when mocked method with ref parameter is used, assert MustHaveHappened fails - unit-testing

Question

Im using FakeItEasy to mock methods for unit tests. One of method (using REF parameter - maybe this is important) saves data in database, so it's mocked as 'Does Nothing'
A.CallTo(() => mockedUserRepository.Save(ref mwbeUserData)).DoesNothing();
. But in this situation last assert fails
A.CallTo(() => mockedUserRepository.Save(ref mwbeUserData)).MustHaveHappened(Repeated.Exactly.Once);
with error:
Assertion failed for the following call:
MobileWallet.Common.DAL.IMwbeUserRepository.Save(<NULL>)
Expected to find it exactly once but found it #0 times among the calls:
1: MobileWallet.Common.DAL.IMwbeUserRepository.Get(userName: \"AAA\")
2: MobileWallet.Common.DAL.IMwbeUserRepository.Save(user: MobileWallet.Common.Repository.MwbeUserData)
Full code of test
[TestMethod]
public void AddUser_MwbeUserObjectReturned()
{
//Arrange
MwbeUserRegistrationIn userRegistrationIn = new MwbeUserRegistrationIn()
{
BirthDate = DateTime.Today,
Email = "www#wp.pl",
FirstName = "Adam",
SecondName = "Ada2",
UserName = "AAA"
};
//mock mockedNotificationService and related:
INotificationService mockedNotificationService = A.Fake<INotificationService>();
//TODO: create notofication service
//mock IMwbeUserRepository and related
IMwbeUserRepository mockedUserRepository = A.Fake<IMwbeUserRepository>();
MwbeReturnData<MwbeUserData> mwbeReturnData = new MwbeReturnData<MwbeUserData>(MwbeResponseCodes.NotFound);
MwbeUserData mwbeUserData = mwbeReturnData.Data;
A.CallTo(() => mockedUserRepository.Get(userRegistrationIn.UserName)).Returns(mwbeReturnData);
A.CallTo(() => mockedUserRepository.Save(ref mwbeUserData)).DoesNothing();
MwbeUserService userService = new MwbeUserService(mockedUserRepository, mockedNotificationService);
//Act
MwbeUser user = userService.AddUser(userRegistrationIn);
//Assert
Assert.IsNotNull(user);
Assert.AreEqual(userRegistrationIn.Email, user.Email);
Assert.AreEqual(userRegistrationIn.UserName, user.UserName);
Assert.AreEqual(userRegistrationIn.FirstName,user.FirstName);
Assert.AreEqual(userRegistrationIn.SecondName, user.SecondName);
Assert.AreEqual(userRegistrationIn.BirthDate, user.BirthDate);
A.CallTo(() => mockedUserRepository.Get(userRegistrationIn.UserName)).MustHaveHappened(Repeated.Exactly.Once);
A.CallTo(() => mockedUserRepository.Save(ref mwbeUserData)).MustHaveHappened(Repeated.Exactly.Once);
}
UPDATE 1:
Please notice that for both calls to this method Save, value of mwbeUserData is the same
MwbeUserData mwbeUserData = mwbeReturnData.Data;
If this is null (but it's not).
Maybe the is problem with syntax for REF parameter? I read that there should be used method called AssignsOutAndRefParameters, but I dont know exactly how to use it.
For now I will use MATCHES parameter to make it more generic.

Answer 1

I believes that it's occurred because you assert against mwbeUserData instance
#Old Fox has the right of it.
You're asking FakeItEasy if mockedUserRepository.Save(ref mwbeUserData) was called with null input (mwbeUserData must be null at the time you make the check). It saw nothing like that, since the MwbeUserData that was passed to mockedUserRepository.Save inside AddUser appears to have been not null.
If you think the test should've passed, perhaps relaxing your constraint will help, as in FakeItEasy: mocked method is not returning expected result.
Also, note that the refness of the parameter has no effect on this behaviour. the same constraint matching would occur for a "plain" parameter. Only out parameters are exempted by default. See Argument Constraints in the FakeItEasy documentation for more information about this, and more on how to use custom argument matchers.