private void CheckItemNull(BackpackItem item)
 {
     if (item == null)
     {
         throw new NullReferenceException("Not valid item!");
     }
 }
        public void updateItem(BackpackItem item)
        {
            CheckItemNull(item);

            if (items.ContainsKey(item.ItemName))
            {
                items.Remove(item.ItemName);
                items.Add(item.ItemName, item);
            }
        }
        public void addItem(Item item, int count)
        {
            BackpackItem backpackItem = new BackpackItem();
            backpackItem.Item = item;

            if (backpackRepository.contains(item.ItemName))
            {
                backpackItem.increaseCount(count);
                backpackRepository.updateItem(backpackItem);
            }
            else
            {
                backpackItem.Count = count;
                backpackRepository.addItem(backpackItem);
            }
        }
        public void addItem(BackpackItem item)
        {
            CheckItemNull(item);

            items.Add(item.ItemName, item);
        }