how to use coverlet

3 min read 05-02-2025

Coverlet is a powerful cross-platform code coverage tool for .NET projects. It integrates seamlessly with various testing frameworks and build systems, providing detailed reports on which parts of your code are executed during testing. This comprehensive guide will walk you through using Coverlet effectively, from installation to interpreting its insightful reports.

Installing Coverlet

Coverlet's installation is straightforward. The recommended method is via the NuGet package manager. You'll need to install it for both your test project and, in some scenarios, your main project.

Installing for your Test Project

Open your test project in Visual Studio or use the .NET CLI. Then, install the coverlet.collector package:

dotnet add package coverlet.collector

This package is essential for collecting coverage data during test runs.

Installing for your Main Project (Sometimes Necessary)

For some projects and testing scenarios (especially with specific frameworks or complex setups), you might need to install the coverlet.msbuild package in your main project as well:

dotnet add package coverlet.msbuild

This package ensures proper integration with the MSBuild process. If you encounter issues, adding this package is a good troubleshooting step.

Running Coverlet with Different Test Runners

Coverlet integrates with popular testing frameworks like xUnit, NUnit, and MSTest. The specific commands vary slightly.

Running Coverlet with dotnet test

The simplest method is to use the dotnet test command with the --collect:"Coverlet" argument:

dotnet test --collect:"Coverlet"

This command runs your tests and collects coverage data using Coverlet. The results will be saved in a default location (typically in your test project's output directory).

Specifying Output Formats

Coverlet supports various output formats, including Cobertura, OpenCover, and lcov. You can specify the desired format using the --format option:

dotnet test --collect:"Coverlet(Format=cobertura)"

This example generates a Cobertura XML report, a format widely compatible with other code coverage tools and CI/CD pipelines.

Specifying Output Directory

By default, Coverlet places the coverage reports in the test project's output directory. You can change this using the --output option:

dotnet test --collect:"Coverlet(OutputFormat=cobertura,OutputDirectory=coverage)"

This creates a coverage directory containing the Cobertura report.

Using Coverlet with other Test Runners (Example: xUnit)

If you're using a test runner other than dotnet test, you may need to configure it to use Coverlet. Consult your test runner's documentation for specific instructions.

Interpreting Coverlet Reports

Once you've run your tests with Coverlet, you'll find a coverage report in the specified location. The exact format depends on the --format option you used. Popular tools like ReportGenerator can process these reports and create human-readable summaries.

Cobertura (XML): This is a common format easily parsed by many CI/CD systems and visualization tools.
OpenCover (XML): Another popular XML-based format offering similar functionality to Cobertura.
lcov (LCOV): A text-based format, often used in Linux environments.

Many IDEs also offer built-in support for displaying code coverage information directly within the code editor. This allows you to quickly identify sections with low coverage and focus on improving your tests.

Advanced Usage and Troubleshooting

Filtering: Coverlet allows you to filter which parts of your code are included in the coverage analysis. This is particularly useful for excluding generated code or third-party libraries.
Modules: Coverlet can handle multiple modules and projects within a solution.
Skipping Code: You can use attributes to tell Coverlet to exclude specific parts of your code from coverage analysis.
Debugging Issues: If you run into problems, ensure you've installed the correct packages in the right projects. Check the Coverlet documentation for detailed troubleshooting steps.

Conclusion

Coverlet is a valuable asset for improving the quality and reliability of your .NET applications. By integrating it into your development workflow, you can gain deep insights into your code's test coverage, ultimately leading to more robust and maintainable software. Remember to explore the advanced options to tailor Coverlet to your specific needs and optimize your testing strategy.