Сериализация типов значений с помощью json.net

Я пытаюсь сделать вызов ajax, чтобы получить штаты из страны, но я продолжаю получать исключение invalidcastException. Я использую MVC4 с json.NET. Я могу без проблем сериализовать объекты в своих тестах, но когда я делаю вызов ajax, я продолжаю получать сообщение об ошибке.

Это тип значения:

[JsonConverter(typeof(ValueObjectConverter))]
public class Code : IValueObject
{
    private readonly string _code;

    private Code(string code)
    {
        _code = code;
    }

    public override string ToString()
    {
        return _code;
    }

    public static implicit operator Code(string code)
    {
        return new Code(code);
    }

    public static implicit operator String(Code code)
    {
        return code.ToString();
    }
}

Это мой объект:

public class CountryState : Entity
{
    public Code CodeWPS { get; set; }
    public Code CodeIAM { get; set; }
    public Name Description { get; set; }
}

Это jsonconverter:

public class ValueObjectConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(IValueObject));
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        return (IValueObject)existingValue;
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var item = (IValueObject)value;
        writer.WriteValue(item.ToString());
        writer.Flush();
    }
}

Это метод API:

    public IEnumerable<CountryState> GetStatesByCountry(string codeType, string countryCodeIam)
    {
        var states = _getAllCountriesQueryHandler.Handle(countryCodeIam);
        return states;
    }

Это ошибка, которую я получаю с firebug:

    {"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 'CodeWPS' on 'CNH.CSCN.BBS.Entities.CountryState'.","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.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, 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 System.Net.Http.Formatting.JsonMediaTypeFormatter.<>c__DisplayClassd.<WriteToStreamAsync>b__c()\r\n   at System.Threading.Tasks.TaskHelpers.RunSynchronously(Action action, CancellationToken token)","InnerException":{"Message":"An error has occurred.","ExceptionMessage":"Unable to cast object of type 'System.String' to type 'CNH.CSCN.BBS.Entities.ValueTypes.Code'.","ExceptionType":"System.InvalidCastException","StackTrace":"   at GetCodeWPS(Object )\r\n   at Newtonsoft.Json.Serialization.DynamicValueProvider.GetValue(Object target)"}}}

ОБНОВИТЬ:

Я добавил контрактный преобразователь:

public class SpecialContractResolver : DefaultContractResolver
{
    protected override IValueProvider CreateMemberValueProvider(MemberInfo member)
    {
        if (member.MemberType == MemberTypes.Property)
        {
            var pi = (PropertyInfo)member;
            if (typeof(IValueObject).IsAssignableFrom(pi.PropertyType))
            {
                return new IValueObjectValueProvider(member, pi.PropertyType);
            }
        }
        else if (member.MemberType == MemberTypes.Field)
        {
            var fi = (FieldInfo)member;
            if (typeof(IValueObject).IsAssignableFrom(fi.FieldType))
            {
                return new IValueObjectValueProvider(member, fi.FieldType);
            }
        }

        return base.CreateMemberValueProvider(member);
    }
}

и IValueProvider:

public class IValueObjectValueProvider : IValueProvider
{
    private readonly object _defaultValue;
    private readonly IValueProvider _underlyingValueProvider;


    public IValueObjectValueProvider(MemberInfo memberInfo, Type underlyingType)
    {
        _underlyingValueProvider = new DynamicValueProvider(memberInfo);
        _defaultValue = underlyingType;
    }

    public void SetValue(object target, object value)
    {
        target = value;
    }

    public object GetValue(object target)
    {
        return target.ToString();
    }
}

Я знаю, что я на правильном пути, я больше не получаю сообщение об ошибке, но такой результат:

Объект { CodeWPS="CNH.CSCN.BBS.Entities.CountryState", CodeIAM="CNH.CSCN.BBS.Entities.CountryState", Description="CNH.CSCN.BBS.Entities.CountryState"}

Я думаю, что мой contractresolver и ivalueprovider еще не на 100% верны...