InteractionTestWithMock.cs

Fact]
public async Task Detail_PassesTheCorrectIdIntoPortfolioByIdQuery()
{
    var mapperFake = A.Fake<IMapper>();
    var mediatorMock = A.Fake<IMediator>();
    var sut = new PortfolioController(mapperFake, mediatorMock);
    
    var portfolioId = 2;
    await sut.Detail(portfolioId);

    A.CallTo(() => mediatorMock.Send(A<PortfolioByIdQuery>.That.Matches(p => p.PortfolioId == portfolioId),
                                      A<CancellationToken>._)).MustHaveHappened();
}