Mocking a generic method with type constraints

Question

I have a method I want to test in a class:

public class ClassUnderTest
{
  public T SetProperty<T>(string value, T object) where T : AnObjectType, IAnotherObjectType
  {
    object.property = value;
    return object;
  }
}

I'm using a mock object type that also implements IAnotherObjectTypelike this:

class MockObjectType: AnObjectType, IAnotherObjectType
{
  (implementation of IAnotherObjectType)
}

I want to set up a mock for my class, and CallBase for the method I'm testing, using my mock object, something like this:

string value = "value";
MockObjectType mockObject = new MockObjectType();

Mock<ClassUnderTest> mockClassUnderTest= new Mock<ClassUnderTest>(MockBehavior.Strict);
mockClassUnderTest.Setup(foo => foo.SetProperty<It.IsSubtype<IAnotherObjectType>>(
    It.IsAny<string>(), It.IsAny<It.IsSubtype<IAnotherObjectType>>()))
  .CallBase();

var result = mockClassUnderTest.Object.SetProperty(value, mockObject);

Assert.That(result.property, Is.EqualTo(value));

But this code won't compile, showing the error:

The type ' Moq.It.IsSubtype<IAnotherObjectType> ' cannot be used as type parameter 'T' in the generic type or method ' SetProperty<T>(string, T) '. There is no implicit reference conversion from ' Moq.It.IsSubtype<IAnotherObjectType> ' to ' AnObjectType '

What am I doing wrong here? I assume this is an issue with the syntax of some kind, so how should I express what I'm trying to do here?

EDIT I've clarified the relationship between the object types that the constraints specify.

I should also clarify that the test I've described is a simplified one. I also want to verify other behaviours of the method, ie calling other mocked methods, etc. Using a mock of the class under test is required for this verification. I just want to know how to CallBase for this method on the mocked class.

Answer 1

Do not mock the subject under test. Mock the dependency to behave as expected when the test is exercised

//Arrange
string value = "value";
var mockObject = new Mock<AnObjectType>();
mockObject.As<IAnotherObjectType>().SetupAllProperties();
// now the mock also implements IAnotherObjectType
mockObject.SetupAllProperties();


ClassUnderTest classUnderTest = new ClassUnderTest();

//Act
var result = classUnderTest.SetProperty(value, mockObject.Object);

//Assert
Assert.That(result.property, Is.EqualTo(value));

Reference Moq Quickstart

Answer 2

It could be tested without mocking the actual method under the test

    public void SetPropertySetsValue()
    {
        string value = "value";
        MockObjectType mockObject = new MockObjectType(); // or create object of ObjectType
        ClassUnderTest classUnderTest = new ClassUnderTest();


        var result = classUnderTest.SetProperty(value, mockObject);
        Assert.AreEqual(value, mockObject.property);
    }

That would verify the value was set by SetProperty() method correctly or not.