Mock.cs

[Fact]
public async Task DetailPassTheCorrectIdIntoPortfolioByIdQuery()
{
   //This is a mock
   var _mediator = new Mock<IMediator>() ;
   var _mapper = new Mock<IMapper>();

    //Arrange
    var sut = new PortfolioController(_mapper.Object, _mediator.Object);

    //Act
    var portfolioId = 2;
    await sut.Detail(portfolioId);

    //Assert
    _mediator.Verify(m => m.Send(It.Is<PortfolioByIdQuery>(s => s.PortfolioId == portfolioId), It.IsAny<CancellationToken>()));
}

 [Fact]
public async Task DetailReturnsCorrectView()
{
   //This is a stub
   var _mediator = new Mock<IMediator>() ;
     var _mapper = new Mock<IMapper>();
   
    //Arrange
    var sut = new PortfolioController(_mapper.Object, _mediator.Object);
    var portfolio = new Portfolio { PortfolioId = 1 };
    _mediator.Setup(m => m.Send(It.IsAny<PortfolioByIdQuery>(), It.IsAny<CancellationToken>())).ReturnsAsync(portfolio);
    _mapper.Setup(m => m.Map<Portfolio, PortfolioViewModel>(It.IsAny<Portfolio>())).Returns(It.IsAny<PortfolioViewModel>());

    //Act
    var result = (ViewResult)await sut.Detail(1);

    //Assert
    result.Should().NotBeNull();
    result.Should().BeOfType<ViewResult>();
    result.ViewName.Should().BeNull();
}