c#读取带命名空间的xml

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <GetAuthorizationCodeResponse xmlns="http://tempuri.org/">
      <GetAuthorizationCodeResult>boolean</GetAuthorizationCodeResult>
    </GetAuthorizationCodeResponse>
  </soap:Body>
</soap:Envelope>

var doc = new XmlDocument();
            var namespaceManager = new XmlNamespaceManager(doc.NameTable);
            namespaceManager.AddNamespace("xsi", "http://www.w3.org/2001/XMLSchema-instance");
            namespaceManager.AddNamespace("xsd", "http://www.w3.org/2001/XMLSchema");
            namespaceManager.AddNamespace("soap", "http://schemas.xmlsoap.org/soap/envelope/");
            namespaceManager.AddNamespace("mynamespace", "http://tempuri.org/");
            doc.LoadXml(result.ResultData);
            var resultNode = doc.SelectSingleNode(@"soap:Envelope/soap:Body/mynamespace:GetAuthorizationCodeResponse/mynamespace:GetAuthorizationCodeResult", namespaceManager);