public RecipeIngredient Create(Guid ingredientId, double grams) { if (grams <= 0) { throw new ArgumentException("Cant add ingredients with grams lower than zero."); } var id = new Identity <Guid>(ingredientId); return(_ingredientRepository.ExistById(id.Value) ? new RecipeIngredient(id, grams) : throw new ArgumentNullException($"Ingredient with Id:{ingredientId} does not exist.")); }