C# object reference not set to an instance of an object in IHttpActionResult

Question

I have had some problems with my code giving a nullreferenceexception. But i don't understand why. In the code below my program gives me a nullrefexc.

The second code part is the variable which the 'var models' should represent. I also tried using the second part code but it also says the same if I use the variable favorites in the IHttpActionResult code.

Exception:

            public IHttpActionResult GetAllFavorites()
            {
                var models = _favoriteModelFactory.CreateFavoritesFromFavoriteModel(_favoriteRepository.GetAll(),
                    _favoriteRepository);
                return Ok(models);
            }

Second part:

        public IEnumerable<Favorite> GetAll()
    {
        IEnumerable<Favorite> favorites = new List<Favorite>
        {
            new Favorite
            {
                StudentId = 1,
                FavoritesIds = {2,5,8},
                FavoriteId = 1
            }
        };
        return favorites;         
    }

Thanks for helping!

Answer 1

Assuming "FavoritesIds" is a List.. It is never instantiated in your code snippet. So when you assign {2,5,8} to it, you get null reference exception.

You can solve this in two ways.

1. In Favourite class constructor

2. In GetAll() Method

   public static IEnumerable<Favorite> GetAll()
           {
               IEnumerable<Favorite> favorites = new List<Favorite>()
           {
               new Favorite
               {
                   StudentId = 1,
                   FavoritesIds = new List<int> {2,5,8},
                   FavoriteId = 1
               }
           };
               return favorites;
           }    
   

OR

public class Favorite
    {
        public Favorite()
        {
            FavoritesIds = new List<int>();
        }
        public int StudentId { get; set; }
        public List<int> FavoritesIds { get; set; }
        public int FavoriteId { get; set; }
    }