SelectSingleNode always returns null?

c# xml xpath namespaces selectsinglenode
bigtv picture bigtv · Nov 19, 2009 · Viewed 7.5k times · Source

Taking this simplifed example of my XML:

<?xml version="1.0"?>
<message xmlns="http://www.mydomain.com/MyDataFeed" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.mydomain.com/MyDataFeed https://secure.mydomain/MyDataFeed/myDataFeed.xsd" requestId="13898" status="1">
<error>Invalid Login</error>
</message>

I am trying to select the 'error' node using SelectSingleNode method, however using the following code it always returns NULL? 

XmlNode errorNode = oss.SelectSingleNode("/message/error");
if (errorNode != null)
     Console.Writeline("There is an error");

From research I have done this is related to namespaces but I simply can't get anything to work. Any advice?

Answer

marc_s picture marc_s · Nov 19, 2009

You're missing the XML namespace defined by the <message> node in your SelectSingleNode call. Assuming oss is an XmlDocument instance, you need to do this:

XmlNamespaceManager nsMgr = new XmlNamespaceManager(oss.NameTable);
nsMgr.AddNamespace("ns", "http://www.mydomain.com/MyDataFeed");

XmlNode errorNode = oss.SelectSingleNode("/ns:message/ns:error", nsMgr);

Marc

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