无法使用存储库模式从数据库中检索数据

Question

我目前正在学习接口和存储库模式，但遇到了困难。我有域名 class "VehicleMake"

  public class VehicleMake
{
    public VehicleMake()
    {

    }

    [Key]
    [Column(Order = 1)]
    public int Id { get; set; }

    [Required]
    [StringLength(50)]
    public string Name { get; set; }

    public virtual ICollection<VehicleModel> VehicleModels { get; set; }
}

我还有一个接口"IVehicleRepository"

    public interface IVehicleRepository
{
    IEnumerable<VehicleMake> All { get; }
}

和实现该接口的 class "VehicleRepository"

public class VehicleRepository : IVehicleRepository
{
    private readonly VehicleDbContext _context;

    public VehicleRepository(VehicleDbContext context)
    {
        _context = context;
    }

    public IEnumerable<VehicleMake> All => _context.VehicleMakes;
}

我的 DbContext class 正在关注

public class VehicleDbContext : DbContext
{
    public VehicleDbContext() : base("VehicleDbContext")
    {

    }

    public DbSet<VehicleMake> VehicleMakes { get; set; }
    public DbSet<VehicleModel> VehicleModels { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {

    }

}

我的"VehicleController"正在关注

 public class VehicleController : Controller
{
    private readonly IVehicleRepository _vehicleRepository;

    public VehicleController()
    {

    }

    public VehicleController(IVehicleRepository vehicleRepository)
    {
        _vehicleRepository = vehicleRepository;
    }

    // GET: VehicleMakes
    public ActionResult Index()
    {
        return View(_vehicleRepository.All);
    }
}

所以问题是，当我尝试在我的控制器中检索 "Vehicle Makes" 时，我得到空引用异常，但我不知道为什么。我是否正确实施了这个？如果不是，正确的方法是什么？

Answer 1

你得到 NullReferenceException 因为它调用你的无参数构造函数，而你没有初始化 _vehicleRepository 字段。您的初始化是在您传递 IVehicleRepository 对象的构造函数中进行的。

我相信，您没有注册依赖注入框架来正确获取实例。因此，您可以简单地在默认构造函数中初始化 _vehicleRepository :

public class VehicleController : Controller
{
     private readonly IVehicleRepository _vehicleRepository;

     public VehicleController()
     {
              _vehicleRepository = new VehicleRepository();
     }

     // GET: VehicleMakes
     public ActionResult Index()
     {
          return View(_vehicleRepository.All);
     }
}

通过以下方式更改您的 VehicleRepository 以支持默认构造函数：

public class VehicleRepository : IVehicleRepository
{
    private readonly VehicleDbContext _context;
    public VehicleRepository() : this(new VehicleDbContext())
    {
    }
    public VehicleRepository(VehicleDbContext context)
    {
        _context = context;
    }

    public IEnumerable<VehicleMake> All => _context.VehicleMakes;
}

无法使用存储库模式从数据库中检索数据

Unable to retrieve data from database using repository pattern

asp.net-mvc

interface

repository-pattern