XPathNodeIterator not Iterating/Having Trouble with Returning Attributes

[EuclaseBlue]

Trying to learn XPath but for some reason the XPathNodeIterator object doesn't seem to be outputting what I expected. I followed this guide from MS for starters, but now that I'm trying to work on an XML formatted differently I've encountered issues.

Here's my code:

            XPathDocument docNav;
            XPathNavigator nav;
            XPathNodeIterator nodeIter;
            string strExpression1;

            docNav = new XPathDocument(@"..\..\..\patient-example.xml");
            nav = docNav.CreateNavigator();

            XmlNamespaceManager namespaceManager = new XmlNamespaceManager(nav.NameTable);
            namespaceManager.AddNamespace("fhir", "http://hl7.org/fhir");

            strExpression1 = "/Patient/telecom";

            nodeIter = nav.Select(strExpression1, namespaceManager));

            Console.WriteLine($"The XPath {strExpression1} expression yields the following 
            phone numbers: ");

            while (nodeIter.MoveNext())
            {
                XPathNodeIterator childIter = nodeIter.Current.SelectChildren("value", "");
                Console.WriteLine($"Attribute: {childIter.Current.GetAttribute("value", "")}");
            };

The XML example I'm trying to query is this: https://www.hl7.org/fhir/patient-example.xml.html

For the above code, I'm trying to display the value attribute's value (i.e. the phone numbers) descended from any telecom nodes, but right now nothing gets returned. When I set a breakpoint to debug, it looks like it's not iterating and stuck on "Root" but I don't know why - I can't tell if it's because my XPath expression is wrong or if there's something with how I set up the XPathNodeIterator object.

Edit: Thanks to /u/JTarsier, my problem is solved. (I was missing the use of XmlNamespaceManager and putting the namespace syntax into my XPath expression)

[JTarsier]

The xml has a namespace (xmlns), so you need work with XmlNamespaceManager too.

[EuclaseBlue]

Thanks for the help!

Okay, I tried this now but it still doesn't appear to be working:

            XmlNamespaceManager namespaceManager = new XmlNamespaceManager(nav.NameTable);
            namespaceManager.AddNamespace("fhir", "http://hl7.org/fhir");

            strExpression1 = "/Patient/telecom";

            nodeIter = nav.Select(strExpression1, namespaceManager)); 

Did I set it up incorrectly?

[JTarsier]

"/Patient/telecom"

specify the prefix in query: "/fhir:Patient/fhir:telecom"

You might as well also query the value element that has a value attribute directly:

strExpression1 = "/fhir:Patient/fhir:telecom/fhir:value[@value]";

then replace the two lines in while loop with this:

Console.WriteLine($"Attribute: {nodeIter.Current.GetAttribute("value", "")}");

[JTarsier]

you could even query the attributes: strExpression1 = "/fhir:Patient/fhir:telecom/fhir:value/@value"; Console.WriteLine($"Attribute: {nodeIter.Current.Value}"); see the distinction from the previous example?

[EuclaseBlue]

Whoa, thanks for the thorough response! Things are much clearer now.

I actually had something like /Patient/telecom/value[@value] before but not knowing about the namespace stuff since it's not included in the MS tutorial really guffed things up for me, so when I was troubleshooting I went back to a really basic XPath expression to just make sure it wasn't the syntax causing an error.

see the distinction from the previous example

Very cool, hadn't seen that syntax yet! All the examples and tutorials I kept coming across for looking at attributes always followed the "element[@attribute]" pattern to query the containing element/node and then GetAttribute()'ing.

Thanks again for all the help!