>>https://msdn.microsoft.com/en-us/library/system.xml.xmlnamespacemanager.hasnamespace(v=vs.110).aspx ?
>
>I thought about that but it seems that one is what we have loaded in the XML namespace manager and not as per the XML.
>
>Thinking of it, I do not think it is possible. The only way to resolve this is to trap it in the code. For example, once the namespace loaded, I will look up for a specific node. If that does not work, it would then be either the node does not exist or the namespace was not properly defined.

Ha. Misread your requirements - thought you just wanted to check whether it was already defined. If you want to populate the NameTable with all of the namespaces used in a specific piece of XML I came across this approach:

 XmlDocument doc = new XmlDocument();
 doc.LoadXml(*whatever*);
XmlNodeList nameSpaceList = doc.SelectNodes(@"//namespace::*[not(. = ../../namespace::*)]");
 var _xmlNSmgr = new XmlNamespaceManager(doc.NameTable);
foreach (XmlNode nsNode in nameSpaceList)
            {
                _xmlNSmgr.AddNamespace(nsNode.LocalName, nsNode.Value);
            }

Only made a quick test but it seems to work......