Unable to retrieve data from database using repository pattern - asp.net-mvc

Question

I'm currently learning interfaces and repository pattern and have difficulties with it. I have domain 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; }
}
I also have an Interface "IVehicleRepository"
public interface IVehicleRepository
{
IEnumerable<VehicleMake> All { get; }
}
and a class "VehicleRepository" which implements that interface
public class VehicleRepository : IVehicleRepository
{
private readonly VehicleDbContext _context;
public VehicleRepository(VehicleDbContext context)
{
_context = context;
}
public IEnumerable<VehicleMake> All => _context.VehicleMakes;
}
My DbContext class is following
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)
{
}
}
And my "VehicleController" is following
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);
}
}
So the problem is that when I try in my controller to retrieve "Vehicle Makes", I get null reference exception and I don't why. Did I implemented this properly? If not, what is the proper way to do this?

Answer 1

You are getting NullReferenceException because it's calling your parameterless constructor where you don't have initialization of _vehicleRepository field. Your initialization is in constructor where you are passing IVehicleRepository object.
I believe, you don't have dependedency injection framework registered for correctly getting instance. So, you can simply initialize _vehicleRepository in default constructor :
public class VehicleController : Controller
{
private readonly IVehicleRepository _vehicleRepository;
public VehicleController()
{
_vehicleRepository = new VehicleRepository();
}
// GET: VehicleMakes
public ActionResult Index()
{
return View(_vehicleRepository.All);
}
}
Change your VehicleRepository to support default constructor in following way :
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;
}