Question

Moq has been driving me a bit crazy on my latest project. I recently upgraded to version 4.0.10827, and I’m noticing what seems to me to be a new behavior.

Basically, when I call my mocked function (MakeCall, in this example) in the code I am testing, I am passing in an object (TestClass). The code I am testing makes changes to the TestClass object before and after the call to MakeCall. Once the code has completed, I then call Moq’s Verify function. My expectation is that Moq will have recorded the complete object that I passed into MakeCall, perhaps via a mechanism like deep cloning. This way, I will be able to verify that MakeCall was called with the exact object I am expecting it to be called with. Unfortunately, this is not what I’m seeing.

I attempt to illustrate this in the code below (hopefully, clarifying it a bit in the process).

1. I first create a new TestClass object. Its Var property is set to "one".
2. I then create the mocked object, mockedObject, which is my test subject.
3. I then call the MakeCall method of mockedObject (by the way, the Machine.Specifications framework used in the example allows the code in the When_Testing class to be read from top to bottom).
4. I then test the mocked object to ensure that it was indeed called with a TestClass with a Var value of "one". This succeeds, as I expected it to.
5. I then make a change to the original TestClass object by re-assigning the Var property to "two".
6. I then proceed to attempt to verify if Moq still thinks that MakeCall was called with a TestClass with a value of "one". This fails, although I am expecting it to be true.
7. Finally, I test to see if Moq thinks MakeCall was in fact called by a TestClass object with a value of "two". This succeeds, although I would initially have expected it to fail.

It seems pretty clear to me that Moq is only holding onto a reference to the original TestClass object, allowing me to change its value with impunity, adversely affecting the results of my testing.

A few notes on the test code. IMyMockedInterface is the interface I am mocking. TestClass is the class I am passing into the MakeCall method and therefore using to demonstrate the issue I am having. Finally, When_Testing is the actual test class that contains the test code. It is using the Machine.Specifications framework, which is why there are a few odd items (‘Because of’, ‘It should…’). These are simply delegates that are called by the framework to execute the tests. They should be easily removed and the contained code placed into a standard function if that is desired. I left it in this format because it allows all Validate calls to complete (as compared to the ‘Arrange, Act Assert’ paradigm). Just to clarify, the below code is not the actual code I am having problems with. It is simply intended to illustrate the problem, as I have seen this same behavior in multiple places.

using Machine.Specifications;
// Moq has a conflict with MSpec as they both have an 'It' object.
using moq = Moq;

public interface IMyMockedInterface
{
    int MakeCall(TestClass obj);
}

public class TestClass
{
    public string Var { get; set; }

    // Must override Equals so Moq treats two objects with the 
    // same value as equal (instead of comparing references).
    public override bool Equals(object obj)
    {
        if ((obj != null) && (obj.GetType() != this.GetType()))
            return false;
        TestClass t = obj as TestClass;
        if (t.Var != this.Var)
            return false;
        return true;
    }

    public override int GetHashCode()
    {
        int hash = 41;
        int factor = 23;
        hash = (hash ^ factor) * Var.GetHashCode();
        return hash;
    }

    public override string ToString()
    {
        return MvcTemplateApp.Utilities.ClassEnhancementUtilities.ObjectToString(this);
    }
}

[Subject(typeof(object))]
public class When_Testing
{
    // TestClass is set up to contain a value of 'one'
    protected static TestClass t = new TestClass() { Var = "one" };
    protected static moq.Mock<IMyMockedInterface> mockedObject = new moq.Mock<IMyMockedInterface>();
    Because of = () =>
    {
        mockedObject.Object.MakeCall(t);
    };

    // Test One
    // Expected:  Moq should verify that MakeCall was called with a TestClass with a value of 'one'.
    // Actual:  Moq does verify that MakeCall was called with a TestClass with a value of 'one'.
    // Result:  This is correct.
    It should_verify_that_make_call_was_called_with_a_value_of_one = () =>
        mockedObject.Verify(o => o.MakeCall(new TestClass() { Var = "one" }), moq.Times.Once());

    // Update the original object to contain a new value.
    It should_update_the_test_class_value_to_two = () =>
        t.Var = "two";

    // Test Two
    // Expected:  Moq should verify that MakeCall was called with a TestClass with a value of 'one'.
    // Actual:  The Verify call fails, claiming that MakeCall was never called with a TestClass instance with a value of 'one'.
    // Result:  This is incorrect.
    It should_verify_that_make_call_was_called_with_a_class_containing_a_value_of_one = () =>
        mockedObject.Verify(o => o.MakeCall(new TestClass() { Var = "one" }), moq.Times.Once());

    // Test Three
    // Expected:  Moq should fail to verify that MakeCall was called with a TestClass with a value of 'two'.
    // Actual:  Moq actually does verify that MakeCall was called with a TestClass with a value of 'two'.
    // Result:  This is incorrect.
    It should_fail_to_verify_that_make_call_was_called_with_a_class_containing_a_value_of_two = () =>
        mockedObject.Verify(o => o.MakeCall(new TestClass() { Var = "two" }), moq.Times.Once());
}

I have a few questions regarding this:

Is this expected behavior?
Is this new behavior?
Is there a workaround that I am unaware of?
Am I using Verify incorrectly?
Is there a better way of using Moq to avoid this situation?

I thank you humbly for any assistance you can provide.

Edit:
Here is one of the actual tests and SUT code that I experienced this problem with. Hopefully it will act as clarification.

// This is the MVC Controller Action that I am testing.  Note that it 
// makes changes to the 'searchProjects' object before and after 
// calling 'repository.SearchProjects'.
[HttpGet]
public ActionResult List(int? page, [Bind(Include = "Page, SearchType, SearchText, BeginDate, EndDate")] 
    SearchProjects searchProjects)
{
    int itemCount;
    searchProjects.ItemsPerPage = profile.ItemsPerPage;
    searchProjects.Projects = repository.SearchProjects(searchProjects, 
        profile.UserKey, out itemCount);
    searchProjects.TotalItems = itemCount;
    return View(searchProjects);
}


// This is my test class for the controller's List action.  The controller 
// is instantiated in an Establish delegate in the 'with_project_controller' 
// class, along with the SearchProjectsRequest, SearchProjectsRepositoryGet, 
// and SearchProjectsResultGet objects which are defined below.
[Subject(typeof(ProjectController))]
public class When_the_project_list_method_is_called_via_a_get_request
    : with_project_controller
{
    protected static int itemCount;
    protected static ViewResult result;
    Because of = () =>
        result = controller.List(s.Page, s.SearchProjectsRequest) as ViewResult;

    // This test fails, as it is expecting the 'SearchProjects' object 
    // to contain:
    // Page, SearchType, SearchText, BeginDate, EndDate and ItemsPerPage
    It should_call_the_search_projects_repository_method = () =>
        s.Repository.Verify(r => r.SearchProjects(s.SearchProjectsRepositoryGet, 
            s.UserKey, out itemCount), moq.Times.Once());

    // This test succeeds, as it is expecting the 'SearchProjects' object 
    // to contain:
    // Page, SearchType, SearchText, BeginDate, EndDate, ItemsPerPage, 
    // Projects and TotalItems
    It should_call_the_search_projects_repository_method = () =>
        s.Repository.Verify(r => r.SearchProjects(s.SearchProjectsResultGet, 
            s.UserKey, out itemCount), moq.Times.Once());

    It should_return_the_correct_view_name = () =>
        result.ViewName.ShouldBeEmpty();

    It should_return_the_correct_view_model = () =>
        result.Model.ShouldEqual(s.SearchProjectsResultGet);
}


/////////////////////////////////////////////////////
// Here are the values of the three test objects
/////////////////////////////////////////////////////

// This is the object that is returned by the client.
SearchProjects SearchProjectsRequest = new SearchProjects()
{
    SearchType = SearchTypes.ProjectName,
    SearchText = GetProjectRequest().Name,
    Page = Page
};

// This is the object I am expecting the repository method to be called with.
SearchProjects SearchProjectsRepositoryGet = new SearchProjects()
{
    SearchType = SearchTypes.ProjectName,
    SearchText = GetProjectRequest().Name,
    Page = Page, 
    ItemsPerPage = ItemsPerPage
};

// This is the complete object I expect to be returned to the view.
SearchProjects SearchProjectsResultGet = new SearchProjects()
{
    SearchType = SearchTypes.ProjectName,
    SearchText = GetProjectRequest().Name,
    Page = Page, 
    ItemsPerPage = ItemsPerPage,
    Projects = new List<Project>() { GetProjectRequest() },
    TotalItems = TotalItems
};

Answer 1

Ultimately, your question is whether a mocking framework should take snapshots of the parameters you use when interacting with the mocks so that it can accurately record the state the system was in at the point of interaction rather than the state the parameters might be in at the point of verification.

I would say this is a reasonable expectation from a logical point of view. You are performing action X with value Y. If you ask the mock “Did I perform action X with value Y”, you expect it to say “Yes” regardless of the current state of the system.

To summarize the problem you are running into:

---

• You first invoke a method on a mock object with a reference type parameter.

• Moq saves information about the invocation along with the reference type parameter passed in.

• You then ask Moq if the method was called one time with an object equal to the reference you passed in.

• Moq checks its history for a call to that method with a parameter that matches the supplied parameter and answers yes.

• You then modify the object that you passed as the parameter to the method call on the mock.

• The memory space of the reference Moq is holding in its history changes to the new value.

• You then ask Moq if the method was called one time with an object that isn’t equal to the reference its holding.

• Mock checks its history for a call to that method with a parameter that matches the supplied parameter and reports no.

---

To attempt to answer your specific questions:

1. Is this expected behavior?

   I would say no.

2. Is this new behavior?

   I don’t know, but it’s doubtful the project would have at one time had behavior that facilitated this and was later modified to only allow the simple scenario of only verifying a single usage per mock.

3. Is there a workaround that I am unaware of?

   I’ll answer this two ways.

   From a technical standpoint, a workaround would be to use a Test Spy rather than a Mock. By using a Test Spy, you can record the values passed and use your own strategy for remembering the state, such as doing a deep clone, serializing the object, or just storing the specific values you care about to be compared against later.

   From a testing standpoint, I would recommend that you follow the principle “Use The Front Door First”. I believe there is a time for state-based testing as well as interaction-based testing, but you should try to avoid coupling yourself to the implementation details unless the interaction is an important part of the scenario. In some cases, the scenario you are interested in will be primarily about interaction (“Transfer funds between accounts”), but in other cases all you really care about is getting the correct result (“Withdraw $10”). In the case of the specification for your controller, this seems to fall into the query category, not the command category. You don’t really care how it gets the results you want as long as they are correct. Therefore, I would recommend using state-based testing in this case. If another specification concerns issuing a command against the system, there may still end up being a front door solution which you should consider using first, but it may be necessary or important to do interaction based testing. Just my thoughts though.

4. Am I using Verify incorrectly?

   You are using the Verify() method correctly, it just doesn’t support the scenario you are using it for.

5. Is there a better way of using Moq to avoid this situation?

   I don’t think Moq is currently implemented to handle this scenario.

Hope this helps,

Derek Greer
http://derekgreer.lostechies.com
http://aspiringcraftsman.com
@derekgreer