Pitfall in FakeItEasy

FakeItEasy

The current project I’m doing is a CQRS based project, using the ncqrs framework. Today I came to a phase I needed some mocking done for some of my domain services. I choose FakeItEasy to handle this for me. So I started by just adding a reference to my DomainScenario project using a Nuget package.

For some of my complexer scenario’s I ran into some issues which took me about about 45 minutes to fix one of my tests. For fixing all other failing tests I had to do the same trick.

Before I explain the issue I ran into with FakeItEasy, I have to mention I use my aggregate roots to prepare my eventstore. My issue is related to this specific way of preparing the eventstore.

For example I have a Participant aggregate root like below.

Participant.cs
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public class Participant : BisAggregateRoot
{
private readonly IUniqueParticipantNrGuard _uniqueFormattedParticipantNrVerifier = NcqrsEnvironment.Get<IUniqueParticipantNrGuard>();

//other members left for convenience
private string _participantNrFormat;
private int _participantNr;
private Gender _gender;
private PartialDate _birthDate;
private string _remark;

public Guid ParticipantId { get { return EventSourceId; } }

// ReSharper disable UnusedMember.Local
private Participant()
// ReSharper restore UnusedMember.Local
{
}

public Participant(Guid participantId, ScientificStudy scientificStudy, string participantNrFormat, int participantNr, Gender gender, PartialDate birthdate)
: base(participantId)
{
_uniqueFormattedParticipantNrVerifier.GuardIsUnique(scientificStudy.ScientificStudyId, participantId,
string.Format(participantNrFormat, participantNr));
ApplyEvent(new ParticipantAddedEvent(Credentials)
{
ParticipantId = participantId,
ParticipantNrFormat = participantNrFormat,
ParticipantNr = participantNr,
ScientificStudyId = scientificStudy.ScientificStudyId,
Gender = gender.ActualValue,
Birthdate = birthdate.ActualValue
});
}

public void Edit(ScientificStudy scientificStudy, string participantNrFormat, int participantNr, Gender gender, PartialDate birthdate, string remark)
{
//Some other business rules
_uniqueFormattedParticipantNrVerifier.GuardIsUnique(scientificStudy.ScientificStudyId, ParticipantId,
string.Format(participantNrFormat, participantNr));
//Some other business rules

ApplyEvent(new ParticipantEditedEvent(Credentials)
{
ParticipantId = ParticipantId,
ParticipantNrFormat = participantNrFormat,
ParticipantNr = participantNr,
Gender = gender.ActualValue,
Birthdate = birthdate.ActualValue
});
}

protected void OnParticipantAdded(ParticipantAddedEvent e)
{
_claimedAliquots = new List<ClaimedAliquot>();
_participantNrFormat = e.ParticipantNrFormat;
_participantNr = e.ParticipantNr;
_gender = new Gender(e.Gender);
_birthDate = new PartialDate(e.Birthdate);
}

protected void OnParticipantEdited(ParticipantEditedEvent e)
{
_participantNrFormat = e.ParticipantNrFormat;
_participantNr = e.ParticipantNr;
_gender = new Gender(e.Gender);
_birthDate = new PartialDate(e.Birthdate);
_remark = e.Remark;
}
}

As you can see both the edit and the construction of this aggregate use the same domain service to validate if the participant number is unique. Keep this in mind because of my eventstore preparation style, because this relates to the problem.

For my eventstore I use some fluent builder thing I built, which uses the domainobjects to generate the actual events.

New.cs
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public static class New
{
public static ScientificStudy ScientificStudy(string nr = "M001", string name = "Maastricht Study", string participantNrFormat = "P{0}", string boxNameFormat = "B{0}")
{
return new ScientificStudy(Default.Id.For.ScientificStudy, nr, name, participantNrFormat, boxNameFormat);
}

public static Participant Participant(ScientificStudy scientificStudy, string participantNrFormat = "P{0}", int participantNr = 1, int gender = 0, string birthdate = "19861117")
{
var realGender = new Gender(gender);
var realBirthDate = new PartialDate(birthdate);
return new Participant(Default.Id.For.Participant, scientificStudy, participantNrFormat, participantNr, realGender, realBirthDate);
}

//other members left for convenience
}

This builder is used like below in my tests. Notice the GetChanges method is a extension method on my aggregateRoots.

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protected override void SetupDependencies()
{
base.SetupDependencies();

var eventStore = NcqrsEnvironment.Get<IEventStore>();
var scientificStudy = New.ScientificStudy();
var scientificStudyEvent = scientificStudy.TrackChanges().GetChanges();
var participantEvents = New.Participant(scientificStudy).GetChanges();
eventStore.Store(Prepare.Events(scientificStudyEvent).ForSourceUncomitted(Default.Id.For.ScientificStudy, Default.Id.Random()));
eventStore.Store(Prepare.Events(participantEvents).ForSourceUncomitted(Default.Id.For.Participant, Default.Id.Random()));
}

For the test I want to do I registered my Fakes in first instance like below, which made my test fail. I am trying to test if the editing causes an ‘FormattedParticipantNrShouldBeUniqueException’ when changing the participantNr into an already excisting participantNr.

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protected override void RegisterFakesInConfiguration(EnvironmentConfigurationWrapper configuration)
{
var commandService = new CommandService();
commandService.RegisterExecutor(new EditParticipantCommandExecutor());

var uniqueParticipantGuard = A.Fake<IUniqueParticipantNrGuard>();
A.CallTo(uniqueParticipantGuard).DoesNothing().Once();
A.CallTo(uniqueParticipantGuard).Throws(
new FormattedParticipantNrShouldBeUniqueException(_formattedParticipantNr));

configuration.Register<ICommandService>(commandService);
configuration.Register(uniqueParticipantGuard);

//other fakes left for convenience

base.RegisterFakesInConfiguration(configuration);
}

After 45 minutes searching and discussing with my colleague, we found a solution.

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protected override void RegisterFakesInConfiguration(EnvironmentConfigurationWrapper configuration)
{
var commandService = new CommandService();
commandService.RegisterExecutor(new EditParticipantCommandExecutor());

var uniqueParticipantGuard = A.Fake<IUniqueParticipantNrGuard>();
A.CallTo(uniqueParticipantGuard).Throws(
new FormattedParticipantNrShouldBeUniqueException(_formattedParticipantNr));
A.CallTo(uniqueParticipantGuard).DoesNothing().Once();

configuration.Register<ICommandService>(commandService);
configuration.Register(uniqueParticipantGuard);

//other fakes left for convenience

base.RegisterFakesInConfiguration(configuration);
}

As you can see the solution was really simple, but hard to find, because debugging didn’t highlighted this was the issue.

Just switch the order of configuring the Mocked/Faked object.

Last but not least I show you my actual testmethod.

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[Then]
public void it_should_throw_an_formatted_participant_nr_should_be_unique_exception_containing_the_formatted_participant_nr()
{
var exception = (FormattedParticipantNrShouldBeUniqueException)CaughtException;
exception.FormattedParticipantNr.Should().Be(_formattedParticipantNr);
}

I’m not sure if I was just doing it wrong or if FakeItEasy just needs a fix to prevent you running into this issue. Hope I made the issue clear and it will save you some time when encountering some same issues.

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