C# 为什么在 web api 中返回一个具有一对多关系的实体会导致错误？

why In web api returning an entity that has a one to many relationship causes an error?

提问by Kas

Guys I have a One to many relation to same class called user,
I am returning one instance of user in web apis get method it works fine as far as I don't have any

伙计们，我与名为 user 的同一个类有一对多的关系，
我在 web apis get 方法中返回一个用户实例，只要我没有，它就可以正常工作

User 

ID  | Name 
0   | A 
1   | B 

Friends Table ( This table is used to build the one to many relationship ) 

User_ID | Friend_ID 
 1      |     0 
 0      |     1 

This is my GetMethod

这是我的 GetMethod

[HttpGet]
        public Models.User   Authenticate()
        {
            try
            {
                return   db.Users.SingleOrDefault(x => x.ID == 0 ) ; 
            } 
            catch
            {
                return null;
            }
        }

If I get and return a User entity then I get the blow error

如果我得到并返回一个用户实体，那么我会收到打击错误

Can anyone please help me to fix this below error ?

任何人都可以帮我解决以下错误吗？

{"Message":"An error has occurred.","ExceptionMessage":"The 'ObjectContent`1' type failed to serialize the response body for content type 'text/html; charset=utf-8'.","ExceptionType":"System.InvalidOperationException","StackTrace":null,"InnerException":{"Message":"An error has occurred.","ExceptionMessage":"Self referencing loop detected with type 'System.Data.Entity.DynamicProxies.Friend_E5C79F4736EB2750392FFC7061B18E9C12F15F9583409603C75C5D1B1F7358D4'. Path 'Friends1[0].User.Friends'.","ExceptionType":"Newtonsoft.Json.JsonSerializationException","StackTrace":" at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.CheckForCircularReference(JsonWriter writer, Object value, JsonProperty property, JsonContract contract, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IWrappedCollection values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IWrappedCollection values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter, Object value)\r\n at Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value)\r\n at Newtonsoft.Json.JsonSerializer.Serialize(JsonWriter jsonWriter, Object value)\r\n at System.Net.Http.Formatting.JsonMediaTypeFormatter.<>c__DisplayClassd.b__c()\r\n at System.Threading.Tasks.TaskHelpers.RunSynchronously(Action action, CancellationToken token)"}}

{"Message":"发生错误。","ExceptionMessage":"'ObjectContent`1' 类型未能序列化内容类型 'text/html; charset=utf-8' 的响应正文。","ExceptionType ":"System.InvalidOperationException","StackTrace":null,"InnerException":{"Message":"发生错误。","ExceptionMessage":"检测到类型为'System.Data.Entity.DynamicProxies的自引用循环.Friend_E5C79F4736EB2750392FFC7061B18E9C12F15F9583409603C75C5D1B1F7358D4'。路径'Friends1[0].User.Friends'.","ExceptionType":"Json"Internal.JsonStackJsonSerializer.CheckForCircularReference(JsonWriter writer, Object value, JsonProperty property, JsonContract contract, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IWrappedCollection values, JsonContract contract, JsonContractContractContainer contract, JsonContractProperty containerProperty) 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.ObjectSerialization.JsonSerializeInserializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n (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 成员, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n 在 Newtonsoft.JsonSerialization. JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContainerContract member, JsonContract valueContainerContract containerContract、JsonProperty containerProperty)\r\n 在 Newtonsoft.Json.Serialization.JsonSerializerInternalWriter。SerializeList(JsonWriter writer, IWrappedCollection values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContractPropertyContainer member, JsonContractProperty) JsonProperty containerProperty)\r\n 在 Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject（JsonWriter 编写器，对象值，JsonObjectContract 合同，JsonProperty 成员，JsonContainerContract collectionContract，JsonProperty containerProperty）\r\n 在 Newtonsoft.Json.Serialization.JsonSerializerInternalValueInternalValue (JsonWriter writer, Object value, JsonContract valueContract, JsonProperty 成员,JsonContainerContract containerContract, JsonProperty containerProperty)\r\n 在 Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter, Object value)\r\n 在 Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value)\r\n在 Newtonsoft.Json.JsonSerializer.Serialize(JsonWriter jsonWriter, Object value)\r\n 在 System.Net.Http.Formatting.JsonMediaTypeFormatter.<>c__DisplayClassd.b__c()\r\n 在 System.Threading.Tasks.TaskHelpers。 RunSynchronously(Action action, CancellationToken token)"}}Serialize(JsonWriter jsonWriter, Object value)\r\n 在 System.Net.Http.Formatting.JsonMediaTypeFormatter.<>c__DisplayClassd.b__c()\r\n 在 System.Threading.Tasks.TaskHelpers.RunSynchronously(Action action, CancellationToken token )"}}Serialize(JsonWriter jsonWriter, Object value)\r\n 在 System.Net.Http.Formatting.JsonMediaTypeFormatter.<>c__DisplayClassd.b__c()\r\n 在 System.Threading.Tasks.TaskHelpers.RunSynchronously(Action action, CancellationToken token )"}}

采纳答案by alexmac

Try to change webApi formatter. Add these lines in WebApiConfig.cs:

尝试更改 webApi 格式化程序。在 WebApiConfig.cs 中添加这些行：

var json = config.Formatters.JsonFormatter;
json.SerializerSettings.PreserveReferencesHandling = Newtonsoft.Json.PreserveReferencesHandling.Objects;
config.Formatters.Remove(config.Formatters.XmlFormatter);

And add this line:

并添加这一行：

json.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore

回答by B-Lat

Apply the [JsonIgnore]attribute to the navigation property you don't want to be serialised. It will still serialise both the parent and child entities but just avoids the self referencing loop.

将该 [JsonIgnore]属性应用于您不想序列化的导航属性。它仍然会序列化父实体和子实体，但只是避免了自引用循环。