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Stryker.NET

Stryker.NET is the Apache-2.0 mutation testing tool for C# and .NET from the Stryker Mutator project, run as the dotnet-stryker global tool with HTML reports, a dashboard and incremental runs.

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Introduction

Stryker.NET brings mutation testing to C#. It is part of the Stryker Mutator family alongside StrykerJS and Stryker4s. It changes your code in small ways (mutants), such as flipping comparison operators, removing statements or altering LINQ methods and string values, and runs your tests against each mutant. Surviving mutants show where tests execute code without really checking it.

You install it as a .NET tool (dotnet tool install -g dotnet-stryker) and run dotnet stryker in a test project folder. It works with xUnit, NUnit and MSTest via VSTest, produces an interactive HTML report, and can publish results to the Stryker Dashboard. Options include mutation levels, thresholds that fail the build, filtering files and mutators, and since/baseline modes that mutate only code changed relative to a git branch, which keeps pull request runs fast. Configuration lives in stryker-config.json. dotnet-stryker 5.1.0 was released on 10 November 2026.

The source code is at stryker-mutator/stryker-net on GitHub under the Apache-2.0 licence, with about 2,100 stars; the most recent commit was on 11 November 2026.

Features

  1. C# mutation testing: Measures test effectiveness beyond coverage.
  2. HTML report and dashboard: Explore surviving mutants visually.
  3. Incremental runs: --since and baseline modes for changed code.
  4. Thresholds: Fail CI when mutation score drops.

Pros

  1. Free to use: Open source under the Apache-2.0 licence, so the core tool costs nothing to use.
  2. Actively maintained: The repository had commits as recent as 11 November 2026.

Cons

  1. Runtime cost: Large solutions take time; use incremental modes.
  2. C# focus: F# and VB.NET are not the main target.
dotnet tool install -g dotnet-stryker
cd tests/Shop.Tests
dotnet stryker
dotnet stryker --since:main --threshold-break 60

stryker-config.json:

{ "stryker-config": { "reporters": ["html", "progress"], "thresholds": { "high": 80, "low": 60, "break": 50 } } }

Resources:

Pricing

Stryker.NET is free and open source under the Apache-2.0 licence.

  1. .NET teams with solid coverage wanting to measure test quality.
  2. Pull request pipelines using --since for fast incremental runs.

Conclusion

Stryker.NET is the leading mutation testing tool for C#, practical for CI thanks to incremental runs and clear reports.

Alternatives

Other tools on CheckOps that cover similar ground. Open a page, or compare one with Stryker.NET side by side.