Proper use of MOQ in a Unit Test

Question

Given the following, is this the proper use of MOQ? I am very new to "mocking", "stubbing", "faking", etc. and just trying to wrap my head around it.

The way I understand it is that this mock is providing a known result, so when I test this service using it, the service reacts properly?

public interface IRepository<T> where T : class
{
    void Add(T entity);
    void Delete(T entity);
    void Update(T entity);
    IQueryable<T> Query();
}

public interface ICustomerService
{
    void CreateCustomer(Customer customer);
    Customer GetCustomerById(int id);
}

public class Customer
{
    public int Id { get; set; }

}

public class CustomerService : ICustomerService
{
    private readonly IRepository<Customer> customerRepository;

    public CustomerService(IRepository<Customer> customerRepository)
    {
        this.customerRepository = customerRepository;
    }

    public Customer GetCustomerById(int id)
    {
        return customerRepository.Query().Single(x => x.Id == id);
    }

    public void CreateCustomer(Customer customer)
    {
        var existingCustomer = customerRepository.Query().SingleOrDefault(x => x.Id == customer.Id);

        if (existingCustomer != null)
            throw new InvalidOperationException("Customer with that Id already exists.");

        customerRepository.Add(customer);
    }
}

public class CustomerServiceTests
    {
        [Fact]
        public void Test1()
        {
            //var repo = new MockCustomerRepository();
            var repo = new Mock<IRepository<Customer>>();
            repo.Setup(x => x.Query()).Returns(new List<Customer>() { new Customer() { Id = 1 }}.AsQueryable());

            var service = new CustomerService(repo.Object);

            Action a = () => service.CreateCustomer(new Customer() { Id = 1 });

            a.ShouldThrow<InvalidOperationException>();

        }
    }

I am using xUnit, FluentAssertions and MOQ.

Answer 1

The way I understand it is that this mock is providing a known result, so when I test this service using it, the service reacts properly?

This statement is correct - the unit test should be verifying that the class you're testing (in this case, CustomerService) is exhibiting the behavior you desire. It's not intended to verify that its dependencies are behaving as expected (in this case, IRepository<Customer>).

Your test is good* - you're setting up your mock for the IRepository and injecting into your SystemUnderTest, and verifying that the CustomerService.CreateCustomer() function is exhibiting the behavior that you expect.

*The overall setup of the test is fine, but I'm not familiar with xUnit, so the final two line's syntax is foreign to me, but it looks like it's correct based on the semantics. For reference, you would do the last two lines in NUnit like so:

Assert.Throws<InvalidOperationException>(() => service.CreateCustomer(...));

Answer 2

The test looks fine to me, the mock just provides a fake repository that returns a hardcoded answer just for the test, so the test only cares about the service you're testing and don't deals with a real-life database or whatever, since you're not testing it here.

I would only add one thing to the test to be even more complete. When you setup method calls on the mocks, make sure they were really called by the system under test. After all, the service is supposed to ask the repo for some object and throw only under a certain return value. Moq in particular provides a syntax for this:

repo.VerifyAll();

What this does is simply checking that the setups you've placed before were actually called at least once. This can protect you from errors where the service just throws the exception right away without calling the repo (easy to spot in examples like yours, but with complex code it's easy to miss a call). With that line, at the end of your test, if your service didn't called the repo asking for the list (and with that specific set of parameters), the test will fail too, even if the exception was properly thrown.