A journey to Property-Based Testing

The purpose of this workshop is to understand by the practice an approach to testing that allows us to identify and check our business invariants. We will discover :

• How it differs from traditional sampling approach (example-based testing)

• Its advantages and limits

Source code

Source code is available here in :

• Scala : Scalatest / Scalacheck

• C# 10 : XUnit / FsCheck / Language-Ext / FluentAssertions

Connection - Calculator

Connect participants to the topic by making them write Unit Tests on the Calculator class :

Debriefing :

Concepts - PBT

• Go through slides 4 to 15 to explain the different concepts :

  • Example-Based Testing vs PBT

  • What is PBT ?

  • Addition properties example

  • PBT by hands

  • Scalacheck / FsCheck / QuickCheck

  • Associated concepts

• Demonstrate how to write properties on the Calculator, you can use the step-by-step guide to prepare yourself

Concrete Practice

Postal Parcel

Open `PostalParcel` :

• Identify properties – 5’

• What are the invariants ?

• Write them in `PostalParcelPropertiesFlatSpec` with ScalaCheck / FsCheck - 15’

• Collective debriefing – 5’

  • Scala

  • C#

Bank withdrawal

• Open `AccountService` :•Identify properties – 5’

• What should we do to check those properties ?

• Demonstrate how you could write properties on those knowing that Command and Account are tightly coupled as inputs :

  • Using properties + input filtering (when)

  • Using properties and Builder

  • How we would write tests on Bank withdrawal in real life (with Parameterized tests

Rental Calculator - PBT as a refactoring tool

• Open `RentalCalculator`

• Imagine this code is running in production and business is happy with it

• Imagine you need to adapt it, but you need to be sure that you don’t introduce any regression

• How PBT can help us ?

• Let them code - 15'

• Collective debriefing - 5'

  • Scala

  • C#

Conclusion

• When to apply PBT in real life

• Associated anti-patterns

• Reflect : Think about your last development, how this approach could have helped you ?

Resources

• ScalaCheck documentation

• Scala exercises on PBT : https://www.scala-exercises.org/scalacheck/generators

• "Functions for nothing, and your tests for free" Property-based testing and F# - George Pollard

• Property based testing - step by step : https://www.leadingagile.com/2018/04/step-by-step-toward-property-based-testing/

Slide deck to support workshop