What is the difference between Stubs, Mocks, Fakes and Spies?

The lines between the different test doubles are blurred when using a Mocking library like Moq.

I use mocks for almost everything, but I think it's still worthwhile to know the difference between Stubs, Mocks, Fakes and Spies. If nothing else, we will know what people are talking about when they use the different terms.

Before we look at all the different test doubles, let's look at the interface that we are mocking for the examples.

public interface IRepository
{
     bool IsValid(int id);
     void Add(int id, bool valid);
}

Stub

A stub is an object that provides (canned) hardcoded values to method calls. It always returns the same output regardless of the input.

Below is the stubbed version using Moq of a service where the OnPing() function always returns "pong" regardless of the input.

var mock = new Mock<IRepository>();
mock.Setup(x => x.IsValid(It.IsAny<int>())).Returns(true);

An example can be an object that needs to grab some data from the database to respond to a method call. Instead of the actual object, we can introduce a stub and define what data should be returned.

Spy

A spy allows us to verify what functions were called, with what arguments, when, and how often.

For example, let's take a repository class, which does not have an interface associated with it.

public class RepositorySpy : IRepository
{
    public boolean IsValidWasCalled = false;

    public Boolean bool IsValid(int id)
    {
        IsValidWasCalled = true;
        return true;
    }
}

In the test, we will assert that IsValidWasCalled was true.

var spy = new RepositorySpy();
Assert.True(spy.IsValidWasCalled);

We should use spies where we don't care about the return values of functions but want to verify what functions were called, with what arguments, when, and how often.

An example could be to be check that we are sending an email only once and with the correct arguments or that all the logs are logged correctly.

I rarely use Spies, and it will become clear why once we look at Mocks.

Mock

A mock is similar to a stub, but the behaviour of the mocked interface can be changed dynamically based on scenarios. It is also similar to a spy as it allows us to verify that a method was called. However, unlike a spy, which has the assertion in the test, the assertion is in the verify method in a mock.

Below is the mocked version using Moq of a service where the IsValid() function behaves differently for different inputs. We also verify that the correct calls were received.

var mock = new Mock<IRepository>();
mock.Setup(x => x.IsValid(1)).Returns(true);
mock.Setup(x => x.IsValid(2)).Returns(false);
mock.Verify(x => x.Add(1), Times.Once);
mock.Verify(x => x.IsValid(It.IsAny<int>())), Times(2));

A mock can be used anywhere where we would use a stub, but we need to verify multiple more specific behaviours.

Fake

A Fake is an object that has a concrete implementation that works similar to the actual implementation. It is a simplified version of production code. It does not affect the behaviour of the test but simplifies the implementation of the test by removing heavyweight dependencies.

It should be used sparingly and only in cases where a Stub or Mock is not usable.

Below is an implementation of a Fake repository

public class FakeRepository : IRepository
{
    private Dictionary<int, bool> _values = new Dictionary<int, bool>();

    public bool IsValid(int id)
    {
        return _values[id];
    }

    public void Add(int id, bool valid)
    {
        _values.Add(id, open);
    }
}

An example could be using AWS S3 in production but adding a fake that saves files to disk for testing purposes.

Conclusion

Hopefully, now, you understand the difference between the test doubles and are not confused when people talk about fakes, stubs or mocks.