Asp.net web api Web API中不应出现XmlMediaTypeFormatter抛出的异常

我正在使用MVC5编写一个诊断网站，它将向多个后端服务发出任意请求，并在浏览器中以Json或XML报告结果

由于结果类型很多，所以我将对象类型的变量传递给Okcontent方法

当我请求Json输出时，它工作正常，但在请求xml输出时，XmlMediaTypeFormatter失败。例外是

The 'ObjectContent`1' type failed to serialize the response body for content type 'application/xml; charset=utf-8'.

Type '<type>' with data contract name '<schema-address>' is not expected. 
Consider using a DataContractResolver or add any types not known statically 
to the list of known types - for example, by using the KnownTypeAttribute 
attribute or by adding them to the list of known types passed to
DataContractSerializer.

内部的例外是

The 'ObjectContent`1' type failed to serialize the response body for content type 'application/xml; charset=utf-8'.

Type '<type>' with data contract name '<schema-address>' is not expected. 
Consider using a DataContractResolver or add any types not known statically 
to the list of known types - for example, by using the KnownTypeAttribute 
attribute or by adding them to the list of known types passed to
DataContractSerializer.

这似乎发生在任何不是简单纯对象的返回类型上，例如，字符串返回类型将正确显示，但即使是具有2个整数属性和2个日期时间属性的简单类也会以这种方式失败。使用调试器深入研究代码表明，对HttpRequestMessage.CreateResponse的调用知道主体内容的类型，如果尝试像所说的那样进行操作，则效果很好

谷歌搜索这个错误导致了大量关于实体框架导航链接和代理的讨论，但这些都不适用于这里，因为它们是简单的返回类，不涉及EntityFramework或导航引用

有人有线索吗

谢谢