TryReadElementFromXml method
Implementation
@override
Future<bool> TryReadElementFromXml(EwsServiceXmlReader reader) async {
  bool result = await super.TryReadElementFromXml(reader);
  if (!result) {
    if (reader.LocalName == XmlElementNames.Constant) {
      this.value = reader.ReadAttributeValue(XmlAttributeNames.Value);
      result = true;
    }
  }
  return result;
}