C# '；ObjectContent`1'；类型未能序列化内容类型'；的响应正文；应用程序/json；字符集=utf-8'；

几个月来，以下web API 2（简化示例）正确响应

http://localhost/TestApi2/api/tests with this. 
[{"Id":1,"Name":"Naples"},{"Id":2,"Name":"Paris"}] 
    //using Newtonsoft.Json;
    namespace TestApi2.Controllers
    {
        public class PersonsController : ApiController
        {
            [HttpGet]
            [Route("api/tests")]
            public HttpResponseMessage GetAll()
            {
                //  This is a dbContext generated from a DatabaseFirst EF Generation
                using (var cleanDbContext = new cleanDbEntities())
                {
                    var tests = cleanDbContext.TestTables;
                    // Return all
                    return Request.CreateResponse(HttpStatusCode.OK, tests, Configuration.Formatters.JsonFormatter);
                }
            }
        }
    }

var tests = cleanDbContext.TestTables.ToList();

然后我更新了我在这个项目上的NUGET 同一个电话的回应是：

<ExceptionMessage>
The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8'.
</ExceptionMessage>
<ExceptionType>System.InvalidOperationException</ExceptionType>
<StackTrace/>

var tests = cleanDbContext.TestTables.ToList();

---

修改查询以显式生成列表修复了该问题。我会将此信息发送到Newtonsoft。这是一款很棒的产品。

您确定没有更新EF软件包吗？可能的重复项您确定没有更新EF软件包吗？可能的重复项

var tests = cleanDbContext.TestTables.ToList();