What is the purpose of unit testing an interface repository

Question

I am unit testing an ICustomerRepository interface used for retrieving objects of type Customer.

• As a unit test what value am I gaining by testing the ICustomerRepository in this manner?
• Under what conditions would the below test fail?
• For tests of this nature is it advisable to do tests that I know should fail? i.e. look for id 4 when I know I've only placed 5 in the repository

I am probably missing something obvious but it seems the integration tests of the class that implements ICustomerRepository will be of more value.

[TestClass]
public class CustomerTests : TestClassBase
{
    private Customer SetUpCustomerForRepository()
    {
        return new Customer()
        {
            CustId = 5,
            DifId = "55",
            CustLookupName = "The Dude",
            LoginList = new[]
            {
                new Login { LoginCustId = 5, LoginName = "tdude" },
                new Login { LoginCustId = 5, LoginName = "tdude2" }
            }
        };
    }

    [TestMethod]
    public void CanGetCustomerById()
    {
        // arrange
        var customer = SetUpCustomerForRepository();
        var repository = Stub<ICustomerRepository>();

        // act
        repository.Stub(rep => rep.GetById(5)).Return(customer);

        // assert
        Assert.AreEqual(customer, repository.GetById(5));
    }
}

Test Base Class

public class TestClassBase
{
    protected T Stub<T>() where T : class
    {
        return MockRepository.GenerateStub<T>();
    }
}

ICustomerRepository and IRepository

public interface ICustomerRepository : IRepository<Customer>
{
    IList<Customer> FindCustomers(string q);
    Customer GetCustomerByDifID(string difId);
    Customer GetCustomerByLogin(string loginName);
}

public interface IRepository<T>
{
    void Save(T entity);
    void Save(List<T> entity);
    bool Save(T entity, out string message);
    void Delete(T entity);
    T GetById(int id);
    ICollection<T> FindAll();
}

Answer 1

I may be missing something but it seems like every single aspect of your test is mocked up?

Generally speaking you only mock up the objects that aren't core to the test. In this case you may use this repository as the source for a function which you expect to do something with the repository to retrieve customer #5 and perform an operation on it.

For example you may mock up a customer repository so that you can call a method that verifies the login of a user. You'd use your mock repository to prevent your unit-test from relying on a real data-source, not to test your mock repository.