stryker-net-mutation

stryker-net-mutation

Overview

Per stryker-net-intro:

"Stryker.NET offers you mutation testing for your .NET Core and .NET Framework projects. It allows you to test your tests by temporarily inserting bugs."

Per stryker-net-intro, Stryker.NET joined the Stryker family in 2018 as the .NET ecosystem mutation testing answer.

When to use

• A .NET test suite (xUnit / NUnit / MSTest) needs mutation-quality verification.
• A C# library needs higher confidence than coverage alone shows.
• The team wants Stryker-family parity across JS + .NET.

Step 1 - Install

dotnet tool install -g dotnet-stryker

Per-project (recommended for CI determinism):

dotnet new tool-manifest
dotnet tool install dotnet-stryker

Step 2 - Run

# From the test project directory
cd MyApp.Tests
dotnet stryker

The first run discovers the test framework, reports baseline coverage, then mutates and re-tests.

Step 3 - Configure via stryker-config.json

{
  "$schema": "https://raw.githubusercontent.com/stryker-mutator/stryker-net/master/src/Stryker.Core/Stryker.Core/stryker-config-schema.json",
  "stryker-config": {
    "project": "../MyApp/MyApp.csproj",
    "test-projects": ["MyApp.Tests.csproj"],
    "mutation-level": "Standard",
    "thresholds": { "high": 80, "low": 60, "break": 50 },
    "concurrency": 4,
    "reporters": ["progress", "html", "cleartext"]
  }
}

mutation-level controls how aggressive mutations are:

• Basic (fewest mutators)
• Standard (default, recommended)
• Advanced
• Complete (most mutators; slowest)

Step 4 - Solution mode

For multi-project solutions:

dotnet stryker --solution path/to/MyApp.sln

Stryker.NET discovers all test projects and mutates the production projects each test references.

Step 5 - CI integration

- uses: actions/setup-dotnet@v4
  with: { dotnet-version: '8.x' }
- run: dotnet tool restore
- run: dotnet stryker --break-at 50
- uses: actions/upload-artifact@v4
  if: always()
  with:
    name: stryker-net-report
    path: StrykerOutput/

Reports land under StrykerOutput/<timestamp>/reports/mutation-report.html.

Step 6 - Mutators (per Standard level)

Common mutators include:

Mutator	Example
Arithmetic operator	+ → -, * → /
Conditional boundary	< → <=, > → >=
Conditional negation	!x → x
Logical operator	&& → ||
Equality	== → !=
Return value	return foo() → return null / return "" / return 0
Statement removal	Foo(); → ;
String literal	"x" → ""

A surviving mutant means the test suite doesn't distinguish the original behavior from the mutated one.

Anti-patterns

Anti-pattern	Why it fails	Fix
Running on the entire solution every PR	Long runtime; team disables.	Scope to changed projects via --project.
mutation-level: Complete from day one	Slowest mode; many irrelevant mutants.	Start Standard; promote to Complete for critical libraries.
Ignoring "no coverage" mutants in the report	Untested code; mutation testing can't measure it.	Add tests OR exclude those files via --mutate.
Skipping --break-at in CI	Mutation score regressions slip through.	Set --break-at to baseline + small headroom (Step 5).
Running against a Debug build	Slower; some mutators behave differently.	Run against Release build for CI gates.

Limitations

• Newer than StrykerJS. Per stryker-net-intro, the .NET variant filled "what developers perceived as a gap"; feature parity with JS is improving but not 1:1.
• Async + concurrency mutation can be tricky. Some mutants produce equivalent code; flag + exclude.
• CI runtime. A medium codebase mutation run takes 10-60 min; not for every PR.

References

• sni - Stryker.NET overview, NuGet distribution, .NET Core + .NET Framework support, history.
• stryker-mutation - JS sibling with the same Stryker model.
• pitest-mutation, mutmut-mutation, mull-mutation - per-language alternatives.
• mutation-survivor-explainer - agent for surviving-mutant analysis.