XPath & Namespaces

Thursday, April 3, 2003

c# xml xpath

I'm sure many out there know this or figured it out eons ago, but it was new to me and took awhile to figure out.

Say you have a RDF style XML document that looks something like:

<?xml version="1.0" encoding="iso-8859-1" ?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:sy="http://purl.org/rss/1.0/modules/syndication/" xmlns:admin="http://webns.net/mvcb/" xmlns="http://purl.org/rss/1.0/">
<channel rdf:about="http://someplace.com/weblog/">
<title>News from Bob</title>
<link>http:// someplace.com/weblog/</link>
<description></description>
<dc:language>en-us</dc:language>
<dc:creator>Bob</dc:creator>
<dc:rights>Copyright 2003 Bob</dc:rights>
<dc:date>2003-03-19T22:30:34-07:00</dc:date>
<admin:generatorAgent rdf:resource="http://www.movabletype.org/?v=2.51" />
<admin:errorReportsTo rdf:resource="mailto:tbob@somewhere.com" />
<sy:updatePeriod>hourly</sy:updatePeriod>
<sy:updateFrequency>1</sy:updateFrequency>
</channel>
<item rdf:about="http://somewhere/weblog/archives/2003/03/19.shtml#how_i_really_feel_about_cheese">
<title>How I really feel about Cheese</title>
<description><p>
I was asked in a comment why I really don't like cheese...</description>
</item>
</rdf:RDF>

Simplistically I thought the following code would work:

XmlNode dateNode = rdfXml.SelectSingleNode("/RDF/channel/date");

But of course it returns Null. So I tried the following:

XmlNode dateNode = rdfXml.SelectSingleNode("/rdf:RDF/channel/dc:date");

Of course this returned Null as well. So it was time to turn to the help file, which didn’t help much. My next best friend is Google, and Google Groups didn’t let me down. Turns out SelectSingleNode is overloaded and takes a XmlNamespaceManager as the second parameter. I now had this:

nsMgr = new XmlNamespaceManager(rdfXml.NameTable);

nsMgr.AddNamespace("rdf", "http://www.w3.org/1999/02/22-rdf-syntax-ns#");

nsMgr.AddNamespace("dc", "http://purl.org/dc/elements/1.1/");

XmlNode dateNode = rdfXml.SelectSingleNode("/rdf:RDF/channel/dc:date", nsMgr);

However, I didn’t know what to do with the default namespace. A little more research lead me to the nugget that there is no such thing as a default namespace in XPath, so all I has to do was add the default namespace with the prefix of my choosing.

nsMgr = new XmlNamespaceManager(feedXml.NameTable);

nsMgr.AddNamespace("rdf", "http://www.w3.org/1999/02/22-rdf-syntax-ns#");

nsMgr.AddNamespace("dc", "http://purl.org/dc/elements/1.1/");

nsMgr.AddNamespace("rss", "http://purl.org/rss/1.0/");

XmlNode dateNode = rdfXml.SelectSingleNode("/rdf:RDF/rss:channel/dc:date", nsMgr);

Now everything works like it should.

Full example (.Net 2.0):

using System.Xml;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            XmlDocument rdfXml = new XmlDocument();
            rdfXml.Load("data.xml");
            XmlNamespaceManager nsMgr = new XmlNamespaceManager(rdfXml.NameTable);
            nsMgr.AddNamespace("rdf", "http://www.w3.org/1999/02/22-rdf-syntax-ns#");
            nsMgr.AddNamespace("dc", "http://purl.org/dc/elements/1.1/");
            nsMgr.AddNamespace("rss", "http://purl.org/rss/1.0/");

            XmlNode dateNode = rdfXml.SelectSingleNode("/rdf:RDF/rss:channel/dc:date", nsMgr);
        }
    }
}

43 Comments

Thank you. I find this very userful.

David - Monday, March 29, 2004 2:28:00 PM

Just what I needed... thanks very much!

Dave - Thursday, June 10, 2004 9:31:00 AM

YES!!! Thank you very much!

PJ - Tuesday, June 13, 2006 10:43:18 AM

Adds the given namespace to the collection.
[Visual Basic]
Public Overridable Sub AddNamespace( _
  ByVal prefix As String, _
  ByVal uri As String _
)
[C#]
public virtual void AddNamespace(
  string prefix,
  string uri
);
Parameters
prefix:
The prefix to associate with the namespace being added. Use String.Empty to add a default namespace.
However, if the XmlNamespaceManager will be used for resolving namespaces in an XPath expression, a prefix must be specified. If an XPath expression does not include a prefix, it is assumed that the namespace URI is the empty namespace. For more information about XPath expressions and the XmlNamespaceManager, refer to the XmlNode.SelectNodes and XPathExpression.SetContext methods.
uri:
The namespace to add.

Kiran Gudupalli - Monday, July 31, 2006 6:30:06 PM

Thank you very much. This is really help full.
Use String.Empty as First paramenter in AddNamespace method to add a default namespace.

Kiran Gudupalli - Monday, July 31, 2006 6:32:20 PM

wow, thx. that really helped.

Joern - Wednesday, September 6, 2006 9:56:30 AM

Exactly what I needed. Thanks so much

Nathan Rose - Thursday, December 21, 2006 2:42:22 AM

thanks, really helped.

Clint - Thursday, March 29, 2007 7:07:22 PM

Thanks!!! Nice, simple explanation - exactly what I was looking for.

Tanya Z - Thursday, September 13, 2007 5:33:53 PM

Thanks a lot for sharing this.

Proved very useful.

Sheron

Sheron - Thursday, September 20, 2007 4:18:56 AM

Thanks for sharing!  Google is also my best friend.  Cheers!

Ramir Borja - Monday, October 22, 2007 9:29:41 AM

Excellent - Google now links directly to this so I didn't have to search as much as you all those years ago!

Robin Massart - Tuesday, November 13, 2007 12:44:48 PM

Excellent!! Thanks alot...

This is exactly what I wanted & it has become my one stop look up for XPath isssue..

Thank you...

Sandeep - Wednesday, December 5, 2007 9:50:53 AM

Great artilce it saved me a lot of time :)))

Lukas F - Wednesday, February 20, 2008 4:36:11 PM

Thanks, it really helps.

Bruce Li - Wednesday, March 12, 2008 10:23:44 AM

Awesome, really clear. I was scatching my head over this one. Thanks for sharing.

Bob Smith - Monday, March 17, 2008 1:58:39 PM

All woking like a dream! Thanks for that.

Phil - Monday, April 21, 2008 1:21:22 PM

Awesome - sorted my problem in a minute,

Cheers!

Mike - Thursday, February 5, 2009 9:55:29 AM

Imran,

You still need to create a namespace manager with the appropriate uri.

var xmlDOM = xmlhttp.responseXML;
var nsMgr = new XmlNamespaceManager(xmlDOM.NameTable);
nsMgr.AddNamespace("wfs", "http://correct.uri");
var tasNodes = xmlDOM.selectNodes('/wfs:FeatureCollection/gml:featureMembers/topp:tasmania_water_bodies', nsMgr );

Wayne Allen - Tuesday, June 23, 2009 9:50:00 PM

duh!! now it doesnt recognize XmlNamespaceManager

XmlNamespaceManager undefined!!!

maaaan this is so annoying :(

Imran - Wednesday, August 12, 2009 5:15:50 AM

Imran,
If you can supply more information I could help more.

Wayne Allen - Wednesday, August 12, 2009 10:44:50 PM

Awesome, thanks for sharing. Exactly what i wanted

Ravi Yellasiri - Wednesday, October 7, 2009 1:04:46 AM

awesome, thanks a lot

maingi4 - Tuesday, November 17, 2009 6:03:59 AM

Thanks! helped a lot

Stefan Gustafsson - Friday, December 4, 2009 8:13:04 AM

Hi Guys, I see some of you are Pro.
In my case i am new to this and don't where to put this code:

nsMgr = new XmlNamespaceManager(feedXml.NameTable);

nsMgr.AddNamespace("rdf", "http://www.w3.org/1999/02/22-rdf-syntax-ns#");

nsMgr.AddNamespace("dc", "http://purl.org/dc/elements/1.1/");

XmlNode dateNode = rdfXml.SelectSingleNode("/rdf:RDF/channel/dc:date", nsMgr);

I know in my case would look a little bit different but i believe, something is missing here. Please try to be more specific.
PS: Do i have to write the code in the "code behind" - page?

Thank you !

Tom J. - Saturday, April 24, 2010 12:44:32 AM

niachipal -

Yes this would go in your code-behind for an aspx page as part of loading and parsing an XML document with namespaces. I.e. Bob, the "dc" part is an XML namespace. See: http://en.wikipedia.org/wiki/XML_namespace

Wayne Allen - Friday, April 30, 2010 4:26:14 PM

Why the heck is this not documented!
Brilliant and thanks

Mark S - Sunday, July 11, 2010 10:33:19 AM

Hey Wayne, thanks for this article. It really helps me with the namespace.

Newb - Friday, August 20, 2010 6:36:33 AM

Thanks! A co-worker was having a similar problem with some XML and I was able to fix it by clearing the namespace.

Andrew - Tuesday, October 5, 2010 11:17:45 AM

Work fine..This is Good article and very rare..

siva - Thursday, December 2, 2010 11:29:26 AM

Thanks!, in 2011 this article is still update date!

Bas - Tuesday, March 22, 2011 2:50:33 PM

Thanks, especially for the bit about handling the "default" namespace.

Phil - Monday, July 25, 2011 7:44:15 PM

nsMgr = new XmlNamespaceManager(feedXml.NameTable);

what is feedXml ?

Wesley isaac - Friday, August 19, 2011 6:59:34 AM

@Wesley - Whoops, after all these years you are the fist to catch this. It should have been rdfXml - fixed.

Wayne Allen - Friday, August 19, 2011 3:47:09 PM

This should be the first search result when looking for reading from Default Name Spaces. This solved my issues with the so called default name spaces thanks!

Alex - Monday, September 5, 2011 10:55:11 AM

I appreciate your so much, can't describe how much time it took me to figure out "Default Namespace"

Karthik - Friday, December 9, 2011 1:38:23 AM

Thanks.

And yeah, whoever designed XML/namespaces/xpath this way should be shot

MH - Monday, February 6, 2012 4:24:21 PM

thanks, it's solved my problem....

bhagya - Thursday, February 16, 2012 1:55:55 PM

thanks, i used it for rubys libxml

Uwe - Thursday, April 19, 2012 8:28:30 AM

Thanks for this, it helped a lot!

OBrady - Thursday, August 23, 2012 1:34:29 AM

Thanks, very useful!

DR - Thursday, September 27, 2012 11:06:33 AM

Exactly what I was looking for. Thanks a lot.

ruperth - Tuesday, October 2, 2012 6:00:13 PM

This was exactly what I needed. I loved your explanations and code.Thank you!

Anna - Tuesday, February 5, 2013 9:46:07 PM

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