public class CompressFilter : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        HttpRequestBase request = filterContext.HttpContext.Request;
 
        // load encodings from header
        QValueList encodings = new QValueList(request.Headers["Accept-Encoding"]);
 
        // get the types we can handle, can be accepted and in the defined client preference
        QValue preferred = encodings.FindPreferred("gzip", "deflate", "identity");
 
        // if none of the preferred values were found, but the
        // client can accept wildcard encodings, we'll default
        // to Gzip.
        if (preferred.IsEmpty && encodings.AcceptWildcard && encodings.Find("gzip").IsEmpty)
 
            preferred = new QValue("gzip");
 
        HttpResponseBase response = filterContext.HttpContext.Response;
        // handle the preferred encoding
        switch (preferred.Name)
        {
            case "gzip":
                response.AppendHeader("Content-encoding", "gzip");
                response.Filter = new GZipStream(response.Filter, CompressionMode.Compress);
                break;
 
            case "deflate":
                response.AppendHeader("Content-encoding", "deflate");
                response.Filter = new DeflateStream(response.Filter, CompressionMode.Compress);
                break;
 
            case "identity":
                break;
            default:
                break;
        }
    }
}

[AcceptVerbs(HttpVerbs.Get)]
[CompressFilter]
public ContentResult SiteMap()
{
    return new ContentResult
               {
                   Content = SyndicationHelper.GetAggregateSiteMap(),
                   ContentType = "application/xml; charset=UTF-8"
               };
}