Web API2序列化异常

我有一个使用EF搭建的web api2应用程序。

我正在尝试序列化以下类(其中包含延迟加载的导航属性):

namespace Models
{
    public class Speciality
    {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int SpecialityId { get; set; }
        public string Name { get; set; }
        public virtual ICollection<Doctor> Doctors { get; set; }
        public virtual ICollection<MedicalFacility> MedicalFacilities { get; set; }
    }
} 

是我的控制器:

namespace FindAMedicService.Controllers
{
    public class SpecialitiesController : ApiController
    {
        private ApplicationDbContext db = new ApplicationDbContext();
        // GET: api/Specialities
        public IQueryable<Speciality> GetSpeciality()
        {
            return db.Speciality;
        }
    }
}

当我尝试访问这个特定的服务以获取"Specialities"列表时，我得到以下错误:

<Error>
<Message>An error has occurred.</Message>
<ExceptionMessage>
The 'ObjectContent`1' type failed to serialize the response body for content type 'application/xml; charset=utf-8'.
</ExceptionMessage>
<ExceptionType>System.InvalidOperationException</ExceptionType>
<StackTrace/>
<InnerException>
<Message>An error has occurred.</Message>
<ExceptionMessage>
Type 'System.Data.Entity.DynamicProxies.Speciality_DC264D6DBBAF52FB19E27F20DCC47DA1141620CEC0CCAF6E2DEF4D8907FA7C8C' with data contract name 'Speciality_DC264D6DBBAF52FB19E27F20DCC47DA1141620CEC0CCAF6E2DEF4D8907FA7C8C:http://schemas.datacontract.org/2004/07/System.Data.Entity.DynamicProxies' is not expected. Consider using a DataContractResolver if you are using DataContractSerializer or add any types not known statically to the list of known types - for example, by using the KnownTypeAttribute attribute or by adding them to the list of known types passed to the serializer.
</ExceptionMessage>
<ExceptionType>
System.Runtime.Serialization.SerializationException
</ExceptionType>
<StackTrace>
at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeAndVerifyType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, Boolean verifyKnownType, RuntimeTypeHandle declaredTypeHandle, Type declaredType) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithXsiType(XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle objectTypeHandle, Type objectType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle, Type declaredType) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.InternalSerialize(XmlWriterDelegator xmlWriter, Object obj, Boolean isDeclaredType, Boolean writeXsiType, Int32 declaredTypeID, RuntimeTypeHandle declaredTypeHandle) at WriteArrayOfSpecialityToXml(XmlWriterDelegator , Object , XmlObjectSerializerWriteContext , CollectionDataContract ) at System.Runtime.Serialization.CollectionDataContract.WriteXmlValue(XmlWriterDelegator xmlWriter, Object obj, XmlObjectSerializerWriteContext context) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.WriteDataContractValue(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle) at System.Runtime.Serialization.XmlObjectSerializerWriteContext.SerializeWithoutXsiType(DataContract dataContract, XmlWriterDelegator xmlWriter, Object obj, RuntimeTypeHandle declaredTypeHandle) at System.Runtime.Serialization.DataContractSerializer.InternalWriteObjectContent(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver) at System.Runtime.Serialization.DataContractSerializer.InternalWriteObject(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver) at System.Runtime.Serialization.XmlObjectSerializer.WriteObjectHandleExceptions(XmlWriterDelegator writer, Object graph, DataContractResolver dataContractResolver) at System.Runtime.Serialization.DataContractSerializer.WriteObject(XmlWriter writer, Object graph) at System.Net.Http.Formatting.XmlMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, HttpContent content) at System.Net.Http.Formatting.XmlMediaTypeFormatter.WriteToStreamAsync(Type type, Object value, Stream writeStream, HttpContent content, TransportContext transportContext, CancellationToken cancellationToken) --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Web.Http.WebHost.HttpControllerHandler.<WriteBufferedResponseContentAsync>d__1b.MoveNext()
</StackTrace>
</InnerException>
</Error>

我没有这个错误之前，当我没有两个导航属性在我的专业类(医生，医疗设施)。

任何帮助都是非常感激的。

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

namespace Models
{
    [Serializable]
    public class Speciality
    {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int SpecialityId { get; set; }
        public string Name { get; set; }
        public virtual ICollection<Doctor> Doctors { get; set; }
        public virtual ICollection<MedicalFacility> MedicalFacilities { get; set; }
    }
} 

public class SpecialitiesController : ApiController
    {
        private ApplicationDbContext db = new ApplicationDbContext();
        public SpecialitiesController()
        {
            db.Configuration.ProxyCreationEnabled = false;
        }
        // GET: api/Specialities
        public IQueryable<Speciality> GetSpeciality()
        {
            return db.Speciality;
        }
    }