ASMX WebService无法处理JSON POST请求，寻求解决方法

问题背景 #

在Visual Studio 2022中创建基于.NET Framework的ASP.NET Web Application（ASMX WebService），使用C# HttpClient发送JSON格式POST请求时，收到“服务器无法处理请求”的SOAP错误，但发送到JSONPlaceholder免费测试API时可正常运行。

错误响应信息 #

Hello, World!
Send msg:
Hello_World.
=====
{"Transmission":"Hello_World"}
=====
Response:
<?xml version="1.0" encoding="utf-8"?><soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><soap:Body><soap:Fault><soap:Code><soap:Value>soap:Receiver</soap:Value></soap:Code><soap:Reason><soap:Text xml:lang="es">System.Web.Services.Protocols.SoapException: El servidor no puede procesar la solicitud. ---> System.Xml.XmlException: Los datos del nivel de raíz no son válidos. línea 1, posición 1.
   en System.Xml.XmlTextReaderImpl.Throw(Exception e)
   en System.Xml.XmlTextReaderImpl.ParseRootLevelWhitespace()
   en System.Xml.XmlTextReaderImpl.ParseDocumentContent()
   en System.Web.Services.Protocols.SoapServerProtocol.SoapEnvelopeReader.Read()
   en System.Xml.XmlReader.MoveToContent()
   en System.Web.Services.Protocols.SoapServerProtocolHelper.GetRequestElement()
   en System.Web.Services.Protocols.Soap12ServerProtocolHelper.RouteRequest()
   en System.Web.Services.Protocols.SoapServerProtocol.Initialize()
   en System.Web.Services.Protocols.ServerProtocolFactory.Create(Type type, HttpContext context, HttpRequest request, HttpResponse response, Boolean&amp; abortProcessing)
   --- Fin del seguimiento de la pila de la excepción interna ---</soap:Text></soap:Reason><soap:Detail /></soap:Fault></soap:Body></soap:Envelope>

客户端代码 #

private static async Task Main(string[] args)
{
    Console.WriteLine("Hello, World!");

    try
    {
        /*
          * Check web service created locally.
          * string wsUrl = "https://localhost:44373/SoapDemo.asmx";
          * string wsUrl = "https://jsonplaceholder.typicode.com/posts";
        */
        string wsUrl = "https://localhost:44373/SoapDemo.asmx";

        // Send a small string.
        string msg = "Hello_World";
        Console.WriteLine("Send msg:\n{0}.", msg);

        // Create connection to Web Service.
        var client = new HttpClient();

        // Prepare the data to send to WS.
        Post postMsg = new Post();
        postMsg.Transmission = msg;
        var data = JsonSerializer.Serialize<Post>(postMsg);

        // Finally send the msg to the WebService.
        Console.WriteLine("=====\n" + data + "\n=====");
        HttpContent content = new StringContent(data, System.Text.Encoding.UTF8, "application/json");

        // Recive a response from the Web Service server.
        var httpResponse = await client.PostAsync(wsUrl, content);

        // if (httpResponse.IsSuccessStatusCode) {

            var result = await httpResponse.Content.ReadAsStringAsync();
            Console.WriteLine("Response:\n{0}", result);
            // var postResult = JsonSerializer.Deserialize<Post>(result);
            // Console.WriteLine("Response:\n{0}", postResult);

        // }

    }
    catch (Exception ex)
    {
        Console.WriteLine("Transmission error:\n{0}", ex.ToString());
    }
}

WebService端ASMX函数代码 #

[WebMethod]
// [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
public bool Transmission()
{
    ABCObject ObjectName = null;
    string contentType = HttpContext.Current.Request.ContentType;

    if (false == contentType.StartsWith("application/json", StringComparison.OrdinalIgnoreCase))
    {
        return false;
    }

    using (System.IO.Stream stream = HttpContext.Current.Request.InputStream)
    using (System.IO.StreamReader reader = new System.IO.StreamReader(stream))
    {
        stream.Seek(0, System.IO.SeekOrigin.Begin);
        string bodyText = reader.ReadToEnd();
        bodyText = bodyText == "" ? "{}" : bodyText;
        var json = Newtonsoft.Json.Linq.JObject.Parse(bodyText);
        ObjectName = Newtonsoft.Json.JsonConvert.DeserializeObject<ABCObject>(json.ToString());
    }

    return true;
}

解决方案 #

1. 启用ASMX的JSON请求支持 #

ASMX默认仅处理SOAP请求，必须给WebMethod添加[ScriptMethod]特性，明确指定请求和响应格式为JSON：

[WebMethod]
[ScriptMethod(ResponseFormat = ResponseFormat.Json, RequestFormat = RequestFormat.Json)]
public bool Transmission(ABCObject requestData)
{
    // 直接使用自动反序列化后的参数，无需手动读取请求流
    if (requestData == null || string.IsNullOrEmpty(requestData.Transmission))
    {
        return false;
    }
    // 此处添加业务处理逻辑
    return true;
}

2. 修正客户端请求地址 #

JSON请求必须指向ASMX的具体方法，地址应为https://localhost:44373/SoapDemo.asmx/Transmission，而非根地址SoapDemo.asmx。

3. 确保数据模型结构一致 #

客户端的Post类与服务端的ABCObject类字段需完全匹配，示例：

// 客户端模型
public class Post
{
    public string Transmission { get; set; }
}

// 服务端模型
public class ABCObject
{
    public string Transmission { get; set; }
}

4. 验证Web.config配置 #

确保配置了ScriptHandler以处理JSON请求：

<system.web>
  <httpHandlers>
    <add verb="*" path="*.asmx" type="System.Web.Script.Services.ScriptHandlerFactory" validate="false"/>
  </httpHandlers>
</system.web>

同时确认compilation节点的targetFramework与项目使用的.NET Framework版本一致。

5. 简化客户端请求代码 #

可使用JsonContent简化JSON内容创建（需引用System.Net.Http.Json NuGet包）：

var postMsg = new Post { Transmission = msg };
var content = JsonContent.Create(postMsg);
var httpResponse = await client.PostAsync("https://localhost:44373/SoapDemo.asmx/Transmission", content);

内容的提问来源于stack exchange，提问作者F A Fernández