如何使用网络 API 格式化日期时间

我在C#中使用Web Api构建了一个简单的Web服务。

我有这个方法：

[Route("secsocdata/{id:int?}")]
[HttpGet]
public HttpResponseMessage getSecSocData(int? id = null)
{
    try
    {
    return Request.CreateResponse(HttpStatusCode.OK, new RCamera((short)status_code.Success, "Ok", getSecSocImages(id)));
    }
    catch (Exception e)
    {
    return Request.CreateResponse(HttpStatusCode.InternalServerError, new RMessage((short)status_code.Exception, HttpStatusCode.InternalServerError.ToString()));
    }
}
[NonAction]
private IQueryable<WSOmniacare.Models.AAHome.ImmaginiSecSoc.ImmaginiSecSocDTO> getSecSocImages(int? id)
{
   var strItem = from u in db_data.CAMERA_SEC_SOC
          where u.ID == id
          select u.Image;
    String imageBas64 = GetString(strItem.First());
    if (id != null)
    {
    return from u in db_data.CAMERA_SEC_SOC 
           where u.ID == id
           select new WSOmniacare.Models.AAHome.ImmaginiSecSoc.ImmaginiSecSocDTO()
           {
           image = imageBas64,
           image_width = u.image_width,
           image_height= u.image_height,
           type = u.type,
           timestamp = u.timestamp.ToString("MM/dd/yyyy HH:mm:ss.fff",
            CultureInfo.InvariantCulture),
           rectangle = new WSOmniacare.Models.AAHome.ImmaginiSecSoc.ImmaginiSecSocDTO.Rectangle()
           {
             rects = from pi in db_data.CAMERA_SEC_SOC_Rectangles 
                where pi.ID_SecSoc  == id
                 select new WSOmniacare.Models.AAHome.ImmaginiSecSoc.ImmaginiSecSocDTO.Rectangle.Rect()
                {   
                height= pi.height,
                width = pi.width,
                x = pi.x,
                y=pi.y
                }
           } 
           };
    }
    return null;
}

因此，如果我尝试调用此 Web 服务，则会出现此错误：

{ "消息"： "发生错误。 "ExceptionMessage"： "'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": "LINQ to Entities does not recognize the method 'System.String ToString(System.String, System.IFormatProvider)' method, and this method cannot be translated into a store expression.", "ExceptionType": "System.NotSupportedException", "StackTrace": " in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.DefaultTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)'r'n in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)'r'n in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator 1.Translate（ExpressionConverter parent， Expression linq）''r' in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression（Expression linq）''r' in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MemberInitTranslator.TypedTranslate（ExpressionConverter parent， MemberInitExpression linq）''r' in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator 1.Translate(ExpressionConverter parent, Expression linq)'r'n in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)'r'n in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input)'r'n in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input, DbExpressionBinding& binding)'r'n in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, DbExpression& source, DbExpressionBinding& sourceBinding, DbExpression& lambda)'r'n in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SelectTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)'r'n in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SequenceMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, SequenceMethod sequenceMethod)'r'n in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)'r'n in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator 1.Translate（ExpressionConverter parent， Expression linq）''r' in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression（Expression linq）''r' in System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.Convert（）''r' in System.Data.Entity.Core.Objects.ELinq.ELinqQueryState.GetExecutionPlan（Nullable 1 forMergeOption)'r'n in System.Data.Entity.Core.Objects.ObjectQuery 1.<>c__DisplayClass7.b__6（）''r' in System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction[T]（Func 1 func, IDbExecutionStrategy executionStrategy, Boolean startLocalTransaction, Boolean releaseConnectionOnSuccess)'r'n in System.Data.Entity.Core.Objects.ObjectQuery 1.<>c__DisplayClass7.b__5（）''r' in System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute[TResult]（Func 1 operation)'r'n in System.Data.Entity.Core.Objects.ObjectQuery 1.GetResults（Nullable 1 forMergeOption)'r'n in System.Data.Entity.Core.Objects.ObjectQuery 1..GetEnumerator>b__0（）''r' in System.Data.Entity.Internal.LazyEnumerator'1.MoveNext（）''r' in Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList（JsonWriter writer， IEnumerable values， JsonArrayContract contract， JsonProperty member， JsonContainerContract collectionContract， JsonProperty containerProperty）''r' inNewtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue（JsonWriter writer， Object value， JsonContract valueContract， JsonProperty member， JsonContainerContract containerContract， JsonProperty containerProperty）''r' in Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject（JsonWriter writer， Object value， JsonObjectContract contract， JsonProperty member， JsonContainerContract collectionContract， JsonProperty containerProperty）''r' inNewtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue（JsonWriter writer， Object value， JsonContract valueContract， JsonProperty member， JsonContainerContract containerContract， JsonProperty containerProperty）''r' in Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize（JsonWriter jsonWriter， Object value， Type objectType）''r' in Newtonsoft.Json.JsonSerializer.SerializeInternal（JsonWriter jsonWriter， Object value， Type objectType）''r' inNewtonsoft.Json.JsonSerializer.Serialize(JsonWriter jsonWriter, Object value)''r' in System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, Encoding effectiveEncoding)''r' in System.Net.Http.Formatting.JsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, Encoding effectiveEncoding)''r' in System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStream(Type type, Object value, StreamwriteStream， HttpContent content）''r' in System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStreamAsync（Type type， Object value， Stream writeStream， HttpContent content， TransportContext transportContext， CancelToken cancelToken）''r'--- Fine traccia dello stack da posizione precedente dove è stata generata l'eccezione ---''r' in System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess（Task task）''r' in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification（Task task）''r' in System.Runtime.CompilerServices.TaskAwaiter.GetResult（）''r' in System.Web.Http.WebHost.HttpControllerHandler.d__1b.移动下一个（）" } }

现在我认为问题出在我插入这行代码时：

timestamp = u.timestamp.ToString("MM/dd/yyyy HH:mm:ss.fff",
                                CultureInfo.InvariantCulture),

如果我使用这个

timestamp = u.timestamp.ToString(),

而不是

timestamp = u.timestamp.ToString("MM/dd/yyyy HH:mm:ss.fff",
                                    CultureInfo.InvariantCulture),

我的代码有效。我该如何修复它？