Solved: Json JsonSerializer.Serialize of Data in NET6

using System.Text.Json;

JsonSerializer.Serialize(menuItems, new JsonSerializerOptions(){IncludeFields = true}); 

using Newtonsoft.Json;

Old Version: with Newtonsoft in Asp.Net and Net Core 1-3
JsonConvert.SerializeObject(menuItems);

Old Version with Devextreme:

JsonConvert.SerializeObject(DataSourceLoader.Load(menuItems, loadOptions));

using Microsoft.AspNetCore.Mvc;

using prada_drive.Models;

using prada_drive.Resources;

//using Newtonsoft.Json;

using System.Text.Json;

namespace .Controllers.Api

{

    //============< Namespace >============

    [Route("api/[controller]")]

    [ApiController]

    public class ApiSideMenu : ControllerBase

    {

        //========< Controller >========

        private LocalWords _localWords;

        public ApiSideMenu(LocalWords localWords)

        {

            _localWords = localWords;          

        }

        [HttpGet("GetMenuItems")]

        public object GetMenuItems() //(DataSourceLoadOptions loadOptions)

        {

            try

            {

                var menuItems = MenuModel.SideMenuItems;

                foreach (var item in menuItems)

                {

                    item.NameCurrentCulture = _localWords.GetWord(item.Name);

                }

           return JsonSerializer.Serialize(menuItems, new JsonSerializerOptions(){IncludeFields = true}); 

                //return JsonConvert.SerializeObject(menuItems);

                //return JsonConvert.SerializeObject(DataSourceLoader.Load(menuItems, loadOptions));

            }

            catch (Exception)

            {

                return BadRequest(_localWords.GetWord("ErrorRetrieveData").ToString());

            }

        }

        //========</ Controller >========

    }

    //============</ Namespace >============

}