C# and Moq: How to validate an instance method is calling the proper secondary instance method?

Question

I have a lot of experience in Mocking in another languages like Ruby, but I am new with Moq in C#, I am trying to validate what for me is the most basic mock: to validate that a method is calling the proper method with the proper parameter.

I have this example Subject class

public class Subject
{
    int _age;

    public Subject(int age)
    {
        _age = age;
    }

    public void MainMethod()
    {
        if(_age <= 13)
        {
            KidMethod();
        }
        else
        {
            AdultMethod();
        }
    }

    public void KidMethod() {}
    public void AdultMethod() {}
}

And I am trying to create this Test:

[Test]
public void MainMethod_ShouldCallKidMethod_IfAgeBelow1()
{
    var subjectMock = new Mock<Subject>(12);
    subjectMock.Verify(subject => subject.KidMethod());
    subjectMock.MainMethod();
}

But obviously I am doing something wrong because I get this error:

error CS1061: 'Mock' does not contain a definition for 'MainMethod' and no accessible extension method 'MainMethod' accepting a first argument of type 'Mock' could be found (are you missing a using directive or an assembly reference?)

I think I am missing some basic understanding of Mocking with Moq solving this example will help me to get it.

I have been checking similar Questions in SO but all the ones I've checked were covering more specific cases and I didn't find anyone solving this general simple case.

Update 1

This is compiling:

[Test]
public void MainMethod_ShouldCallKidMethod_IfAgeBelow1()
{
    var subjectMock = new Mock<Subject>(12);
    subjectMock.Object.MainMethod();
    subjectMock.Verify(subject => subject.KidMethod());
}

But now I have the error:

System.NotSupportedException : Unsupported expression: subject => subject.KidMethod() Non-overridable members (here: Subject.KidMethod) may not be used in setup / verification expressions.

Looks like that Moq can not be used with methods that can't be overridden 1

Is there no way to do what I am trying?

Answer 1

The point is that what you're asking for is not what you should be doing. Yes, mocking framework can verify method calls, on mocks. You mock dependencies.

You don't have a dependency, you want to test that a method on the class you want to test calls another method on the class you want to test.

That's not what mocking nor unit testing are for, you're doing the wrong thing here. A method calling another method on the same class is an implementation detail, and not something that you should test. What matters is the output of the method, or the state of the object after the method call. Test that.

See for example How to use Moq in unit test that calls another method in same class.

Answer 2

Your example doesn't make much sense to me: you are trying to mock out certain methods of the same class you are testing. Here is a more typical example.

Your class might use some interface:

public class Subject
{
    private readonly IHelper _helper;
    int _age;

    public Subject(IHelper helper, int age)
    {
        _helper = helper;
        _age = age;
    }

    public void MainMethod()
    {
        if(_age <= 13)
        {
            _helper.KidMethod();
        }
        else
        {
            _helper.AdultMethod();
        }
    }
}

So your test would mock out that interface. This way we are only testing Subject in isolation.

[Test]
public void MainMethod_ShouldCallKidMethod_IfAgeBelow1()
{
    // Arrange.
    var helperMock = new Mock<IHelper>();
    var subject = new Subject(helperMock.Object, 12);

    // Act.
    subject.MainMethod();

    // Assert.
    helperMock.Verify(h => h.KidMethod());
}