Test a method that does something as well as throws an exception - nunit

Question

I'm getting started with TDD (and MoQ). I have a method that takes in an Order and creates a unique CustomerId for that Order.
public class CustomerService
{
public CustomerService(IOrderRepository orderRepo)
{
_orderRepo = orderRepo;
}
public string Create(Order order)
{
//1. Save the Order that comes in, irrespective of being valid or not.
_orderRepo.Save(order);
//2. If the order is invalid, throw an exception.
if (!isValid(order))
throw new ArgumentException();
//3. Create a Customer, generate a unique CustomerId and return that.
return createCustomerAndGenerateCustomerId(order);
}
}
The following test doesn't seem to be working correctly:
[Test]
[ExpectedException(typeof(ArgumentException))]
public void Create_WhenPassedInvalidOrder_StillPersistsIt()
{
//Arrange
var order = new Order(); //This is invalid, as it has some mandatory fields missing
var mockOrderRepo = new Mock<IOrderRepository>();
var customerService = new CustomerService(mockOrderRepo.Object);
//Act
customerService.Create(order); //This should call Save() on the order repo, but still throw an exception.
//Assert
mockOrderRepo.Verify(o => o.Save(order), Times.Once());
}
This test is always passing, even if I don't call _orderRepo.Save(). What am I doing wrong?

Answer 1

You cannot use ExpectedException in this scenario because Nunit will try/catch the exception on the test level so your mockOrderRepo.Verify never gets called.
So you manually need to try catch your customerService.Create call - and if you want manually assert on the thrown exception - or you the Assert.Throws if you're using Nunit 2.5 or higher:
[Test]
public void Create_WhenPassedInvalidOrder_StillPersistsIt()
{
//Arrange
var order = new Order();
var mockOrderRepo = new Mock<IOrderRepository>();
var customerService = new CustomerService(mockOrderRepo.Object);
//Act
Assert.Throws<ArgumentException>(() => customerService.Create(order));
//Assert
mockOrderRepo.Verify(o => o.Save(order), Times.Once());
}