XPath 1.0, which is what MS implements, does not have the idea of a default namespace. So try this:
XDocument xdoc = XDocument.Load(@"C:\SampleXML.xml");
XmlNamespaceManager xnm = new XmlNamespaceManager(new NameTable());
xnm.AddNamespace("x", "http://demo.com/2011/demo-schema");
Console.WriteLine(xdoc.XPathSelectElement("/x:Report/x:ReportInfo/x:Name", xnm) == null);