Favor real dependencies for unit testing

Favor real dependencies for unit testing

If you’ve worked with unit testing, you’ve probably used dependency injection to be able to decouple objects and control their behavior while testing them. You’ve probably injected mocks or stubs into the system under test in order to define repeatable, deterministic unit tests.

Such a test might look like this:

public async Task AcceptWhenInnerManagerAccepts()
    var r = new Reservation(
    var mgrTD = new Mock();
    mgrTD.Setup(mgr => mgr.TrySave(r)).ReturnsAsync(true);
    var sut = new RestaurantManager(
    var actual = await sut.Check(r);

(This C# test uses xUnit.net 2.4.1 with Moq 4.14.1.)

Such tests are brittle. They break easily and therefore increase your maintenance burden.

Why internal dependencies are bad

As the above unit test implies, the RestaurantManager relies on an injected IReservationsManager dependency. This interface is an internal implementation detail. Think of the entire application as a blue box with two objects as internal components:

An application contains many internal building blocks. The above illustration emphasizes two such components, and how they interact with each other.

What happens if you’d like to refactor the application code? Refactoring often involves changing how internal building blocks interact with each other. For example, you might want to change the IReservationsManager interface.

When you make a change like that, you’ll break some of the code that relies on the interface. That’s to be expected. Refactoring, after all, involves changing code.

When your tests also rely on internal implementation details, refactoring also breaks the tests. Now, in addition to improving the internal code, you also have to fix all the tests that broke.

Using a dynamic mock library like Moq tends to amplify the problem. You now have to visit all the tests that configure mocks and adjust them to model the new internal interaction.

This kind of friction is likely to deter you from refactoring in the first place. If you know that a warranted refactoring will give you much extra work fixing tests, you may decide that it isn’t worth the trouble. Instead, you leave the production code in a suboptimal state.

Is there a better way?

Functional core

In order to find a better alternative, you must first understand the problem. Why use test doubles (mocks and stubs) in the first place?

Test doubles serve a major purpose: They enable us to write deterministic unit tests.

Unit tests should be deterministic. Running a test multiple times should produce the same outcome each time (ceteris paribus). A test that succeeds on a Wednesday shouldn’t fail on a Saturday.

By using a test double each test can control how a dependency behaves. In Working Effectively with Legacy Code, Michael Feathers likens a test to a vise. It’s a tool to fix a particular behavior in place.

Test doubles, however, aren’t the only way to make tests deterministic.

A better alternative is to make the production code itself deterministic. Imagine, for example, that you need to write code that calculates the volume of a frustum. As long as the frustum doesn’t change, the volume remains the same number. Such a calculation is entirely deterministic.

Write your production code using mostly deterministic operations. For example, instead of the above RestaurantManager, you can write an immutable class with a method like this:

public bool WillAccept(
    DateTime now,
    IEnumerable existingReservations,
    Reservation candidate)
    if (existingReservations is null)
        throw new ArgumentNullException(nameof(existingReservations));
    if (candidate is null)
        throw new ArgumentNullException(nameof(candidate));
    if (candidate.At < now)
        return false;
    if (IsOutsideOfOpeningHours(candidate))
        return false;
    var seating = new Seating(SeatingDuration, candidate.At);
    var relevantReservations =
    var availableTables = Allocate(relevantReservations);
    return availableTables.Any(t => t.Fits(candidate.Quantity));

This example, like all code in this article, is from my book Code That Fits in Your Head. Despite implementing quite complex business logic, it’s a pure function. All the helper methods involved (IsOutsideOfOpeningHours, Overlaps, Allocate, etc.) are also deterministic.

The upshot is that deterministic operations are easy to test. For instance, here’s a parametrized test of the happy path:

[Theory, ClassData(typeof(AcceptTestCases))]
public void Accept(MaitreD sut, DateTime now, IEnumerable reservations)
    var r = Some.Reservation.WithQuantity(11);
    var actual = sut.WillAccept(now, reservations, r);

This code snippet doesn’t show the test case data source (AcceptTestCases), but it’s a small helper class that produces seven test cases that supply values for sut, now, and reservations.

This test method is typical of unit tests of pure functions:

  1. Prepare input value(s)
  2. Call the function
  3. Compare the expected outcome with the actual value

If you recognize that structure as the Arrange Act Assert pattern, you’re not wrong, but that’s not the main point. What’s worth noticing is that despite non-trivial business logic, no test doubles (i.e. mocks or stubs) are required. This is one of many advantages of pure functions. Since they are already deterministic, you don’t have to introduce artificial seams into the code to enable testing.

Writing most of a code base as deterministic functions is possible, but requires practice. This style of programming is called functional programming (FP), and while it may require effort for object-oriented programmers to shift perspective, it’s quite the game changer—both because of the benefits to testing, and for other reasons.

Even the most idiomatic FP code base, however, must deal with the messy, non-deterministic real world. Where do input values like now and existingReservations come from?

Imperative shell

A typical functional architecture tends to resemble the Ports and Adapters architecture. You implement all business and application logic as pure functions and push impure actions to the edge.

At the edge, and only at the edge, you allow impure actions to take place. In the example code that runs through Code That Fits in Your Head, this happens in controllers. For example, this TryCreate helper method is defined in a ReservationsController class:

private async Task TryCreate(
    Restaurant restaurant,
    Reservation reservation)
    using var scope =
        new TransactionScope(TransactionScopeAsyncFlowOption.Enabled);
    var reservations = await Repository
        .ReadReservations(restaurant.Id, reservation.At)
    var now = Clock.GetCurrentDateTime();
    if (!restaurant.MaitreD.WillAccept(now, reservations, reservation))
        return NoTables500InternalServerError();
    await Repository.Create(restaurant.Id, reservation)
    return Reservation201Created(restaurant.Id, reservation);

The TryCreate method makes use of two impure, injected dependencies: Repository and Clock.

The Repository dependency represents the database that stores reservations, while Clock represents some kind of clock. These dependencies aren’t arbitrary. They’re there to support unit testing of the application’s imperative shell, and they have to be injected dependencies exactly because they’re sources of non-determinism.

It’s easiest to understand why Clock is a source of non-determinism. Every time you ask what time it is, the answer changes. That’s non-deterministic, because the textbook definition of determinism is that the same input should always produce the same output.

The same definition applies to databases. You can repeat the same database query, and over time receive different outputs because the state of the database changes. By the definition of determinism, that makes a database non-deterministic: The same input may produce varying outputs.

You can still unit test the imperative shell, but you don’t have to use brittle dynamic mock objects. Instead, use Fakes.


In the pattern language of xUnit Test Patterns, a fake is a kind of test double that could almost serve as a “real” implementation of an interface. An in-memory “database” is a useful example:

public sealed class FakeDatabase :

While implementing IReservationsRepository, this test-specific FakeDatabase class inherits ConcurrentDictionary>, which means it can leverage the dictionary base class to add and remove reservations. Here’s the Create implementation:

public Task Create(int restaurantId, Reservation reservation)
        new Collection { reservation },
        (_, rs) => { rs.Add(reservation); return rs; });
    return Task.CompletedTask;

And here’s the ReadReservations implementation:

public Task> ReadReservations(
    int restaurantId,
    DateTime min,
    DateTime max)
    return Task.FromResult>(
        GetOrAdd(restaurantId, new Collection())
            .Where(r => min <= r.At && r.At <= max).ToList());

The ReadReservations will return the reservations already added to the repository with the Create method. Of course, it only works as long as the FakeDatabase object remains in memory, but that’s sufficient for a unit test:

[InlineData(1049, 19, 00, "juliad@example.net", "Julia Domna", 5)]
[InlineData(1130, 18, 15, "x@example.com", 

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One thought on “Favor real dependencies for unit testing

  1. Aditya avatar

    Amen to the idea.

    – Prefer real objects, or fakes over mocks. It will make your tests usually more robust.

    – Use mocks when you must: to avoid networking, or other flaky things such as storage.

    – Use mocks for “output only objects”, for example listeners, or when verifying the output for some logging. (But, prefer a good fake)

    – Use mocks when you “need to get shit done”, it’s the easiest way to add tests in an area that has almost none, and the code is not designed to be easily testable. But remember this is tech debt, and try to migrate towards real objects over time.

    That’s my short advice I told many times. So might as well comment with it here.