Why model in Entity Framework 6 can not serialize the data? - entity-framework

Question

I try to get data from model and pass to client with json format. I wrote this code but when execute the code, I get an internal server error. When debugging code, data is shown such as this image.
Please advice.
[HttpPost]
public ActionResult GetCity(int idCountry)
{
TravelEnterAdminTemplate.Models.LG.MyJsonResult myresult = new Models.LG.MyJsonResult();
try
{
List<City> citylist = new List<City>();
var citystable = db.Cities.Where(p => p.CountryId == idCountry).ToList();
if (citystable != null)
{
foreach (var city in citystable)
{
myresult.obj = citystable;
}
myresult.Result = true;
}
else
{
myresult.Result = false;
myresult.message = "داده ای یافت نشد";
}
}
catch (Exception e)
{
errorlog.Error("DeleteShopping", "157", e.Source.ToString(), e.Message);
myresult.Result = false;
myresult.message = "خطا در بارگذاری اطلاعات";
}
return Json(myresult, JsonRequestBehavior.AllowGet);
}
City Entity :
namespace TravelEnterAdminTemplate.Models.dbModel
{
using System;
using System.Collections.Generic;
public partial class City
{
public City()
{
this.ArtPlaces = new HashSet<ArtPlace>();
this.Historicals = new HashSet<Historical>();
this.Hospitals = new HashSet<Hospital>();
this.Hotels = new HashSet<Hotel>();
this.PhotoTables = new HashSet<PhotoTable>();
this.PhotoTables1 = new HashSet<PhotoTable>();
this.Restaurants = new HashSet<Restaurant>();
this.Shoppings = new HashSet<Shopping>();
}
public int Id { get; set; }
public string NameFA { get; set; }
public string NameEN { get; set; }
public int CountryId { get; set; }
public virtual ICollection<ArtPlace> ArtPlaces { get; set; }
public virtual Country Country { get; set; }
public virtual ICollection<Historical> Historicals { get; set; }
public virtual ICollection<Hospital> Hospitals { get; set; }
public virtual ICollection<Hotel> Hotels { get; set; }
public virtual ICollection<PhotoTable> PhotoTables { get; set; }
public virtual ICollection<PhotoTable> PhotoTables1 { get; set; }
public virtual ICollection<Restaurant> Restaurants { get; set; }
public virtual ICollection<Shopping> Shoppings { get; set; }
}
}