您可以考虑以下选项:

1. 确定文档是否包含命名空间，并根据它构造 xpath 查询
2. 使用与命名空间无关的 xpath，例如 local-name()，这将忽略命名空间

选项 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;
     }
 }

选项 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;
     }
}