Cannot deserialize the current JSON object, WHY?

问题

I'm trying to use a WebApi to get a list of Employees from my data base, using this code: This is the code of my client MVC Application:

        string u = "http://localhost:1411/api/EmployeeAPI";
        Uri uri = new Uri(u);

        HttpClient httpClient = new HttpClient();

        Task<HttpResponseMessage> response = httpClient.GetAsync(uri);

        Task.WaitAll(response);

        HttpResponseMessage resposta = response.Result;

        var msg = resposta.Content.ReadAsStringAsync().Result;

        Employee[] employees = JsonConvert.DeserializeObject<Employee[]>(msg);

        return View(employees);

And this is the code of my WebAPI:

    public IEnumerable<Employee> GetEmployees()
    {
        return db.Employees.AsEnumerable();
    }

But this error keeps popping up and I can't understand why:

Cannot deserialize the current JSON object (e.g. {"name":"value"}) into type 'DataAccess.Employee[]' because the type requires a JSON array (e.g. [1,2,3]) to deserialize correctly. To fix this error either change the JSON to a JSON array (e.g. [1,2,3]) or change the deserialized type so that it is a normal .NET type (e.g. not a primitive type like integer, not a collection type like an array or List) that can be deserialized from a JSON object. JsonObjectAttribute can also be added to the type to force it to deserialize from a JSON object. Path 'Message', line 1, position 11.

My Employee Class:

namespace DataAccess
{
    using System;
    using System.Collections.Generic;

    public partial class Employee
    {
        public Employee()
        {
            this.Products = new HashSet<Product>();
        }

        public int EmployeeId { get; set; }
        public string Title { get; set; }
        public string FirstName { get; set; }
        public string MiddleName { get; set; }
        public string LastName { get; set; }
        public byte[] rowguid { get; set; }
        public System.DateTimeOffset ModifiedDate { get; set; }

        public virtual ICollection<Product> Products { get; set; }
    }
}

The Json output I'm not really sure how to get it

The msg variable content:

My msg variable returns

"{\"Message\":\"An error has occurred.\",\"ExceptionMessage\":\"The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; 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.ProductSubCategory_9EC9A3706390DE6A3B51F713F0DDAC2162AFB5B3FAB8F8587C9A865333A7729A'. Path '[0].Products[0].ProductSubCategory.ProductCategory.ProductSubCategories'.\",\"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.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.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.Serialize(JsonWriter jsonWriter, Object value)\r\n at Newtonsoft.Json.JsonSerializer.SerializeInternal(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)\"}}"

回答1:

The best way to stop that error from occuring is:

In your WebApi project go to the App_Start folder and add to the WebApiConfig this four lines of code:

config.EnableSystemDiagnosticsTracing();
config.Formatters.JsonFormatter.SerializerSettings.PreserveReferencesHandling = PreserveReferencesHandling.None;
config.Formatters.JsonFormatter.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;
config.Formatters.JsonFormatter.SerializerSettings.Formatting = Formatting.Indented;
config.Formatters.Remove(config.Formatters.XmlFormatter);

and make sure you have instaled the Json nuget package

回答2:

Read the error, "Cannot deserialize the current JSON object (e.g. {"name":"value"}) into type 'DataAccess.Employee[]' because the type requires a JSON array (e.g. [1,2,3]) to deserialize correctly."

 var msg = resposta.Content.ReadAsStringAsync().Result;

 Employee[] employees = JsonConvert.DeserializeObject<Employee[]>(msg);

You need to make sure above "msg" is an actual JSONArray.

{ "key" : "Value" }

is an JSONObject,

[{ "key" : "Value" }]

is an JSONArray.

来源：https://stackoverflow.com/questions/24126675/cannot-deserialize-the-current-json-object-why