C# 配置EF接口报错：ObjectContent`1”类型未能序列化内容类型“application/json; charset=utf-8”的响应正文

WebApi项目中，配置了EntityFramework，一顿LINQ操作后接口调试出现以下错误：

{
  "Message": "出现错误。",
  "ExceptionMessage": "“ObjectContent`1”类型未能序列化内容类型“application/json; charset=utf-8”的响应正文。",
  "ExceptionType": "System.InvalidOperationException",
  "StackTrace": null,
  "InnerException": {
    "Message": "出现错误。",
    "ExceptionMessage": "Self referencing loop detected with type 'System.Data.Entity.DynamicProxies.TimerServices_HoldDe_5887B15F712A690D23D5730B5D23D92E1914B018B8A19561126379653006AF93'. Path '[0].TimerServices_Base.TimerServices_HoldDetail'.",
    "ExceptionType": "Newtonsoft.Json.JsonSerializationException",
    "StackTrace": "   在 Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.CheckForCircularReference(JsonWriter writer, Object value, JsonProperty property, JsonContract contract, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   在 Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n   在 Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   在 Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n   在 Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   在 Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n   在 Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   在 Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n   在 Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   在 Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter, Object value, Type objectType)\r\n   在 Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value, Type objectType)\r\n   在 System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, Encoding effectiveEncoding)\r\n   在 System.Net.Http.Formatting.JsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, Encoding effectiveEncoding)\r\n   在 System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, HttpContent content)\r\n   在 System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStreamAsync(Type type, Object value, Stream writeStream, HttpContent content, TransportContext transportContext, CancellationToken cancellationToken)\r\n--- 引发异常的上一位置中堆栈跟踪的末尾 ---\r\n   在 System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   在 System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   在 System.Web.Http.WebHost.HttpControllerHandler.<WriteBufferedResponseContentAsync>d__22.MoveNext()"
  }
}

开始单看下面这句，以为是WebApi不支持json类型的返回

ObjectContent`1”类型未能序列化内容类型“application/json; charset=utf-8”的响应正文。

参考了：https://www.cnblogs.com/zoujinhua/p/10772321.html

尝试在Register方法里添加配置：

public static void Register(HttpConfiguration config)
{
　　// Web API 配置和服务
　　// 添加这句：
　　GlobalConfiguration.Configuration.Formatters.XmlFormatter.SupportedMediaTypes.Clear();

　　// Web API 路由
　　config.MapHttpAttributeRoutes();

结果当然是——没有用。

回到异常重新研究，才发现重点其实是在这一句！

Self referencing loop detected with type 'System.Data.Entity.DynamicProxies.TimerServices_HoldDe_5887B15F712A690D23D5730B5D23D92E1914B018B8A19561126379653006AF93'. Path '[0].TimerServices_Base.TimerServices_HoldDetail'.",

TimerServices_Base.TimerServices_HoldDetail（EF定义的导航属性），出现了循环自引用。

因为A里有B的导航属性，B里也有A，互相引用来来回回无穷尽也……

于是参考大佬的方法：https://www.cnblogs.com/songjl/p/9603830.html

做如下处理：

找到App_Start下的WebApiConfig.cs文件（这里用到的是.Net Framework4.8，其他版本结构可能不同）

还是在注册方法里加一句配置代码：

public static void Register(HttpConfiguration config)
{
    // Web API 配置和服务
    // 添加这句：
    config.Formatters.JsonFormatter.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore;

    // Web API 路由
    config.MapHttpAttributeRoutes();

    config.Routes.MapHttpRoute(
        name: "DefaultApi",
        routeTemplate: "api/{controller}/{action}/{id}",
        defaults: new { id = RouteParameter.Optional }
    );
}        

问题解决！