Mutation Testing with Stryker.NET

Run Stryker mutation tests to evaluate test suite effectiveness. Mutation testing finds code that can be changed without failing tests—indicating weak coverage.

Arguments

If provided, $ARGUMENTS is the path to the test project or solution. Otherwise, search for test projects.

Workflow

1. Find the Test Project

If no path provided, search for test projects:

bash

find . -name "*.Tests.csproj" -o -name "*.UnitTests.csproj" | head -20

2. Check Stryker Installation

bash

dotnet stryker --version

If not installed:

bash

dotnet tool install -g dotnet-stryker

3. Check for Configuration

Look for stryker-config.json in the test project directory—it may have project-specific settings.

4. Run Stryker

From the test project directory:

bash

dotnet stryker

With options for faster runs:

bash

# Only mutate files changed since main
dotnet stryker --since:main

# Target specific files
dotnet stryker --mutate "src/Services/**/*.cs"

5. Interpret Results

Console output shows:

code

- Killed:    150  (mutants detected by tests - good)
- Survived:   25  (mutants NOT detected - weak tests)
- Timeout:     5  (caused infinite loop - usually OK)
- No Coverage: 10  (code not covered by tests)

Mutation score: 78.95%

Score interpretation:

Score	Quality
80%+	Excellent
60-80%	Good, some gaps
40-60%	Fair, significant weaknesses
<40%	Poor coverage

6. Find Surviving Mutants

Check the HTML report at StrykerOutput/reports/mutation-report.html for:

•Which files have survivors
•Exact line numbers and mutation types
•What was changed

Common survivor patterns indicate:

•Boundary conditions not tested (> → >= survives)
•Missing negative tests (if (x) → if (true) survives)
•Return values not verified
•Exception paths not tested

Reporting

Provide:

•Summary: Overall score, killed/survived/no-coverage counts
•Weak spots: Files with lowest scores, specific surviving mutations
•Recommendations: Specific tests to add based on survivors

Example:

code

Mutation Score: 72%
- Killed: 145, Survived: 28, No Coverage: 12

Weak spots:
- UserService.cs (65%): Line 45 `age > 18` → `age >= 18` survived (boundary not tested)
- OrderValidator.cs (78%): Line 23 `&&` → `||` survived (logic branch not covered)

Recommendations:
- Add test for age exactly 18
- Add test where first condition true, second false

Troubleshooting

Too slow: Use --since:main or --mutate to limit scope

No tests found: Run from test project directory, check --project points to source project

Memory issues: Reduce concurrency with --concurrency 2