Using Fakes with Constructor Injection and Action Parameters

[TestClass, Isolated]
public class SystemTypeServiceTests
{
    SystemTypeService service;
    SystemTypeRepository fakeSystemTypeRepository = Isolate.Fake.Instance<SystemTypeRepository>(Members.MustSpecifyReturnValues);

    public SystemTypeServiceTests()
    {
        Isolate.Fake.StaticMethods(typeof(UnitOfWork));
        Isolate.WhenCalled(() => UnitOfWork.Do(null)).DoInstead(context =>
        {
            var action = context.Parameters[0] as Action;
            action.Invoke();
        });
        service = new SystemTypeService(fakeSystemTypeRepository);
    }

    [TestMethod, Isolated]
    public void Retrieve_ValidCode_ReturnsObject()
    {
        string code = "25";
        var expected = new SystemType { Id = 1L, ClientId = 25L, Code = "25", Description = "Test Type" };
        Isolate.WhenCalled(() => fakeSystemTypeRepository.RetrieveByCode(code)).WillReturn(expected);

        var actual = service.Retrieve(code);

        Assert.AreEqual(expected, actual);
    }
}

public SystemType Retrieve(string code)
{
    Check.ForPatternMismatch(code, SystemTypeRepository.CodeFormat, "code");

    SystemType result = null;
    UnitOfWork.Do(() =>
    {
        result = repository.RetrieveByCode(code);
    });
    return result;
}