XPath查询总是只在SingleNodeSelect中的某些查询上返回NULL

<?xml version="1.0" encoding="utf-16"?>
<users>
  <user number="0772247157">
    <step stepnumber="1">complete</step>
    <step stepnumber="2">complete</step>
    <step stepnumber="3">complete</step>
  </user>
  <user number="0772247158">
    <step stepnumber="1">complete</step>
    <step stepnumber="2">complete</step>
  </user>
  <user number="0772247159">
    <step stepnumber="1">complete</step>
  </user> 
</users>

查询,例如

//user[@number='0772243950'] 和 //user[@number=0772243950]/step[last()]

但是,下面的函数总是返回null。它与XPathVisualizer配合得非常好,我还与一个在线XPath计算器进行了反复检查。

public bool checkStepExists(string Number, string StepNumber)
{
    string XPathQuery = "//user[@number=" + Number + "]/step[@stepnumber=" + StepNumber + "]";

    XmlNode Search = SettingsFile.SelectSingleNode(XPathQuery);      

    if (Search == null)
       return false;
    else
       return true;            
}