"；对象未设置为对象的实例“；使用在一个实例中有效但在另一个实例不起作用的代码

我在asp.net 4.6上使用VS'15。

我还在为我的应用程序建模，在以下位置找到了N层设计：

使用实体框架实现通用数据访问层

我可以从User表返回数据，但是当使用基于User代码的代码从另一个表返回数据时，我会得到null返回。

我的BLL代码工作：

using System;
using System.Collections.Generic;
using System.Linq;
using Library.DataAccessLayer;
using Library.Model;
namespace Library.BusinessLogicLayer
{
    public interface IBusinessLogicLayer_User
    {
        IList<UsersModel> GetAllUsers();
        UsersModel GetUserByAapNetUserID(string _UserID);
        void UpdateUser(params UsersModel[] _UserModel);
    }
    public class BusinessLogicLayer_User : IBusinessLogicLayer_User
    {
        private readonly IUsersRepository _UsersRepository;
        public BusinessLogicLayer_User()
        {
            _UsersRepository = new UsersRepository();
        }
        public BusinessLogicLayer_User(IUsersRepository userRepository)
        {
            _UsersRepository = userRepository;
        }
        public IList<UsersModel> GetAllUsers()
        {
            return _UsersRepository.GetAll();
        }
        public UsersModel GetUserByAapNetUserID(string _UserID)
        {
            return _UsersRepository.GetSingle(u => u.AspNetUserID.Equals(_UserID));
        }
        public void UpdateUser(params UsersModel[] _UsersModel)
        {
            _UsersRepository.Update(_UsersModel);
        }
    }
}

我的BLL代码不起作用：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Library.DataAccessLayer;
using Library.Model;
namespace Library.BusinessLogicLayer
{
    public interface IBusinessLogicLayer_AreaSecurity
    {
        AreaSecurityModel GetAreaSecurityByUserID(string _AspNetUserID);
        void AddAreaSecurity(params AreaSecurityModel[] _AreaSecurityModel);
        void UpdateAreaSecurity(params AreaSecurityModel[] _AreaSecurityModel);
        IList<AreaSecurityModel> GetAllAreaSecurity();
    }
    public class BusinessLogicLayer_AreaSecurity : IBusinessLogicLayer_AreaSecurity
    {
        private readonly IAreaSecurityRepository _AreaSecurityRepository;
        public BusinessLogicLayer_AreaSecurity()
        {
            _AreaSecurityRepository = new AreaSecurityRepository();
        }
        public BusinessLogicLayer_AreaSecurity(IAreaSecurityRepository areaSecurityRepository)
        {
            _AreaSecurityRepository = areaSecurityRepository;
        }
        public void AddAreaSecurity(params AreaSecurityModel[] _AreaSecurityModel)
        {
            _AreaSecurityRepository.Add(_AreaSecurityModel);
        }
        public IList<AreaSecurityModel> GetAllAreaSecurity()
        {
            return _AreaSecurityRepository.GetAll();
        }
        public AreaSecurityModel GetAreaSecurityByUserID(string _AspNetUserID)
        {
            return _AreaSecurityRepository.GetSingle(r => r.AspNetUserID.Equals(_AspNetUserID));
        }
        public void UpdateAreaSecurity(params AreaSecurityModel[] _AreaSecurityModel)
        {
            _AreaSecurityRepository.Update(_AreaSecurityModel);
        }
    }
}

这是调用有效和无效的东西的代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using Library.BusinessLogicLayer;
using Library.Model;

namespace IdahoFalls9thWardBulletin.Controllers
{
    public class ApplicationBaseController : Controller
    {
        protected override void OnActionExecuted(ActionExecutedContext filterContext)
        {
            if (User != null)
            {
                var context = new ApplicationDbContext();
                var username = User.Identity.Name;
                if (!string.IsNullOrEmpty(username))
                {
                    var user = context.Users.SingleOrDefault(u => u.UserName == username);
                    BusinessLogicLayer_User _BusinessLogicLayer_User = new BusinessLogicLayer_User();
                    var UserDetails = _BusinessLogicLayer_User.GetUserByAapNetUserID(user.Id);
                    ViewData.Add("FirstName", UserDetails.FirstName);
                    BusinessLogicLayer_AreaSecurity _BusinessLogicLayer_AreaSecurity = new BusinessLogicLayer_AreaSecurity();
                    AreaSecurityModel _AreaSecurityModel = _BusinessLogicLayer_AreaSecurity
                        .GetAllAreaSecurity()
                        .ToList()
                        .FirstOrDefault();
                    ViewData.Add("AreaSecurity", _AreaSecurityModel);
                }
            }
            base.OnActionExecuted(filterContext);
        }
        public ApplicationBaseController()
        {
        }
    }
}

我不明白的是，当我执行_BusinessLogicLayer_AreaSecurity.GetAllAreaSecurity().ToList().FirstOrDefault()时，它实际上返回了所有记录的列表。

但是，当我执行_BusinessLogicLayer_AreaSecurity.GetAllAreaSecurity().GetAreaSecurityByUserID(user.Id)时，它返回空值。

我不知道我在这里错过了什么。这基本上是我第一次尝试MVC代码优先。