using System;

using System.Collections.Generic;

using System.Text;

using System.Linq;

using System.Xml.Serialization;

using System.Xml;

using System.Xml.Linq;

using System.Xml.XPath;

using System.IO;

using System.Data;

using System.Runtime.Serialization;

using System.Runtime.Serialization.Formatters.Binary;

using System.IO.Compression;

​

using NUnit.Framework; //NUnit.Framework.AssertionException

​

//https://stackoverflow.com/questions/63406761/selectnodes-in-xml-returns-empty#63406761

​

#if false

You have a three problems here: 1) the xpath query should be `xpath="WellBore/MetaData/Data_Set"` or `xpath="/WELL_SET/WellBore/MetaData/Data_Set"` not `xpath="/WellBore/MetaData/Data_Set"`.  The leading `/` means that the xpath is absolute and should start with the root element name - but the root node is named `<WELL_SET>` not `<WellBore>`.  2) To get the text value of the `<Data_Set>` node use `path.InnerText`.  3) `</Metadata>` should be `</MetaData>`.  See: https://dotnetfiddle.net/WJaQaK.

#endif

    public class TestClass

    {

        const string defaultPrefix = "default";

        public static void Test()

        {

            foreach (var xpathXml in GetXpathXml())

            {

                var xpathDoc = XDocument.Parse(xpathXml); // XDocument.Load(FILENAME);

                var dataDoc = XDocument.Parse(GetDataXml()); //XDocument.Load(XMLFILE);

​

                var root = dataDoc.Root;

​

                // Add all named namespaces in the root element to the XmlNamespaceManager, using their prefixes, 

                // or "default" for the default namespace, if present.

                var nsmgr = new XmlNamespaceManager(new NameTable());

                foreach (var attr in root.Attributes().Where(a => a.IsNamespaceDeclaration))

                    nsmgr.AddNamespace(attr.Name == "xmlns" ? defaultPrefix : attr.Name.LocalName, attr.Value);

​

                Console.WriteLine("\nQuerying data document: ");

                Console.WriteLine(dataDoc);

                Console.WriteLine("Using Xpath document:");

                Console.WriteLine(xpathDoc);

                var dt = new DataTable();

                foreach (var col in xpathDoc.Descendants("mapping").Select(x => (string)x.Attribute("column")).Distinct())

                {

                    dt.Columns.Add(col, typeof(string));

                }

​

                foreach (var record in xpathDoc.Descendants("mappings"))

                {

                    var row = dt.Rows.Add();

                    foreach (var field in record.Elements("mapping"))

                    {

                        var xpath = (string)field.Attribute("xpath");

                        if (!string.IsNullOrEmpty(xpath))

                        {

                            foreach (var path in root.XPathSelectElements(xpath,nsmgr))

                            {

                                var s = path.Value;

                                // Process the value somehow

                                Console.WriteLine("Found: {0}", s);

                            }

                        }                

                    }

                }   

            }

        }

        static string GetDataXml() => 

            @"<WELL_SET xmlns:xsd=""http://www.w3.org/2001/XMLSchema"" xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns="""">

  <WellBore>

     <MetaData>

      <Data_Set>TXRRC</Data_Set>

     </MetaData>

  </WellBore>

</WELL_SET>";

        static IEnumerable<string> GetXpathXml() =>

            new []

            {

            @"<mappings>

        <mapping column=""SOURCE"" xpath=""/WELL_SET/WellBore/MetaData/Data_Set""/>

</mappings>",

            @"<mappings>

        <mapping column=""SOURCE"" xpath=""WellBore/MetaData/Data_Set""/>

</mappings>",

            @"<mappings>

        <mapping column=""SOURCE"" xpath=""/WellBore/MetaData/Data_Set""/>

</mappings>",

            };

    }

​

    public static class XmlSerializationHelper

    {

        public static string GetOuterXml(this XmlNode node, bool indent = true)

        {

            if (node == null)

                return null;

            using (var textWriter = new StringWriter())

            {

                var settings = new XmlWriterSettings

                {

                    Indent = indent,

                    OmitXmlDeclaration = true,

                };

                using (var xmlWriter = XmlWriter.Create(textWriter, settings))

                {

                    node.WriteTo(xmlWriter);

                }

                return textWriter.ToString();

            }

        }

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Roslyn 2.0 Compiler; Environment version: " + Environment.Version);

            Console.WriteLine();

​

            try

            {

                TestClass.Test();

            }

            catch (Exception ex)

            {

                Console.WriteLine("Uncaught exception: ");

                Console.WriteLine(ex);

                throw;

            }

        }

    }

​

​