public TransactionItem GetTransactionItem(int transactionItemId)
        {
            TransactionItem item = null;

            if (_transactionItems.ContainsKey(transactionItemId))
            {
                item = _transactionItems[transactionItemId];
            }
            else
            {
                throw new Exception("Item does not exist.");
            }

            return(item.Clone());
        }
 public int AddTransactionItem(TransactionItem item)
 {
     item.Id = _transactionItemId++;
     _transactionItems.Add(item.Id, item.Clone());
     return(item.Id);
 }