public void WalletAddCashBackError()
{
Wallet wallet = new Wallet();
Action action = () => { wallet.AddCahsBack(null); };
Assert.ThrowsException <Exception>(action);
}
public void WalletAddCashBackSuccess()
{
Wallet wallet = new Wallet();
int currentBalance = wallet.Balance;
var cashback = new List <ICoin>();
cashback.Add(new Coin(1, 2));
cashback.Add(new Coin(2, 1));
cashback.Add(new Coin(5, 1));
cashback.Add(new Coin(10, 1));
wallet.AddCahsBack(cashback);
Assert.AreEqual(currentBalance + cashback.Sum(c => c.CoinValue * c.CoinsCount), wallet.Balance);
}