private void ExecuteCommandAdd(object parameter)
        {
            int q;

            Int32.TryParse(quantity, out q);
            Coin coin = parameter as Coin;

            for (int i = 0; i < q; i++)
            {
                if (coin != null)
                {
                    Repo.AddCoin(coin);
                }
            }
            Total    = repo.TotalValue().ToString();
            Quantity = String.Empty;
        }