Nunit, NCrunch and NSubstitute - Always 2 tests failing with UnexpectedArgumentMatcherException, which change (quasi) randomly

Question

I have a text fixture which uses NUnit 3.4.1, NSubstitute 1.10.0 and NCrunch 2.23.0.2

There are at any point in time 2 faling tests in this fixture. Which test fail seems to vary every time I change something. Not all tests end up failing some time or another, but most do, and the problem is always an NSubstitute exception on a line like:

// _clock is initialized as _clock = Substitute.For<IClock>();
// the dates in the Returns statement change on every test
_clock.Now.Returns(new DateTime(2015, 1, 1));

I add NCrunch to this mix because all tests seem to pass with the Resharper 2016 test runner. Mostly.

The exception I invariably get is:

NSubstitute.Exceptions.UnexpectedArgumentMatcherException : Argument matchers 
    (Arg.Is, Arg.Any) should only be used in place of member arguments. 
Do not use in a Returns() statement or anywhere else outside of a member call.

Which is clear enough, except that on mosts of the tests I don't use any Arg.Is or Arg.Any.

IClock interface

Here's the IClock inerface in all it's glory. Now is a getter-only property, but that shouldn't be a problem for NSubstitute, should it?

public interface IClock
{
    DateTime Now { get; }
}

Full Fixture, coming up

Sorry for the enormous amount of code coming up, but I don't want to assume it's due to one test or the other, so here goes:

[TestFixture]
public class AuctionTests : TestBase
{
    #region Fields

    AuctionService _auctionService;
    IClock _clock;
    Ride _ride;
    IMailService _mailer;

    #endregion

    [SetUp]
    public void Init()
    {
        _clock = Substitute.For<IClock>();
        _mailer = Substitute.For<IMailService>();

        _ride = new Ride
        {
            StartAuction = new DateTime(2016, 2, 12, 19, 0, 23),
            PriceForCustomer = 20m,
            InitialAuctionPrice = 15m,
            HighestAuctionPrice = 19m
        };

        SetupData(_ride);

        _auctionService = new AuctionService(RavenSession, _clock, _mailer);
    }

    [Test]
    public void Auction_rejects_price_when_price_is_higher_then_HighestAuctionPrice()
    {
        const decimal price = 90m;
        _clock.Now.Returns(new DateTime(2015, 1, 1));

        var result = _auctionService.Accept(_ride.Id, price);

        result.Should().BeFalse();
    }

    [Test]
    public void Auction_rejects_price_when_inactive()
    {
        _clock.Now.Returns(new DateTime(2016, 2, 12, 20, 1, 23));

        var result = _auctionService.Accept(_ride.Id, Arg.Any<decimal>());

        result.Should().BeFalse();
    }

    [Test]
    public void Auction_is_inactive_when_current_time_is_before_auction_startDate()
    {
        _clock.Now.Returns(new DateTime(2016, 2, 12, 18, 0, 23));

        var result = _auctionService.Accept(_ride.Id, Arg.Any<decimal>());

        Assert.IsFalse(result);
    }

    [Test]
    public void Auction_is_active_when_current_time_is_exactly_auction_startDate()
    {
        _clock.Now.Returns(_ride.StartAuction);

        var result = _auctionService.Accept(_ride.Id, _ride.InitialAuctionPrice);

        result.Should().BeTrue();
    }

    [Test]
    public void Action_price_is_valid_at_auction_start_time_if_equal_to_initial_price()
    {
        var price = _ride.InitialAuctionPrice;

        _clock.Now.Returns(_ride.StartAuction);

        var result = _auctionService.Accept(_ride.Id, price);

        result.Should().BeTrue();
    }

    [Test]
    public void Action_price_is_valid_at_auction_end_time_if_equal_to_highest_possible_price()
    {
        var price = _ride.HighestAuctionPrice;

        _clock.Now.Returns(_ride.StartAuction.AddMinutes(60));

        var result = _auctionService.Accept(_ride.Id, price);

        result.Should().BeTrue();
    }

    [Test]
    public void Action_price_is_invalid_if_not_within_time_parameters()
    {
        var price = 12m;

        _clock.Now.Returns(_ride.StartAuction.AddMinutes(30));

        var result = _auctionService.Accept(_ride.Id, price);

        result.Should().BeFalse();
    }

    [Test]
    [Ignore("Uitzoeken hoeveel seconde vertraging wenselijk is")]
    public void Accept_takes_delay_in_requests_into_account()
    {
        var price = 17m;

        _clock.Now.Returns(_ride.StartAuction.AddMinutes(30).AddSeconds(30));

        var result = _auctionService.Accept(_ride.Id, price);

        result.Should().BeTrue();
    }

    [Test]
    public void Ride_is_saved_with_accepted_price()
    {
        var price = 17m;

        var date = _ride.StartAuction.AddMinutes(30);
        _clock.Now.Returns(date);

        var result = _auctionService.Accept(_ride.Id, price);

        Assert.IsTrue(result);

        var dbRide = RavenSession.Load<Ride>(_ride.Id);

        price.IsSameOrEqualTo(dbRide.AcceptedAuctionPrice);
    }

    [Test]
    public void On_Start_Auction_InitialAuctionPrice_should_be_fifteen_percent_of_PriceForCustomer()
    {
        _ride.PriceForCustomer = 100;

        _auctionService.StartAuction(_ride.Id);

        Assert.AreEqual(85m, _ride.InitialAuctionPrice);
    }

    [Test]
    public void On_Start_Auction_send_email_to_priorityPartners()
    {
        var priorityPartner = new Partner { Priority = true, Email = "[email protected]" };

        SetupData(priorityPartner, new Partner { Priority = false });

        _auctionService.StartAuction(_ride.Id);

        _mailer.Received(1).SendAuctionEmail(Arg.Any<string>(), _ride);
    }
}

Is there anything (obvious) that I'm missing? The quasi randomness of it all seems to point at code not being properly re-initialised for every test, but I completely fail to see how.

Any help is much appreciated.

Answer 1

These lines attempt to use an argument matcher with a non-substitute (_auctionService):

var result = _auctionService.Accept(_ride.Id, Arg.Any<decimal>());

Argument matchers can only be used with substitutes, not with standard values created via new.