WebAPI2 + Angular http GET - 返回 IEnumerable<Categories>。未能序列化内容类型“application/json”的响应正文

Question

当我们说话时我正在学习这个，发现其中一些非常奇怪和困难;)

这是 Angular 中的其余服务调用：

$scope.categories = [];
    $scope.selectedCategory = null;

    $http({
            method: 'GET',
            url: './api/file/GetCategories',
            accept: 'application/json'
        })
        .success(function(result) {
        $scope.categories = result;
    });

这是我要获取的服务：(C# - FileController.cs)

public IEnumerable<Category> GetCategories()
    {
        return FileServices.GetCategoriesForUser();

    }

文件服务方法：

public static IEnumerable<Category> GetCategoriesForUser()
    {
        User currentUser = UserServices.GetLoggedInUser();
        IEnumerable<Category> userCategories;
        using (var context = new PhotoEntities())
        {
            userCategories = context.Categories.Where(c => c.UserId == currentUser.Id);
        }
        return userCategories;
    }

问题在于它可能根本无法将其识别为 JSON，但是，唉！这是整个响应错误消息：

{"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":"Error getting value from 'Categories' on 'System.Data.Entity.DynamicProxies.User_523AFE24D26493BB74E43EAA23AAEBFD40D7EABFCAA3A6105EB951B365E60A04'.","ExceptionType":"Newtonsoft.Json.JsonSerializationException","StackTrace":"   at Newtonsoft.Json.Serialization.DynamicValueProvider.GetValue(Object target)\r\n   at Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.CalculatePropertyValues(JsonWriter writer, Object value, JsonContainerContract contract, JsonProperty member, JsonProperty property, JsonContract& memberContract, Object& memberValue)\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, Type objectType)\r\n   at Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value, Type objectType)\r\n   at Newtonsoft.Json.JsonSerializer.Serialize(JsonWriter jsonWriter, Object value)\r\n   at System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, Encoding effectiveEncoding)\r\n   at System.Net.Http.Formatting.JsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, Encoding effectiveEncoding)\r\n   at System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, HttpContent content)\r\n   at System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStreamAsync(Type type, Object value, Stream writeStream, HttpContent content, TransportContext transportContext, CancellationToken cancellationToken)\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.GetResult()\r\n   at System.Web.Http.WebHost.HttpControllerHandler.<WriteBufferedResponseContentAsync>d__1b.MoveNext()","InnerException":{"Message":"An error has occurred.","ExceptionMessage":"The ObjectContext instance has been disposed and can no longer be used for operations that require a connection.","ExceptionType":"System.ObjectDisposedException","StackTrace":"   at System.Data.Entity.Core.Objects.ObjectContext.get_Connection()\r\n   at System.Data.Entity.Core.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)\r\n   at System.Data.Entity.Core.Objects.ObjectQuery`1.Execute(MergeOption mergeOption)\r\n   at System.Data.Entity.Core.Objects.DataClasses.EntityCollection`1.Load(List`1 collection, MergeOption mergeOption)\r\n   at System.Data.Entity.Core.Objects.DataClasses.EntityCollection`1.Load(MergeOption mergeOption)\r\n   at System.Data.Entity.Core.Objects.DataClasses.RelatedEnd.Load()\r\n   at System.Data.Entity.Core.Objects.DataClasses.RelatedEnd.DeferredLoad()\r\n   at System.Data.Entity.Core.Objects.Internal.LazyLoadBehavior.LoadProperty[TItem](TItem propertyValue, String relationshipName, String targetRoleName, Boolean mustBeNull, Object wrapperObject)\r\n   at System.Data.Entity.Core.Objects.Internal.LazyLoadBehavior.<>c__DisplayClass7`2.<GetInterceptorDelegate>b__1(TProxy proxy, TItem item)\r\n   at System.Data.Entity.DynamicProxies.User_523AFE24D26493BB74E43EAA23AAEBFD40D7EABFCAA3A6105EB951B365E60A04.get_Categories()\r\n   at GetCategories(Object )\r\n   at Newtonsoft.Json.Serialization.DynamicValueProvider.GetValue(Object target)"}}}

Answer 1

尝试将此添加到 App_Start 文件夹中配置文件中的 Register() 函数中：

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

我遇到了类似的问题，这为我解决了问题。摘自：http://social.msdn.microsoft.com/Forums/vstudio/en-US/a5adf07b-e622-4a12-872d-40c753417645/

Answer 2

我已经养成了返回 HttpResponseMessage 的习惯，这样我就可以直接控制状态和内容类型。这里有一些代码可以为你做到这一点。

public class StuffController : ApiController
{
    public HttpResponseMessage Get()
    {
        List<SomeModel> models = ReadModels();
        return GetJsonResponse(models);
    }
}

public static HttpResponseMessage GetJsonResponse(object serializableObject)
{
    string jsonText = JsonConvert.SerializeObject(serializableObject, Formatting.Indented);
    HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.OK);
    response.Content = new StringContent(jsonText, Encoding.UTF8, "text/plain");

    return response;
}

Answer 3

只需使用列表：

public static List<Category> GetCategoriesForUser()
    {
        User currentUser = UserServices.GetLoggedInUser();
        IEnumerable<Category> userCategories = new List<Category>();
        using (var context = new PhotoEntities())
        {
            userCategories.addRange(context.Categories.Where(c => c.UserId == currentUser.Id).toList());
        }
        return userCategories;
    }

IEnumerable 是一个接口，不能创建接口实例。

编辑：更改为 addRange 方法。 并将您的服务从 IEnumerable 更改为 List：

public List<Category> GetCategories()