What are fakes in unit testing

What are fakes in unit testing: mocks vs stubs

Do you know what are fakes? Are stubs and mocks the same thing? Do you know if you need any of them? Once I made the exact same questions. Let’s see what are fakes in unit testing.

In unit testing, fakes or test doubles are classes or components that replace external dependencies. Fakes simulate successful or failed scenarios to test the logic around the real dependencies they replace.

The best analogy to understand fakes are flight simulators. With a flight simulator, teachers create flight and environment conditions to train and test their pilot students in controlled scenarios.

Fakes are like flight simulators. Fakes return values, throw exceptions or record method calls to test the code around it. They create the conditions to test our code in controlled scenarios.

Female aerospace engineer conducts flight simulator
Fakes are like flight simulators. Photo by ThisisEngineering RAEng on Unsplash

An example of Fakes

Let’s move to an example. In our last post when we wrote tests that use DateTime.Now, we slightly covered the concept of fakes. In that post, we wrote a validator for credit cards. It looks something like this:

public class CreditCardValidator : AbstractValidator<CreditCard>
    public CreditCardValidator(ISystemClock systemClock)
        // Rest of code here...
        var now = systemClock.Now;

public interface ISystemClock
    System.DateTime Now { get; }

public class SystemClock : ISystemClock
    public DateTime Now
        => DateTime.Now;

We created a ISystemClock interface with a Now property to replace DateTime.Now inside our validator. Then, in the unit tests, instead of using SystemClock with the real date and time, we wrote a FixedDateClock class to always return the same date and time.

This is one of the tests where wrote that uses the FixedDateClock class.

public class CreditCardValidationTests
    public void CreditCard_ExpiredYear_ReturnsInvalid()
        var when = new DateTime(2021, 01, 01);
        var clock = new FixedDateClock(when);
        var validator = new CreditCardValidator(clock);

        var request = new CreditCardBuilder()
        var result = validator.TestValidate(request);


Well, that FixedDateClock is a fake. It replaces the SystemClock holding the real date and time with a testable alternative. With that fake in place, we make our tests use any date and time we want instead of the real date and time.

To be more precise, the FixedDateClock is a stub. But, let’s find out about stubs and mocks.

What’s the difference between Mocks and Stubs?

Now that we know what fakes are, let’s see two types of fakes: mocks and stubs. This is the difference between them.

Both mocks and stubs are fakes or test doubles. Stubs provide values or exceptions to the code under test and mocks are used to assert that a method was called with the right parameters.

OrderService example

To better understand the difference between mocks and stubs, let’s use another example. Let’s process online orders with an OrderService class.

This OrderService checks if an item has stock available to then charge a credit card. Imagine it uses an online payment processing software and a microservice to find the stock of an item. We use two interfaces, IPaymentGateway and IStockService, to represent the two dependencies. Something like this,

public class OrderService 
    private readonly IPaymentGateway _paymentGateway;
    private readonly IStockService _stockService;

    public OrderService(IPaymentGateway paymentGateway, IStockService stockService)
        _paymentGateway = paymentGateway;
        _stockService = stockService;

    public OrderResult PlaceOrder(Order order)
        if (!_stockService.IsStockAvailable(order))
            throw new OutOfStockException();


        return new PlaceOrderResult(order);

To test the OrderService class, we should check two things:

Let’s write a test for the scenario of an item in stock.

Fake for available stock

First, we need a fake that returns if there’s stock available for any order. Let’s call it: AlwaysAvailableStockService. It looks like this:

public class AlwaysAvailableStockService : IStockService
    public bool IsStockAvailable(Order order)
        return true;

As its name implies, it will always return stock for any order we pass. It simply returns true.

Fake for payment gateway

Second, the OrderService works if it charges a credit card. But, we don’t want to charge a real credit card every time we run our test.

Let’s use a fake to record if the payment gateway was called or not. Let’s name this fake: FakePaymentGateway. It looks like this:

public class FakePaymentGateway : IPaymentGateway
    public bool WasCalled;

    public void ProcessPayment(Order order)
        WasCalled = true;

It has a public field WasCalled we set to true when the method ProcessPayment() is called. This way we can assert if the payment gateway was called.

Now that we have AlwaysAvailableStockService and FakePaymentGateway in place, let’s write the actual test.

public class OrderServiceTests
    public void PlaceOrder_ItemInStock_CallsPaymentGateway()
        var paymentGateway = new FakePaymentGateway();
        var stockService = new AlwaysAvailableStockService();
        var service = new OrderService(paymentGateway, stockService);

        var order = new Order();


The AlwaysAvailableStockService fake is there to provide a value for our test. It’s a stub. And, the FakePaymentGateway is used to assert that the OrderService called the method to charge a credit card. It’s a mock. Actually, we could call it MockPaymentGateway.

Again, stubs provides values and mocks are used to assert.

When to use fakes in our tests? Use fakes when in your unit tests when you depend on external systems you don’t control. For example, third-party APIs and message queues. Assert the right call were made or the right messages were sent.

Notice, we used the UnitOfWork_Scenario_ExpectedResult naming convention in our test. For the expect result part, we used the keyword “Calls”. It shows we expect the OrderService to call a payment gateway to charge credit cards.

For more naming conventions, check 4 test naming conventions

Other types of fakes: dummies, stubs, spies and mocks

We learned about mocks and stubs. But, there are more types of fakes or doubles.

The book xUnit Patterns presents a broader category of fakes. It uses: dummies, stubs, spies and mocks. Let’s quickly go through them.

Dummies are used to respect the signature of methods and classes under test. A dummy is never called inside the code under test. A null value or a null object, like NullLogger, in a class constructor are dummies when we’re testing one of the class methods that doesn’t use that parameter.

Stubs feed our code under test with indirect input values. We use stubs when the real dependencies aren’t available in the test environment or when using one will have side effects. Like charging a credit card. For “xUnit Patterns” stubs are exactly the same as what we described earlier.

Spies are observation points added to the code under test. We use spies to check if the code under test called another component or not, and the parameters it used. According to “xUnit Patterns”, mocks we wrote earlier are actually spies.

Mocks are testable replacements that check if they were used correctly. We use mocks when we know in advanced the parameters the code under test will use. With mocks, we set the expected parameters to be used before calling the code under test. Then, we use a verification method in the mock itself to check if the mock was called with those exact same parameters.

Don’t get confused with all these terms. Let’s stick to the types of fakes presented in the book The Art of Unit Testing (My takeaways here). In there, there are only two types of fakes or test doubles: stubs and mocks. Everything else is a fake. Easier!

Voilà! That’s what fakes are in unit testing. Remember, stubs provide values for our tests and mocks assert that calls were made. That’s the difference between them.

Often, we use the terms fake, stubs and mocks interchangeably. And sometimes we use the term “mocking” to mean the replacement of external components with testable equivalents. But, we have seen there’s a distinction between all these terms.

If you want to start writing fakes with a mocking library, read my post on how to write fakes with Moq. Also, check these tips to write better stubs and mocks.

If you’re new to unit testing, read Unit Testing 101, 4 common mistakes when writing your first tests and 4 test naming conventions.

Happy testing!