Question

I’m trying to save my Photo class that has a byte[] File field. When trying to save it using context is throws the error

Object reference not set to an instance of an object.

But when debugging I can see that it is not null. I can see all of the properties of the the class including the values in the byte array.

 public class PhotoRepository 
    {
        private static BlogContext _ctx;

        public PhotoRepository()
        {
            _ctx = new BlogContext();
        }

        public static void Save(Photo p)
        {
            _ctx.Photos.Add(p);
            _ctx.SaveChanges();
        }
    }

controller

 public class PhotoController : Controller
    {
        public ActionResult Index()
        {
            using (var ctx = new BlogContext())
            {
                return View(ctx.Photos.AsEnumerable());
            }
        }

        public ActionResult Upload()
        {
            return View(new Photo());
        }

        [HttpPost]
        public ActionResult Upload(PhotoViewModel model)
        {
            var photo =  new Photo();//Mapper.Map<PhotoViewModel, Photo>(model);
            if (ModelState.IsValid)
            {

                photo.AlternateText = model.AlternateText;
                photo.Description = model.Description;
                photo.File = MapStreamToFile(model.File);
                photo.Name = model.Name;
                PhotoRepository.Save(photo);
                return RedirectToAction("Index");
            }
            return View(photo);
        }

        public byte[] MapStreamToFile(HttpPostedFileBase file)
        {
            using (var stream = file.InputStream)
            {
                var memoryStream = stream as MemoryStream;
                if (memoryStream == null)
                {
                    memoryStream = new MemoryStream();
                    stream.CopyTo(memoryStream);
                }
                return memoryStream.ToArray();
            }
        }
    }

Photo

 public class Photo
    {
        public int Id { get; set; }

        public Byte[] File { get; set; }

        public string Name { get; set; }

        public string Description { get; set; }

        public string AlternateText { get; set; }
    }

PhotoViewModel

 public class PhotoViewModel
    {
        public int Id { get; set; }

        public HttpPostedFileBase File { get; set; }

        public string Name { get; set; }

        public string Description { get; set; }

        public string AlternateText { get; set; }
    }

Answer 1

I think the problem is that _ctx is null. Notice that you declare it and Save static, but _ctx is only instantiates in public PhotoRepository(), which is the constructor. As long as it should really be static, instantiate it statically instead of in the constructor:

 public static class PhotoRepository 
    {
        private static BlogContext _ctx = new BlogContext();

        public static void Save(Photo p)
        {
            _ctx.Photos.Add(p);
            _ctx.SaveChanges();
        }
    }

I also changed the class to static, since I only see it contains static members. This may not be correct if you intend more for this class.

Edit: (thanks @pst) I see from looking at your code more, I think this might really be a better design:

 public class PhotoRepository : IDisposable
    {
        private BlogContext _ctx = new BlogContext();

        public void Save(Photo p)
        {
            _ctx.Photos.Add(p);
            _ctx.SaveChanges();
        }

        void IDisposable.Dispose() { _ctx.Dispose(); }
    }

And then always be sure to dispose PhotoRepository when you’re done with it. The reason for my suggested change here is that BlogContext is disposable, and is used with using in another place.