mutation-testing

Mutation Testing

Add a third validation layer to Outside-In TDD workflow. Acceptance tests verify WHAT (observable behavior), Domain tests verify HOW (business rules), mutation testing verifies tests actually catch bugs.

Core Concept

Mutation testing introduces deliberate bugs (mutants) into source code, then runs the test suite. If tests fail, the mutant is killed ✓. If tests pass despite the bug, the mutant survives ✗ (test gap found).

Source code → introduce mutation → run tests
                                     ├── tests FAIL → mutant killed ✓
                                     └── tests PASS → mutant survived ✗

A project with 100% code coverage can still have a 60% mutation score — meaning 40% of introduced bugs go undetected.

When to Use

Run mutation testing after the relevant test baseline is green:

1. ✅ Core behavior tests pass
2. ✅ Rule-focused tests pass
3. 🧬 Mutation testing — verify tests detect regressions

Never run on red baseline — mutation assumes tests work correctly first.

Approach for .NET/C#

Primary: Stryker.NET (Recommended)

For .NET projects, Stryker.NET is the established mutation framework with excellent C# support. No config file needed — all options are passed via CLI.

Install (only if not already available):

# Check first — if this succeeds, skip installation entirely. Do NOT manipulate PATH.
dotnet stryker --version

# Only run if the above command fails (tool not found)
dotnet tool install -g dotnet-stryker

Run on changed code only (default workflow — use after every story):

# Mutate only files changed since main — fast, targeted
dotnet stryker \
  --project src/YourProject.Domain/YourProject.Domain.csproj \
  -tp tests/YourProject.UnitTests/YourProject.UnitTests.csproj \
  --mutate "**/*.cs" --mutate "!**/*Marker.cs" --mutate "!**/DependencyInjection.cs" \
  --since:main \
  --break-at 100 \
  -r json