Maybe.

But first, I’ll start by clarifying that developers asking that question usually mean “test doubles“, instead of “mocks”.

Some situations that might indicate code smell in tests include:

• using several test doubles to test one thing
• complex test doubles setup

Both cases aren’t only an indication of a code smell in the test; they often indicate a bigger problem: code smell in the design!

If the system under test (SUT) requires too many test doubles to satisfy its dependencies, it likely violates the Interface Segregation Principle

A client should never be forced to implement an interface that it doesn’t use, or clients shouldn’t be forced to depend on methods they do not use.

Take the example below:

public class SomeClass(Dependency1 Dependency1,   
Dependency2 Dependency2, Dependency3 Dependency3, 
Dependency4 Dependency4)
{ 
 public void SomeMethod()  
 {    
   if (dependency1.CheckThis() && 
       dependency2.CheckThat() && 
       dependency3.CheckSomethingElse() &&   
       dependency4.FinalCheck())    
   {
       // do the thing...    
   }
  }
}

To write a unit test for SomeMethod, we would need mock each one of the 4 dependencies.

By inspecting how those dependencies are used, we could identify a new abstraction that offers what SomeClass needs:

public class SomeClass(Dependency Dependency)
{  
  public void SomeMethod()  
  {   
   if (dependency.CanIDoTheThing())     
   {     
     // do the thing...    
   }  
  }
}

Now there’s only one dependency to deal with in tests, and it’s easier to learn what that dependency provides.

An example of code smell

Here’s a test I’ve run into many years ago:

According to the name of the test, it verifies that the “PUT method returns OK and PortfolioModel when service response is successful“.

When I’ve read through the test, these considerations came to mind:

• Number of stubs and mocks (mockPortfolioService, portModel, response, portfolioModel)
• Overuse of Arg.Any
• Arg.Any hiding the meaning/intent of several parameters
• Unclear what “Configure” is all about (what does it mean to return true from it?)
• What’s the difference between portModel and portfolioModel? Why are both needed?
• The file had about 40 tests that looked very similar to this one in terms of mocks and stubs; a product of copy-and-paste.

After raising the issues with the developers on the team, we identified the design issues that resulted in tests having to be written that way. The tests were rewritten to isolate and call out the issue, and a design change was proposed.

“Is it worth writing test code for application logic (as opposed to business logic)?”

• Yes.
• Not all of it.
• Not always.

Test what yields business value.
Making the development effort more efficient may yield business value.

If application logic is directly related to business value, it needs automated tests.

If lack of tests for application logic delays development efforts (including manual testing), then it’s worth writing tests.

An example…

As a developer, I like being able to take an API contract designed by the team (the URI to the endpoint and the shape of the input and output) and write a quick test for it that we can use to make sure the endpoint works as expected.

This is what such tests look like:

On the left, we see the test. On the right, we see the expected payload and response.

This integration test verifies that:

• The route exists
• The json payload can be handled
• The response gets serialized into the expected json

But not only that, it also verifies any middleware that exists between the route and the controller, so things like authorization, model binders, dependency registrations, etc.

We either find a test harness, or build one, to make such tests easy to write, so there’s no reason not to.

The example above:

• Does not need any special tool
• Is written in plain C# and xUnit.net

In summary, when deciding what we should write tests for, “application logic” also comes into consideration.

That’s another common question: Is TDD something you do sometimes or all the time?

The short answer is neither. Or, “it depends”.

But let’s explore the long answer…

When I started learning TDD, yes, I’d do it all the time.

“But have you always worked on projects where you could do TDD all the time?!”

Most certainly I have NOT!

There are times when I can’t do TDD on a project.
That doesn’t prevent me from still doing it on the side.
I learn and practice it on my own time.

TDD became something I do most of the time, but never all the time.

So when do I NOT do TDD?

There are situations when I specifically choose not to do TDD. Here are some that come to mind…

Exploring a library or framework

When trying to learn what a library or framework can do for me, and how I’m supposed to use it.

Once I identify ways I’ll use it, I often refactor those tests into Given-When-Then, documenting my understanding and assumptions, so I can later remember what parts of it I’m using.

That approach also provides a safety net when consuming updates to those packages (identifying breaking changes and such).

Exploring approaches

Sometimes I need to implement features that currently lack clarity, so I want to gather feedback from stakeholders as soon as possible. I may try a few different approaches and won’t do TDD.

I will, however, use BDD (Behavior Driven Development) to describe, to the best of my ability, the feature we’re building.

Solving small problems

If a problem is too small and yields very little value, I may skip TDD.

Pitfalls of TDD

Remember Design Patternitis? That’s something most developers face when we learn Design Patterns; we start trying to apply them everywhere! As mentioned earlier, TDD is not something to be done always, everywhere, every time.

Another situation I see often is “copy-and-paste inheritance”; tests are initially written carefully following TDD, but then every new test comes from copy-and-paste, without any effort going into refactoring the test code. This is a pitfall that happens in most automated tests, and tests written following TDD can also suffer from it.

But BDD…

I try to do BDD most of the time, even if it means writing the stories and scenarios on a napkin.

Many times I don’t even know how I’ll actually implement the specs/tests, but I still write those English sentences before trying to write any code. No GWT, No Code!

So what’s the different between Test-Driven Development (TDD) and Behavior-Driven Development (BDD)? I’ve written about that before, but I think that post lacks something more illustrative.

For one thing, I forgot to mention: I’m in the camp that thinks of the last D as design, instead of development. That matters.

I’ve heard developers say things like:

• “I don’t do TDD anymore, now I’m BDD all the way!”, or
• “I only do TDD, because I can’t use Cucumber, SpecFlow, or any one of those BDD tools.”

Here’s how I see it…

At some point, I learned to write unit/integration tests in this manner:

Through TDD, I learned to write such test before writing the actual code.

Then I learned about “BDD-style” tests, so I refactored tests such as the one above into the one below:

That was my transition from Arrange-Act-Assert (AAA) to Given-When-Then (GWT).

Then I learned that BDD and TDD go hand-in-hand:

No special tools, languages, test frameworks. In the example above, just plain C# and xUnit.net.

In summary:

• BDD specifying the desired outcome
• TDD specifying the desired approach to achieve the desire outcome

Is Test Driven Development (TDD) practical when working with a lot of legacy code?

Yes, it is.

There’s an opportunity to practice Test Driven Development (TDD) whenever we write new code in a legacy system. As an example, when writing a new component, service, feature, etc., which is going to be called from the legacy system.

But what if the new code is to be written right in the middle of existing legacy code?

There’s still an opportunity for TDD.

Think what the new code is supposed to do.

Maybe it’s a new if-block that checks some variables in the existing code. That if-block asks the system a question; could that question be implemented as a method, a function, or similar approach, and then called from the existing code?

Or maybe the new lines will perform some sort of task, which is an opportunity to follow TDD to design and implement the new task, and then call it from the legacy code.

What about testing the existing legacy code?

There’s a possibility that the current state of the legacy code is so poor that even changing it to call any new code is so hard that a decision is made to only change the old code, and therefore, leaving no room for TDD.

If that’s the case, there’s still value in at least trying to write characterization tests for the existing code. That’s one scenario where using code coverage can yield benefits, as opposed to the bad way many people use it.

The existing system may be massive, so it’s important to know what we should write tests for.

What do we get from that?

The experience acquired with writing tests for legacy code makes us stronger TDD practitioners; as we experience the pain of writing tests for code that wasn’t implemented and designed with testability in mind, we also apply those lessons when designing new solutions.

But what if I’m working on a greenfield project and there’s no legacy code here?

Isn’t there? Are you sure? Many developers think of legacy code as code written a long, long time ago, often in defunct languages.

Michael Feathers defined legacy code as “code without tests”.

I’ve learned to think of legacy code as “code nobody wants to deal with, with or without tests”.

Sometimes a decision is made NOT to write tests (topic for another post). We can still write the code in ways that people won’t feel compelled to run away from it, even if there are no tests yet.

Sometimes we write tests within a sprint, but we do a poor job at writing those, so tests become “code nobody wants to deal with”.

In case you haven’t already, make sure to read and apply techniques from Michael Feathers’ Working Effectively with Legacy Code.