SelectSingleNode returning null for known good xml node path using XPath

Question

Consider this simple XML document  The serialized XML shown here is the result of an XmlSerializer from a complex POCO object whose schema I have no control over    lt My RootNode xmlns xsi  http   www w3 org 2001 XMLSchema-instance  xmlns xsd  http   www w3 org 2001 XMLSchema  xmlns    gt     lt id root  2 16 840 1 113883 3 51 1 1 1  extension  someIdentifier  xmlns  urn hl7-org v3    gt      lt creationTime xsi nil  true  xmlns  urn hl7-org v3    gt         lt  My RootNode gt    The goal is to extract the value of the extension attribute on the id node  In this case  we are using the SelectSingleNode method  and given an XPath expression as such   XmlNode idNode   myXmlDoc SelectSingleNode   My RootNode id      idNode is evaluated to null at this point in the debugger  string msgID   idNode Attributes GetNamedItem  extension   Value    The problem is that the SelectSingleNode method returns null for the given XPath expression   Question  any ideas on this XPath query s correctness  or why this method call   XPath expression would return a null value  Perhaps the namespaces are part of the problem

User · Accepted Answer

I strongly suspect the problem is to do with namespaces. Try getting rid of the namespace and you'll be fine - but obviously that won't help in your real case, where I'd assume the document is fixed.

I can't remember offhand how to specify a namespace in an XPath expression, but I'm sure that's the problem.

EDIT: Okay, I've remembered how to do it now. It's not terribly pleasant though - you need to create an XmlNamespaceManager for it. Here's some sample code that works with your sample document:

using System;
using System.Xml;

public class Test
{
    static void Main()
    {
        XmlDocument doc = new XmlDocument();
        XmlNamespaceManager namespaces = new XmlNamespaceManager(doc.NameTable);
        namespaces.AddNamespace("ns", "urn:hl7-org:v3");
        doc.Load("test.xml");
        XmlNode idNode = doc.SelectSingleNode("/My_RootNode/ns:id", namespaces);
        string msgID = idNode.Attributes["extension"].Value;
        Console.WriteLine(msgID);
    }
}

User · Answer

The rule to keep in mind is  if your document specifies a namespace  you MUST use an XmlNamespaceManager in your call to SelectNodes   or SelectSingleNode    That s a good thing    See the article Advantages of namespaces   Jon Skeet does a great job in his answer showing how to use XmlNamespaceManager   This answer should really just be a comment on that answer  but I don t quite have enough Rep Points to comment

User · Answer

just use   id instead of  id  It works fine in my code

User · Answer

Roisgoen s answer worked for me  but to make it more general  you can use a RegEx     Substitute  My RootNode  for whatever your root node is string strRegex      lt My RootNode   lt xmlns gt  s xmlns   s     gt      gt    var myMatch   new Regex strRegex  RegexOptions None  Match myXmlDoc InnerXml   if  myMatch Success        var grp   myMatch Groups  xmlns        if  grp Success                myXmlDoc InnerXml   myXmlDoc InnerXml Replace grp Value                 I fully admit that this is not a best-practice answer  but but it s an easy fix and sometimes that s all we need

User · Answer

This should work in your case without removing namespaces    XmlNode idNode   myXmlDoc GetElementsByTagName  id   0

User · Answer

Just to build upon solving the namespace issues  in my case I ve been running into documents with multiple namespaces and needed to handle namespaces properly   I wrote the function below to get a namespace manager to deal with any namespace in the document   private XmlNamespaceManager GetNameSpaceManager XmlDocument xDoc                XmlNamespaceManager nsm   new XmlNamespaceManager xDoc NameTable           XPathNavigator RootNode   xDoc CreateNavigator            RootNode MoveToFollowing XPathNodeType Element           IDictionary lt string  string gt  NameSpaces   RootNode GetNamespacesInScope XmlNamespaceScope All            foreach  KeyValuePair lt string  string gt  kvp in NameSpaces                        nsm AddNamespace kvp Key  kvp Value                      return nsm

User · Answer

Well    I had the same issue and it was a headache  Since I didn t care much about the namespace or the xml schema  I just deleted this data from my xml and it solved all my issues  May not be the best answer  Probably  but if you don t want to deal with all of this and you ONLY care about the data  and won t be using the xml for some other task  deleting the namespace may solve your problems   XmlDocument vinDoc   new XmlDocument    string vinInfo    your xml string   vinDoc LoadXml vinInfo    vinDoc InnerXml   vinDoc InnerXml Replace  xmlns   http   tempuri org

User · Answer

If you want to ignore namespaces completely  you can use this   static void Main string   args        string xml             lt My RootNode xmlns xsi   http   www w3 org 2001 XMLSchema-instance   xmlns xsd   http   www w3 org 2001 XMLSchema   xmlns      gt  n                  lt id root   2 16 840 1 113883 3 51 1 1 1   extension   someIdentifier   xmlns   urn hl7-org v3     gt  n                  lt creationTime xsi nil   true   xmlns   urn hl7-org v3     gt  n              lt  My RootNode gt         XmlDocument doc   new XmlDocument        doc LoadXml xml        XmlNode idNode   doc SelectSingleNode     local-name    My RootNode     local-name    id

User · Answer

Sorry  you forgot the namespace  You need   XmlNamespaceManager ns   new XmlNamespaceManager myXmlDoc NameTable   ns AddNamespace  hl7   urn hl7-org v3    XmlNode idNode   myXmlDoc SelectSingleNode   My RootNode hl7 id   ns       In fact  whether here or in web services  getting null back from an XPath operation or anything that depends on XPath usually indicates a problem with XML namespaces

[c#] SelectSingleNode returning null for known good xml node path using XPath

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