Mock is operator

Question

Is it possible to mock the is operator? In my code I have logic depending on what type of class it is, but the value itself is stored in the class as the common interface.

Basically, what I want to do is return true by defining the mock to be of IValue, but return true for is Value

public class Value : IValue
{
}

public interface IValue
{
}
public class Tester
{
    public bool CheckIfValue(IValue value)
    {
        return value is Value;
    }
}

[Test]
public void TestIfValue()
{
    Tester tester = new Tester();

    var value = MockRepository.GenerateStub<IValue>();
    // can I add anything here which will make CheckIfValue return true?

    bool isValue = tester.CheckIfValue(value);
    Assert.That(isValue, Is.True);
}

Answer 1

No it is not possible to "mock" the is operator, is will return only true in the following cases:

1. The object being checked is an instance of the type being checked for.
2. The object being checked is a sub class of the type being checked for.
3. The object being checked implements the interface type being checked for.

Your design is bad, you shouldn't require any IValue if you actually need it to be an instance of Value.

The point is to depend on abstractions and not the implementation, that's why you have the IValue interface to start with. Tester shouldn't care if it gets a Value, MockValue or anything else, it should just depend on the methods and properties defined in the IValue interface.

The point to creating a mock is to test the behavior of your Tester class in a specific scenario, the mock allows you to specify the behavior of the object Tester receives for that test without having to create lots of dummy classes such as class FakeValueWhichDoes... : IValue in order to test it.