OrderTests.cs

[Property(Arbitrary = new[]
                      {
                          typeof (Generator)
                      })]
public void DiscountIsApplied_UponCheckout(decimal discount,
                                           decimal price)
{
    var productRequest = new ProductRequest()
                         {
                             Discount = discount,
                             Price = price,
                             ProductId = (ProductId)1,
                             Quantity = 2
                         };

    var orderRequest = CreateDefaultOrderRequest(productRequest);

    var order = Order.OrderFactory.CreateFrom(Guid.NewGuid(), orderRequest);

    var amountToPay = order.Checkout();

    var expectedAmountToPay = CalculateAmountToPay(order);

    amountToPay.Should().Be(expectedAmountToPay);
}