There's a known issue in .NET with the default XML namespace - contrary to what is defined in the XML standards, in .NET you cannot use a string.Empty
as the prefix - you need to use something else.
Try this:
XPathNavigator nav = xmlDoc.CreateNavigator();
XmlNamespaceManager nsMgr = new XmlNamespaceManager(nav.NameTable);
nsMgr.AddNamespace("def", @"http://www.w3.org/2005/Atom"); <== Give this a prefix!
nsMgr.AddNamespace("dxp", @"http://schemas.google.com/analytics/2009");
nsMgr.AddNamespace("openSearch", @"http://a9.com/-/spec/opensearch/1.1/");
XmlNodeList nodeList = xmlDoc.SelectNodes("/def:feed/def:entry", nsMgr);
Now, this list does have two nodes in it - right?