How to use XPath to search an element with namespace

XPathWithNamespace.cs

XDocument d = XDocument.Parse("<p:root xmlns:p='P'><p:e/></p:root>");

XmlNamespaceManager r = new XmlNamespaceManager(new NameTable());

r.AddNamespace("p", "P");

XElement e = d.XPathSelectElement("//p:e", r);

Console.WriteLine(e);