LINQ to Entities 无法识别方法 'System.String ToString()'.尝试解析日期时间时

我正在尝试从实体查询中获取自定义数据集，但幸运的是我收到以下错误：

LINQ to Entities 无法识别方法 System.String ToString(( 方法，并且此方法无法转换为存储表达式。 下面是返回数据的自定义类：

public class featuredlist
{
    public string kodg { get; set; }
    public string excursion { get; set; }
    public string excursiondate { get; set; }
}
public IQueryable<featuredlist> GetFeatured(string fdate)
{
   try
   {
     var fd = DateTime.Parse(fdate);
     var uname = HttpContext.Current.User.Identity.Name;
     var uid = (from x in db.xEmployee where x.Сотрудник.Contains(uname) 
                                             select x.Kod).FirstOrDefault();
     var fl = (from a in db.xGrafik
                         join b in db.xExcursion on a.Excursion equals b.Kod
                         join c in db.xDate on a.КодД equals c.Kod
                         join d in db.xOrder on a.Kod equals d.КодГ
                         where c.Дата == fd & d.Продал == uid
                         orderby c.Дата descending
                         select new featuredlist
                         {
                             kodg = a.Kod.ToString(),
                             excursion = b.Название,
                             //Probably error happend here.
                             excursiondate = 
                             (c.Дата) != null ? 
                                         c.Дата.Value.ToShortDateString :
                                         "01.01"
                         })
                           .AsQueryable();
                return fl.Distinct();
    }
    catch (DbEntityValidationException e)
    {
      foreach (var eve in e.EntityValidationErrors)
      {
        var path = @"d:'error.txt";
        using (StreamWriter sw = File.CreateText(path))
        {
           sw.WriteLine("Entity of type '"{0}'" in state '"{1}'" has 
                         the following validation errors:",
           eve.Entry.Entity.GetType().Name, eve.Entry.State);
        }
        using (StreamWriter sw1 = File.CreateText(@"d:'error1.txt"))
              foreach (var ve in eve.ValidationErrors)
              {
                sw1.WriteLine("- Property: '"{0}'", Error: '"{1}'"",
                              ve.PropertyName, ve.ErrorMessage);
              }
      }
      throw;
    }               
}

异常消息："LINQ to Entities 无法识别方法'System.String ToString(('方法，并且此方法无法转换为存储表达式。 堆栈跟踪： " в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.DefaultTranslator.Translate(ExpressionConverter parent， MethodCallExpression call( в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent， MethodCallExpression linq( в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator 1.Translate(ExpressionConverter parent, Expression linq) в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq) в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MemberInitTranslator.TypedTranslate(ExpressionConverter parent, MemberInitExpression linq) в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator 1.Translate(ExpressionConverter parent， Expression linq( вSystem.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq( в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda， DbExpression input( в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda， DbExpression input， DbExpressionBinding& binding( вSystem.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.translate(ExpressionConverter parent， MethodCallExpression callcall call， DbExpression& source， DbExpressionBinding& sourceBinding， DbExpression& lambda( в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SelectTranslator.Translate(ExpressionConverter parent， MethodCallExpression call( вSystem.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SequenceMethodTranslator.Translate(ExpressionConverter parent， MethodCallExpression call， SequenceMethod sequenceMethod( в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent， MethodCallExpression linq( в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator 1.Translate(ExpressionConverter parent, Expression linq) в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq) в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateSet(Expression linq) в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.UnarySequenceMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpression call) в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SequenceMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, SequenceMethod sequenceMethod) в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq) в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator 1.Translate(ExpressionConverterparent， Expression linq( в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq( в System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.Convert(( в System.Data.Entity.Core.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable 1 forMergeOption) в System.Data.Entity.Core.Objects.ObjectQuery 1.<>c__DisplayClassb.b__a(( в System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction[T](Func 1 func, IDbExecutionStrategy executionStrategy, Boolean startLocalTransaction, Boolean releaseConnectionOnSuccess) в System.Data.Entity.Core.Objects.ObjectQuery 1.<>c__DisplayClassb.b__9(( вSystem.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute[TResult](Func 1 operation) в System.Data.Entity.Core.Objects.ObjectQuery 1.GetResults(Nullable 1 forMergeOption) в System.Data.Entity.Core.Objects.ObjectQuery 1..GetEnumerator>b__0(( в System.Lazy 1.CreateValue() в System.Lazy 1.LazyInitValue(( в System.Lazy 1.get_Value() в System.Data.Entity.Internal.LazyEnumerator 1.MoveNext(( в Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer， IEnumerable values， JsonArrayContract contract， JsonProperty member， JsonContainerContract collectionContract， JsonProperty containerProperty( в Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer， Object value， JsonContract valueContract， JsonProperty member， JsonContainerContract containerContract， JsonProperty containerProperty( в Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter， Object value， Type objectType( в Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter， Object value， Type objectType( в Newtonsoft.Json.JsonSerializer.Serialize(JsonWriter jsonWriter， Object value( в System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStream(Type type， Object value， Stream writeStream， Encoding Effective Encoding( в System.Net.Http.Formatting.JsonMediaTypeFormatter.WriteToStream(Type type， Object value， Stream writeStream， Encoding Effective Encoding( в System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStream(Type type， Object value， Stream writeStream， HttpContent content( вSystem.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStreamAsync(Type type， Object value， Stream writeStream， HttpContent content， TransportContext transportContext， CancelToken cancelToken( --- Конец трассировка стека из предыдущего расположения， где возникло исключение --- в System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task( в System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task( вSystem.Runtime.CompilerServices.TaskAwaiter.GetResult(( в System.Web.Http.WebHost.HttpControllerHandler.d__1b.移动下一个((">

kodg = SqlFunctions.StringConvert(a.Kod),

public class featuredlist
        {
            public int kodg { get; set; }
            public string excursion { get; set; }
            public DateTime? excursiondate { get; set; }
        }