Mutation Testing with Stryker.NET

Mutation testing changes production code in small ways (> to >=, + to -, a removed statement) and re-runs the tests. A mutant that survives means no test noticed the change.

Setup

Install Stryker.NET as a local tool so every developer and CI uses the same version.

dotnet new tool-manifest
dotnet tool install dotnet-stryker

What to target

Good targets (deterministic)	Avoid
Scoring engines, parsers, validators	Actor tests: timing-sensitive, flaky mutations
State classes (Apply)	Architecture tests: no production logic
Mapping extensions	Persistence DTOs: extend-only records
	Streams: timing-dependent

Configuration

One stryker-config.json per test project.

{
  "stryker-config": {
    "project": "MyApp.Scoring.csproj",
    "test-projects": ["../MyApp.Scoring.Tests/MyApp.Scoring.Tests.csproj"],
    "thresholds": { "high": 80, "low": 65, "break": 65 },
    "ignore-mutations": ["string"],
    "reporters": ["html", "progress", "json"],
    "mutate": ["!**/Telemetry.cs"]
  }
}

• break: 65 fails the run when the score drops below it. Start there and raise it gradually.
• The string mutator is ignored because log and wire text mutations are noise.

Equivalent mutants

Some mutants cannot be killed because the behavior is identical (for example i < n versus i != n in a loop that cannot overshoot).

Do not weaken production code to kill a mutant

Document equivalent mutants explicitly (a list next to the config, or a // Stryker disable once <mutator> : <reason> comment) instead of restructuring correct code.

// Stryker disable once Equality : loop bound is unreachable either way
for (var i = 0; i < items.Count; i++) { ... }

Running

Locally, use a Docker container for a consistent environment:

docker run --rm -v "$PWD:/src" -w /src/src/MyApp.Scoring.Tests \
  mcr.microsoft.com/dotnet/sdk:10.0 \
  sh -c "dotnet tool restore && dotnet stryker -f stryker-config.json"

In CI, run the tool directly with --local where the job already has the SDK:

dotnet stryker -f stryker-config.json --local

CI

Mutation runs are slow, so schedule them weekly and never on pull requests. Use a matrix with one job per target project.

name: mutation
on:
  schedule: [{ cron: "0 3 * * 0" }]
  workflow_dispatch:
jobs:
  stryker:
    runs-on: ubuntu-latest
    strategy:
      fail-fast: false
      matrix:
        project: [Scoring, Search, RuleSet]
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-dotnet@v4
        with: { dotnet-version: 10.0.x }
      - run: dotnet tool restore
      - working-directory: src/MyApp.${{ matrix.project }}.Tests
        run: dotnet stryker -f stryker-config.json --local
      - uses: actions/upload-artifact@v4
        if: always()
        with:
          name: stryker-${{ matrix.project }}
          path: src/MyApp.${{ matrix.project }}.Tests/StrykerOutput

Workflow

Read the HTML report, pick surviving mutants in boundary logic first, add a test that pins the exact value, re-run. Treat the score as a signal, not a goal.