调用XContainer.Element返回null，XML属性获取失败问题排查

我编写了postXMLData方法用于发送POST请求并获取XML格式的响应。在接收到响应后，我尝试获取名称以“f”开头的属性，却遇到错误System.Xml.Linq.XContainer.Element(...) returned null。请问我哪里出错了？

postXMLData方法代码 #

public static XmlDocument postXMLData(string xml)
{
    var request = (HttpWebRequest)WebRequest.Create(Requests.url);
    byte[] bytes;
    bytes = System.Text.Encoding.ASCII.GetBytes(xml);
    request.ContentType = "text/xml; encoding='utf-8'";
    request.ContentLength = bytes.Length;
    request.Method = "POST";
    Stream requestStream = request.GetRequestStream();
    requestStream.Write(bytes, 0, bytes.Length);
    requestStream.Close();
    HttpWebResponse response;
    response = (HttpWebResponse)request.GetResponse();

    if (response.StatusCode == HttpStatusCode.OK)
    {
        using (var streamReader = new StreamReader(response.GetResponseStream()))
        {
            var responseText = streamReader.ReadToEnd();
            var result = new XmlDocument();
            result.LoadXml(responseText);
            return result;
        }
    }

    throw new Exception();
}

请求返回的响应XML #

<response result="0">
    <prov>
        <getStatus result="0">
            <pay date="2023-02-08T19:44:33+03:00" fatal="false" id="8022140013003" result="0" status="2" uid="26775263057008" value-date="2023-02-08T19:44:40+03:00">
            </pay>
        </getStatus>
    </prov>
</response>

获取属性的代码 #

XmlDocument doc = postXMLData(Requests.getStatus("08022140013003"));
XElement e = XElement.Load(new XmlNodeReader(doc));
Console.WriteLine(e); //此处正常输出

IEnumerable<XAttribute> attrs1 = e.Element("response").Element("prov").Element("getStatus").Element("pay")?.Attributes().Where(a => a.Name.LocalName.StartsWith("f"));
Console.WriteLine(attrs1);
Console.ReadKey();

错误原因及修正方案 #

核心错误 #

你加载后的XElement e本身就是XML的根节点<response>，此时调用e.Element("response")会尝试找根节点下名为response的子元素，而根节点的子元素是<prov>，所以这一步直接返回null，后续链式调用自然报错。

修正后的获取属性代码 #

// 直接从根节点e开始查找子元素prov
IEnumerable<XAttribute> attrs1 = e.Element("prov")
                                 .Element("getStatus")
                                 .Element("pay")
                                 ?.Attributes()
                                 .Where(a => a.Name.LocalName.StartsWith("f"));

额外优化建议 #

1. 避免XmlDocument与XElement混用
   可以直接在postXMLData方法中返回XElement，省去类型转换步骤，同时优化资源释放：

   public static XElement postXMLData(string xml)
   {
       var request = (HttpWebRequest)WebRequest.Create(Requests.url);
       // 用UTF8编码，和请求头的utf-8保持一致，避免乱码
       byte[] bytes = System.Text.Encoding.UTF8.GetBytes(xml);
       request.ContentType = "text/xml; encoding=utf-8";
       request.ContentLength = bytes.Length;
       request.Method = "POST";
       // 使用using自动释放流资源
       using (Stream requestStream = request.GetRequestStream())
       {
           requestStream.Write(bytes, 0, bytes.Length);
       }
       using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
       {
           if (response.StatusCode == HttpStatusCode.OK)
           {
               using (var streamReader = new StreamReader(response.GetResponseStream()))
               {
                   var responseText = streamReader.ReadToEnd();
                   return XElement.Parse(responseText);
               }
           }
           throw new Exception("请求未返回成功状态");
       }
   }
   

   调用时简化为：

   XElement e = postXMLData(Requests.getStatus("08022140013003"));
   IEnumerable<XAttribute> attrs1 = e.Element("prov")
                                    .Element("getStatus")
                                    .Element("pay")
                                    ?.Attributes()
                                    .Where(a => a.Name.LocalName.StartsWith("f"));
   

2. 编码统一
   原代码中用Encoding.ASCII处理XML字节，可能导致非ASCII字符乱码，建议替换为Encoding.UTF8，与请求头中声明的utf-8编码匹配。

内容的提问来源于stack exchange，提问作者John Paul Johns