I'm sorry I am new, but I have a lot of question. :)
var cartItem = storeDB.Carts.SingleOrDefault(
c => c.CartId == shoppingCartId &&
c.AlbumId == album.AlbumId);
Could you explain that why use SingleOrDefault not Single?
according to the underside: if (cartItem == null) so it also maybe return null while we use the "Default"?