问题

I'm trying to parse an Atom feed programmatically. I have the atom XML downloaded as a string. I can load the XML into an XmlDocument. However, I can't traverse the document using XPath. Whenever I try, I get null.

I've been using this Atom feed as a test: http://steve-yegge.blogspot.com/feeds/posts/default

Calling SelectSingleNode() always returns null, except for when I use "/". Here is what I'm trying right now:

using (WebClient wc = new WebClient())
{
    string xml = wc.DownloadString("http://steve-yegge.blogspot.com/feeds/posts/default");
    XmlNamespaceManager nsMngr = new XmlNamespaceManager(new NameTable());
    nsMngr.AddNamespace(string.Empty, "http://www.w3.org/2005/Atom");
    nsMngr.AddNamespace("app", "http://purl.org/atom/app#");
    XmlDocument atom = new XmlDocument();
    atom.LoadXml(xml);
    XmlNode node = atom.SelectSingleNode("//entry/link/app:edited", nsMngr);
}

I thought it might have been because of my XPath, so I've also tried a simple query of the root node since I knew the root should work:

// I've tried both with & without the nsMngr declared above
XmlNode node = atom.SelectSingleNode("/feed");

No matter what I do, it seems like it can't select anything. Obviously I'm missing something, I just can't figure out what. What is it that I need to do in order to make XPath work on this Atom feed?

EDIT

Although this question has an answer, I found out this question has an almost exact duplicate: SelectNodes not working on stackoverflow feed

回答1:

While the C# implementation may allow default namespaces (I don't know), the XPath 1.0 spec doesn't. So, give "Atom" its own prefix:

nsMngr.AddNamespace("atom", "http://www.w3.org/2005/Atom");

And change your XPath appropriately:

XmlNode node = atom.SelectSingleNode("//atom:entry/atom:link/app:edited", nsMngr);

回答2:

Load XML from a string and lookup for any 'Errors/Error' nodes.

XmlDocument xmlDoc = new XmlDocument();
xmlDoc.LoadXml(xmlResult);            
XmlNamespaceManager nm = new XmlNamespaceManager(xmlDoc.NameTable);
nm.AddNamespace("ns", "http://somedomain.com/namespace1/2"); //ns - any name, make sure it is same in the below line

XmlNodeList errors = xmlDoc.SelectNodes("/ns:*//ns:Errors/ns:Error", nm);

-Mathulan

来源：https://stackoverflow.com/questions/501171/no-nodes-selected-from-atom-xml-document-using-xpath

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