为什么LINQ to XML Query不工作(Amazon S3)

Xml名称空间:

    XNamespace ns = "http://s3.amazonaws.com/doc/2006-03-01/";
    var contents = from content in doc.Descendants(ns + "Contents")
                   select new { Key = content.Element(ns + "Key").Value,
                       ETag = content.Element(ns + "ETag").Value };