C#: XPath validation

Question

I am working on a project and I need to extend the XPath validation. This is based on already existing code (that somebody else wrote) and apparently comes from the following code project:

http://www.codeproject.com/script/Content/ViewAssociatedFile.aspx?rzp=%2FKB%2FXML%2FXPathEditor%2F%2Fxpatheditor_src.zip&zep=XPathEditor%2Fexpressionvalidator.cs&obid=11150&obtid=2&ovid=2

What I understand is that here they make use of an XML document strSample (line 378) to validate the XPath expression. I now need to extend this so that expression like /caseFileModel/DateToResearch + xs:dayTimeDuration('P30s') also can be validated. When I now try to parse such an expression I get the following error:

"Namespace Manager or XsltContext needed. This query has a prefix, variable, or user-defined function."

I tried using a Namespace manager:

XmlNamespaceManager nsmgr = new XmlNamespaceManager(xmlDoc.NameTable);
nsmgr.AddNamespace("xs", "http://www.w3.org/2005/xpath-functions");

and replaced line 252 with

case XPathResultType.Number: strResult = string.Format("{0}", xpathNav.Evaluate(expression, nsmgr)); 
break;

but then I got another error:

"XsltContext is needed for this query because of an unknown function."

So my question is whether somebody can explain me how the validation happens and how I can extend it?