Read from xml files with or without a namespace using XmlDocument

Improving Vadim's Option1, but using XDocument (instead of XmlDocument) API and F# instead of C#:

let fileStream = File.Open(fileLocation, FileMode.Open)
let xDocument = XDocument.Load fileStream
let nsOpt =
    let nsString = xDocument.Root.Name.Namespace.ToString()
    if String.IsNullOrEmpty nsString then
        Console.Error.WriteLine "Warning: no namespace URL found in xmlns attrib"
        None
    else
        let nsManager = XmlNamespaceManager(NameTable())
        let nsPrefix = "x"
        nsManager.AddNamespace(nsPrefix, nsString)
        if nsString <> "http://schemas.sample.data.org/2005" then
            Console.Error.WriteLine "Warning: the namespace URL doesn't match expectations, query may result in no elements"
        Some(nsManager, sprintf "%s:" nsPrefix)

let query = "//{0}Demo/{0}Items"
let nodes =
    match nsOpt with
    | None ->
        let fixedQuery = String.Format(query, String.Empty)
        xDocument.XPathSelectElements fixedQuery
    | Some(nsManager, nsPrefix) ->
        let fixedQuery = String.Format(query, nsPrefix)
        xDocument.XPathSelectElements(fixedQuery, nsManager)
for node in nodes do
    ...

You could consider the following options:

1. determine whether document contains namespace and depending on it construct xpath query
2. use namespace agnostic xpath such as local-name(), which will ignore the namespace

Option 1

var xmlDoc = new XmlDocument();
xmlDoc.Load(fileLocation);
//determine  whether document contains namespace
var namespaceName = "ns";
var namespacePrefix = string.Empty;
XmlNamespaceManager nameSpaceManager = null;
if (xmlDoc.FirstChild.Attributes != null)
{
    var xmlns = xmlDoc.FirstChild.Attributes["xmlns"];
    if (xmlns != null)
    {
          nameSpaceManager = new XmlNamespaceManager(xmlDoc.NameTable);
          nameSpaceManager.AddNamespace(namespaceName, xmlns.Value);
          namespacePrefix = namespaceName + ":";
    }
}

XmlNodeList nodeList = xmlDoc.SelectNodes(string.Format("/{0}Demo/{0}Items",namespacePrefix), nameSpaceManager);
if (nodeList != null)
{
    foreach (XmlNode childNode in nodeList)
    {
       string first = childNode.SelectSingleNode(namespacePrefix + "First", nameSpaceManager).InnerText;
       string second = childNode.SelectSingleNode(namespacePrefix + "Second", nameSpaceManager).InnerText;
       string third = childNode.SelectSingleNode(namespacePrefix +  "Third", nameSpaceManager).InnerText;
     }
 }

Option 2

XmlNodeList nodeList = xmlDoc.SelectNodes("/*[local-name() = 'Demo']/*[local-name() = 'Items']");
if (nodeList != null)
{
    foreach (XmlNode childNode in nodeList)
    {    
        string first = childNode.SelectSingleNode("*[local-name() = 'First']").InnerText;
        string second = childNode.SelectSingleNode("*[local-name() = 'Second']").InnerText;
        string third = childNode.SelectSingleNode("*[local-name() = 'Third']").InnerText;
     }
}