public async Task AddCouponByCodeAsync(string couponCode)
{
if (_coupons.Find(o => o.Code == couponCode) != null)
{
throw new ArgumentException("Coupon code is already used.");
}
else
{
ICoupon coupon = await _couponService.GetByCodeAsync(couponCode);
if (coupon == null)
{
throw new ArgumentException("Invalid coupon code.");
}
await AddCouponAsync(coupon);
}
}