Constructor arguments cannot be passed for interface mocks - MOQ

Question

So I have a unit test which uses a method in a class called ImpositionCalculatorManager. In this class I use dependency injection so I can access the methods on another class called ImpositionCalculatorRepository.

The ImpositionCalculatorManager looks like this:

 public class ImpositionCalculatorManager : IImpositionCalculatorManager {
        private readonly IImpositionCalculatorRepository _impositionCalculatorRepository;
        public ImpositionCalculatorManager(IImpositionCalculatorRepository impositionCalculatorRepository) {
            _impositionCalculatorRepository = impositionCalculatorRepository;
        }
        public ComboBoxItem[] ReturnInkDataSource() {
            return _impositionCalculatorRepository.ReturnInkDataSource();
        }

        public ComboBoxItem[] ReturnCoatingDataSource() {
            return _impositionCalculatorRepository.ReturnCoatingDataSource();
        }
    }
}

In my unit test I mock the ImpositionCalculatorManager and pass in a mocked version ImpositionCalculatorRepository into the constructor for the manager class.

However I get the error Constructor arguments cannot be passed for interface mocks.

This is what my unit test fixture looks like:

 public class ImpositionCalculatorPresenterTestFixture {
        private ImpositionCalculatorPresenter _impositionCalculatorPresenter;
        private readonly SystemVariablesPresenter _systemVariablesPresenter;
        private readonly Mock<IImpositionCalculatorManager> _mockImpositionCalculatorManager;
        private readonly Mock<ISystemVariablesView> _mockSystemVariablesView;
        private readonly Mock<IPrintingDesignManager> _mockPrintingDesignManager;
        private readonly Mock<ISystemVariablesManager> _mockSystemVariablesManager;
        private readonly Mock<IImpositionCalculatorRepository> _mockImpositionCalculatorRepo;
        private Mock<IImpositionFormView> _mockView;

        public ImpositionCalculatorPresenterTestFixture() {
            _mockImpositionCalculatorRepo = new Mock<IImpositionCalculatorRepository>();
//Error occurs on the below line
            _mockImpositionCalculatorManager = new Mock<IImpositionCalculatorManager>(_mockImpositionCalculatorRepo.Object);
            _mockSystemVariablesView = new Mock<ISystemVariablesView>();
            _mockPrintingDesignManager = new Mock<IPrintingDesignManager>();
            _mockSystemVariablesManager = new Mock<ISystemVariablesManager>();
            _systemVariablesPresenter = new SystemVariablesPresenter(_mockSystemVariablesView.Object, _mockSystemVariablesManager.Object);
        }

 [TestMethod]
        public void PopulateInkDataSources_ApplicationOnLoad_InkDataSourcesPopulatedWithDataFromJSON() {
            //Arrange
            SetupImpostionPresenter();
            _mockView.SetupProperty(r => r.InkSideOneDataSource);
            _mockView.SetupProperty(r => r.InkSideTwoDataSource);
            _mockImpositionCalculatorManager.Setup(r => r.ReturnInkDataSource())
                .Returns<ComboBoxItem[]>
                (x => new ComboBoxItem[] {
                        new ComboBoxItem("Test", 1 ),
                        new ComboBoxItem("Test 2", 2)
                });

            //Act
            _mockView.Raise(r => r.FormOnLoad += null, new EventArgs());

            //Assert
            Assert.IsTrue(_mockImpositionCalculatorManager.Object.ReturnInkDataSource()
                == _mockView.Object.InkSideOneDataSource && _mockImpositionCalculatorManager.Object.ReturnInkDataSource()
                == _mockView.Object.InkSideTwoDataSource
                );
        }

private void SetupImpostionPresenter() {
            _mockView = new Mock<IImpositionFormView>();
            _impositionCalculatorPresenter = new ImpositionCalculatorPresenter(_mockView.Object,
            _mockImpositionCalculatorManager.Object, _mockSystemVariablesManager.Object, _systemVariablesPresenter,
            _mockPrintingDesignManager.Object);
        }
    }
}

I've looked on stack overflow and people are saying that it's because an interface doesn't have a constructor and only the class does and that I shouldn't have to pass anything into the constructor but when I remove the constructor arguments from the mock, I receive the error Parameter count mismatch when trying to setup a method from the manager.

I would just like to mock the manager so I can setup a method to return the values which I have already set.

Why will this not work?

Answer 1

ok I reproduced your Parameter count mismatch problem. Try these two changes:

1) (as previously discussed) create your manager without any constructor and without any other dependencies; and

2) fix the Func given to the ReturnInkDataSource mock: change this:

    _mockImpositionCalculatorManager.Setup(r => r.ReturnInkDataSource())
            .Returns<ComboBoxItem[]>
            (x => new ComboBoxItem[] {
                    new ComboBoxItem("Test", 1 ),
                    new ComboBoxItem("Test 2", 2)
            });

to

    _mockImpositionCalculatorManager.Setup(r => r.ReturnInkDataSource())
            .Returns(
                () => new ComboBoxItem[] {
                        new ComboBoxItem("Test", 1 ),
                        new ComboBoxItem("Test 2", 2)
                      }
              );

First change is foundational DI/mocking -- we don't need mocks for mocks.

Second change is Moq specific -- it's necessary because the params for the lambda given to Returns() must match the params provided in Setup() -- see Moq + Unit Testing - System.Reflection.TargetParameterCountException: Parameter count mismatch

Answer 2

I'm assuming you're trying to test the concrete ImpositionCalculatorPresenter implementation? I'm also assuming that this ImpositionCalculatorPresenter implementation is expected to respond to a FormOnLoad event.

If my assumptions are correct, then your _mockImpositionCalculatorManager.Setup(r => r.ReturnInkDataSource()).Returns<ComboBoxItem[]> (...)) should be sufficient and would not require a backing IImpositionCalculatorRepository mock or impl or anything -- your goal of "I would just like to mock the manager" seems to be handled by this moq setup, which fakes the guts of the manager which the ImpositionCalculatorPresenter needs.