title: Throw an exception: Namespace Manager or XsltContext needed
body: <p>I ask a question about using XPath function <a href="https://stackoverflow.com/questions/1781938/c-xpath-to-select-node-with-attribute-containing-a-substring">there</a></p>
  
  <p>But I meet a problem about this exception: </p>
  
  <pre><code>[System.Xml.XPath.XPathException] = {"Namespace Manager or XsltContext needed. This query has a prefix, variable, or user-defined function."}
  </code></pre>
  
  <p>The xml is as follows:</p>
  
  
  
  <pre><code>&lt;x-config&gt;
      &lt;!--user location--&gt;
      &lt;x-list&gt;
        &lt;Country code="TW,UK,MY" /&gt;
        &lt;Country code="US,CA,MX" /&gt;
        &lt;Country code="IN,PR,VI,IR" /&gt;
        &lt;Country code="Others" /&gt; 
      &lt;/x-list&gt;
  </code></pre>
  
  <p>And I use following code to retrieve matches:</p>
  
  <pre><code>XmlNode countryNode = cdnConfig.SelectSingleNode(
      string.Format("x-config/x-list/Country[fn:contains(@code, {0})]", countryCode)
  );
  </code></pre>
  
  <p>I am a newbie of XPath. Could anybody elaborate more on this and give some workaround?</p>
  
  <p>Thanks a lot.</p>