Property-based testing in C#

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Property-based testing With examples in C# @jovaneyck.bsky.social

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Replaying failed properties

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Getting started

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PBT pro == What input to generate + What properties to write + When (not) to PBT

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Generating random inputs

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Generating random inputs

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Generating random inputs

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Generating random inputs

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Constraining generators

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Combining generators

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Combining generators

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DnD generators

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1d6?

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1d6!

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1d6 distribution

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2d6?

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!2d6

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2d6!

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2d6 v2

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PBT pro == What input to generate + What properties to write + When (not) to PBT

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Heuristics for “good properties”

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generalizing unit tests John Hughes - Building on developers' intuitions (...) | Lambda Days 19

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Generalizing unit tests

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Extracting domains from an email address

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Start at the end

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Serialization/deserialization

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Serialization/deserialization

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Inverse operations There and back again

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Fuzzing You can throw anything at it

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Commutativity Different paths, same destination https://fsharpforfunandprofit.com/posts/property-based-testing-2/

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Invariants Some things never change https://fsharpforfunandprofit.com/posts/property-based-testing-2/

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Idempotence The more things change, the more they stay the same https://fsharpforfunandprofit.com/posts/property-based-testing-2/

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Let’s build Google

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Let’s build Google

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Let’s build Google

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Metamorphic testing

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Heuristics for good properties ● Generalized unit tests ● “Start at the end” ● Inverse operations: “There and back again” ● Fuzzing: “You can throw everything at it” ● Commutativity: “Different paths, same destination” ● Invariants: “Some things never change” ● Idempotence: “The more things change, the more they stay the same” ● Metamorphic properties

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PBT pro == What input to generate + What properties to write + When (not) to PBT

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Effort Confidence

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When to PBT ● When high confidence is worth the effort ○ Critical code ● Problems with very large input/state spaces ○ Parsers ● Data structures, DDD aggregates/VO’s, monads ( ) ○ anything with clear and complete laws or invariants ● Fuzzy/non-deterministic problems ○ might not have deterministic or knowable input->output relationships ○ e.g. classifiers, LLM’s ● Legacy code ○ Writing properties will quickly (in)validate mental model of the SUT ● Stateful systems that can be modeled

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When to PBT Mysteries of Dropbox, Property-based testing of a Distributed Synchronization Service, John Hughes et al.

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When to PBT Delhaize rekent al maanden 'spookproducten' aan bij sommige klanten: "Ik moest betalen voor bak trappist die ik nooit kocht"

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When (not) to PBT ● Test-Driven Development ○ It’s possible & fun BUT ○ Often even harder to do PBT in combination with TDD due to the friction between “universal properties’ and “incremental development”

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Criteria Unit test Property Effort easy to write needs more cases you need to list them takes more effort to come up with good properties and generators One property covers lots (and lots) of cases Execution speed Fast 100/10.000x slower Bugs they catch depends on the test list edge cases, “+1 QA” in team Document the system examples business rules and invariants Keeping test code focused test data builders, custom assertions generators and properties Unit test vs. property-based test

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Jessica Kerr’s thoughts on property-based testing “See how much thought goes into a property-based test? and this is a simple specification! I don’t recommend writing these for all your code – only the really important stuff.” “Property-based tests are best combined with example-based tests. Examples help you start organizing your thoughts, and they’re easier for future-you to read and understand when you come back to this code later. Humans think in examples. Programs don’t extrapolate. Property-based thinking and property-based testing can bridge between us and the computer. Math, it’s a tool.” https://jessitron.com/2013/04/25/property-based-testing-what-is-it/

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OO tax

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OO tax

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PBT pro == What input to generate + What properties to write + When (not) to PBT