ASP.NET Web Api返回HTML内容发布至IIS 8后序列化异常如何解决？

我之前也碰到过类似的问题，本地调试好好的，一发布到IIS就出序列化错误，咱们来一步步解决它。

先看你写的API代码：

[HttpGet]
public HttpResponseMessage test()
{
    var response = new HttpResponseMessage(HttpStatusCode.OK);
    response.Content = new StringContent("<html><body>HELLO</body></html>");
    response.Content.Headers.ContentType = new MediaTypeHeaderValue("text/html");
    return response;
}

发布到Windows Server 2012的IIS8后出现的错误信息是：

<Error>
  <script/>
  <script id="tinyhippos-injected"/>
  <Message>An error has occurred.</Message>
  <ExceptionMessage>
    The 'ObjectContent`1' type failed to serialize the response body for content type 'application/xml; charset=utf-8'.
  </ExceptionMessage>
  <ExceptionType>System.InvalidOperationException</ExceptionType>
  <StackTrace/>
  <InnerException>
    <Message>An error has occurred.</Message>
    <ExceptionMessage>
      Type 'System.Net.Http.StringContent' cannot be serialized. Consider marking it with the DataContractAttribute attribute, and marking all of its members you want serialized with the DataMemberAttribute attribute. If the type is a collection, consider marking it with the CollectionDataContractAttribute. See the Microsoft .NET Framework documentation for other supported types.
    </ExceptionMessage>
    <ExceptionType>
      System.Runtime.Serialization.InvalidDataContractException
    </ExceptionType>
    <StackTrace>
      at System.Runtime.Serialization.DataContract.DataContractCriticalHelper.ThrowInvalidDataContractException(String message, Type type)
      at System.Runtime.Serialization.DataContract.DataContractCriticalHelper.CreateDataContract(Int32 id, RuntimeTypeHandle typeHandle, Type type)
      at System.Runtime.Serialization.DataContract.DataContractCriticalHelper.GetDataContractSkipValidation(Int32 id, RuntimeTypeHandle typeHandle, Type type)
      at System.Runtime.Serialization.XmlObjectSerializerContext.GetDataContract(RuntimeTypeHandle typeHandle, Type type)
      at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithXsiType(XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle objectTypeHandle, Type objectType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle, Type declaredType)
      at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
      at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerializeReference(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle)
      at WriteHttpResponseMessageToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , ClassDataContract )
      at System.Runtime.Serialization.ClassDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context)
      at System.Runtime.Serialization.XmlObjectSerializerWriteContext.WriteDataContractValue(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle)
      at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithoutXsiType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle)
      at System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
      at System.Runtime.Serialization.DataContractSerializer.InternalWriteObject(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
      at System.Runtime.Serialization.XmlObjectSerializer.WriteObjectHandleExceptions(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver)
      at System.Runtime.Serialization.DataContractSerializer.WriteObject(XmlWriter writer, Object graph)
      at System.Net.Http.Formatting.XmlMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, HttpContent content)
      at System.Net.Http.Formatting.XmlMediaTypeFormatter.WriteToStreamAsync(Type type, Object value, Stream writeStream, HttpContent content, TransportContext transportContext, CancellationToken cancellationToken)
      --- End of stack trace from previous location where exception was thrown ---
      at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
      at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
      at System.Web.Http.WebHost.HttpControllerHandler.<WriteBufferedResponseContentAsync>d__22.MoveNext()
    </StackTrace>
  </InnerException>
</Error>

问题原因 #

这个错误的核心是Web API框架没有直接返回你设置的HTML内容，反而尝试序列化HttpResponseMessage对象本身。本地调试正常是因为本地请求的Accept头大概率优先接受text/html，框架直接返回了内容；但IIS环境下的请求可能默认Accept是application/xml，触发了框架的XML序列化逻辑，而StringContent类型本身不支持XML序列化，所以报错了。

解决方法 #

给你三种靠谱的解决方式，按需选择：

方法一：改用IHttpActionResult返回（最推荐）

ASP.NET Web API推荐用IHttpActionResult来封装响应，它会自动处理内容类型，避免序列化问题：

[HttpGet]
public IHttpActionResult Test()
{
    var htmlContent = "<html><body>HELLO</body></html>";
    // 直接指定内容类型为text/html
    return Content(HttpStatusCode.OK, htmlContent, new MediaTypeHeaderValue("text/html"));
}

方法二：用ResponseMessageResult包装HttpResponseMessage

如果你习惯用HttpResponseMessage，可以用ResponseMessage方法告诉框架直接返回这个响应，不要序列化：

[HttpGet]
public IHttpActionResult Test()
{
    var response = new HttpResponseMessage(HttpStatusCode.OK);
    // 这里最好加上编码，避免中文乱码
    response.Content = new StringContent("<html><body>HELLO</body></html>", Encoding.UTF8, "text/html");
    return ResponseMessage(response);
}

方法三：全局禁用XML格式化器（适合全API不需要XML响应的场景）

如果你的API主要返回HTML或JSON，不需要XML格式，可以在WebApiConfig.cs里移除XML格式化器：

public static void Register(HttpConfiguration config)
{
    // 移除XML格式化器，框架就不会尝试用XML序列化响应
    config.Formatters.Remove(config.Formatters.XmlFormatter);
    
    // 其他路由配置
    config.MapHttpAttributeRoutes();
    config.Routes.MapHttpRoute(
        name: "DefaultApi",
        routeTemplate: "api/{controller}/{id}",
        defaults: new { id = RouteParameter.Optional }
    );
}

优先选方法一或方法二，它们不会影响其他API的行为；方法三适合整个API都不需要XML响应的场景。

内容的提问来源于stack exchange，提问作者Passa4