Write Tests First–But Not ALL Tests First

I’ve been preaching hard about test-driven development and the importance of writing tests first. I can feel the culture beginning to shift as people are slowly starting to buy in, but I had an interesting discovery yesterday.

I was invited to a meeting by some developers that wanted me to walk through how I would’ve used test-driven development to write tests and develop a new project that they had recently started. It was essentially a data validation project that retrieved data from the database, checked it against a set of rules, and recorded any violations. We were reviewing a Controller class that was responsible for orchestrating the operation.

“Okay,” I said, “What is this thing supposed to do?”

The developers told me it retrieves records, validates each record, and saves the validation results. So, without knowing anything more than that, I figured there were at least two external dependencies: an IDataAccess responsible for retrieving records and saving the result and an IValidator that does the data validation/rule-checking. I drew a diagram on the whiteboard to show the relationships between the Controller and these two components.

I explained that since we know the dependencies and how we expect them to be used, we can begin to write tests. We also need to know how our application should react when the dependencies are missing. I started to rattle off some tests:

  • ProcessRecords_NullDataAccess_ThrowsArgumentNullException
  • ProcessRecords_NullValidator_ThrowsArgumentNullException
  • ProcessRecords_DataAccessReturnsNonEmptyList_ValidatesEachRecord
  • Etc.

The group was with me, but they quickly shifted focus to what tests were needed for the DataAccess class. And the tests for its dependencies. And everything else.

“Whoa, whoa, WHOA. None of that matters for this. All we care about is this method,” I say.

“Well, yes, but we want to do test-driven development. We thought the goal was to have all of our tests written first so we can go back and implement them.”

That’s when I had my epiphany. When I’m telling people to write tests first, they think I mean write ALL tests first. This is not the case! It would be virtually impossible to think about every code decision and execution path for an entire method/class/application upfront, and I think that’s where there’s been a disconnect. I can look at the finished code and come up with all the tests, but there is no way I could’ve come up with every single test for every single method before ever writing any code.

I went to another small team of developers and asked them if they also thought I meant “all tests first.” They did. It’s disappointing to know that I was sending the wrong message, but I’m glad I have something to address that will hopefully result in more passengers on the TDD train.

When you’re getting started with test-driven development, don’t try to write every single test first. Don’t even try to write as many tests as you can think of. You just want to write tests as you go. What does this method need to do next? Write a failing test, then write the implementation to make it pass. Red, green, refactor, baby! I’m also exchanging my “tests first” mantra for a new one: “test as you go!”

Author: Adam Prescott

I'm enthusiastic and passionate about creating intuitive, great-looking software. I strive to find the simplest solutions to complex problems, and I embrace agile principles and test-driven development.

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