    PublishDate = p.PostedOn
                }
    };
    return new RssResult(feed);
}