CartService.cs

public class CartService : ICartService
{
    //...
    CheckoutService checkoutDomainService; 
 

    public CartService(... CheckoutService checkoutDomainService ...)
    {
        //... 
        this.checkoutDomainService = checkoutDomainService;
    }

    public CheckOutResultDto CheckOut(Guid customerId)
    {
        CheckOutResultDto checkOutResultDto = new CheckOutResultDto();

        Cart cart = this.cartRepository.FindOne(new CustomerCartSpec(customerId));
        this.validateCart(customerId, cart);

        Customer customer = this.customerRepository.FindById(cart.CustomerId);

        Nullable<CheckOutIssue> checkOutIssue = 
            this.checkoutDomainService.CanCheckOut(customer, cart);

        if (!checkOutIssue.HasValue)
        {
            Purchase purchase = this.checkoutDomainService.Checkout(customer, cart);
            checkOutResultDto = Mapper.Map<Purchase, CheckOutResultDto>(purchase);
            this.unitOfWork.Commit();
        }
        else
        {
            checkOutResultDto.CheckOutIssue = checkOutIssue;
        }

        return checkOutResultDto;
    }
}