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Coverlet integration with VSTest (a.k.a. Visual Studio Test Platform)

Supported runtime versions:

Since version 8.0.0

  • .NET Core >= 8.0
  • .NET Framework >= 4.7.2

As explained in quick start section, to use collectors you need to run SDK v8.0.414 (LTS) or newer and your project file must reference coverlet.collector and a minimum version of Microsoft.NET.Test.Sdk.

A sample project file looks like:

<Project Sdk="Microsoft.NET.Sdk">
  <!-- Target only .NET 8 and .NET 9 (no .NET 10) -->
  <PropertyGroup>
    <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
    <TestingPlatformDotnetTestSupport>false</TestingPlatformDotnetTestSupport>
  </PropertyGroup>
  <ItemGroup>
    <!-- Minimum version 18.4.0 -->
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.4.0" />
    <!-- Update this reference when new version is released -->
    <PackageReference Include="coverlet.collector" Version="10.0.1" />
  ...
  </ItemGroup>
...
</Project>

The reference to coverlet.collector package is included by default with xunit template test (dotnet new xunit), you only need to update the package for new versions like any other package reference.

With correct reference in place you can run coverage through default dotnet test CLI verbs:

dotnet test --collect:"XPlat Code Coverage"

or

dotnet publish
...
  ... -> C:\project\bin\Debug\net8.0\testdll.dll
  ... -> C:\project\bin\Debug\net8.0\publish\
...
dotnet vstest C:\project\bin\Debug\net8.0\publish\testdll.dll --collect:"XPlat Code Coverage"

As you can see in case of vstest verb you must publish project before.

At the end of tests you'll find the coverage file data under default VSTest platform directory TestResults

Attachments:
  C:\git\coverlet\Documentation\Examples\VSTest\HelloWorld\XUnitTestProject1\TestResults\bc5e983b-d7a8-4f17-8c0a-8a8831a4a891\coverage.cobertura.xml
Test Run Successful.
Total tests: 1
     Passed: 1
 Total time: 2.5451 Seconds

You can change the output directory using the standard dotnet test switch --results-directory

Note

By design VSTest platform will create your file under a random named folder(guid string) so if you need stable path to load file to some gui report system(i.e. coveralls, codecov, reportgenerator etc..) that doesn't support glob patterns or hierarchical search, you'll need to manually move resulting file to a predictable folder

Coverlet options supported by VSTest integration

⚠️At the moment VSTest integration doesn't support all features of msbuild and .NET tool, for instance show result on console, report merging and threshold validation. We're working to fill the gaps.

Tip

Some alternative solutions to merge coverage files

  • use dotnet-coverage tool and merge multiple coverage files

    dotnet-coverage merge artifacts/coverage/**/coverage.cobertura.xml -f cobertura -o artifacts/coverage/coverage.xml*

  • use dotnet-reportgenerator-globaltool to create a HTML report and a merged coverage file

    reportgenerator -reports:"**/*.cobertura.xml" -targetdir:"artifacts\reports.cobertura" -reporttypes:"HtmlInline_AzurePipelines_Dark;Cobertura"

Default option (if you don't specify a runsettings file)

Without specifying a runsettings file and calling coverlet by just the name of the collector, the result of the generated coverage output is by default in cobertura format.

dotnet test --collect:"XPlat Code Coverage"

The output format of the coverage report can also be changed without a runsettings file by specifying it in a parameter. The supported formats are lcov, opencover, cobertura, teamcity, json (default coverlet proprietary format).

dotnet test --collect:"XPlat Code Coverage;Format=json"

It is even possible to specify the coverage output in multiple formats.

dotnet test --collect:"XPlat Code Coverage;Format=json,lcov,cobertura"

Advanced Options (Supported via runsettings)

These are a list of options that are supported by coverlet. These can be specified as datacollector configurations in the runsettings.

Option Summary
Format Coverage output format. These are either cobertura, json, lcov, opencover or teamcity as well as combinations of these formats.
Default is cobertura.
Exclude Exclude from code coverage analyzing using filter expressions.
Default is [coverlet.*]*,[xunit.*]*,[NUnit3.*]*,[Microsoft.Testing.*]*,[Microsoft.Testplatform.*]*,[Microsoft.VisualStudio.TestPlatform.*]*. User-supplied filters are appended to these defaults.
ExcludeByAttribute Exclude a method, an entire class or assembly from code coverage decorated by an attribute.
Default is no additional attributes, but ExcludeFromCoverageAttribute and ExcludeFromCodeCoverageAttribute are always excluded.
ExcludeByFile Ignore specific source files from code coverage.
Default is none.
Include Explicitly set what to include in code coverage analysis using filter expressions.
Default is no include filters (all coverable modules except excluded ones).
IncludeDirectory Explicitly set which directories to include in code coverage analysis.
Default is none.
SingleHit Specifies whether to limit code coverage hit reporting to a single hit for each location.
Default is false.
UseSourceLink Specifies whether to use SourceLink URIs in place of file system paths.
Default is false.
IncludeTestAssembly Include coverage of the test assembly.
Default is false.
SkipAutoProps Neither track nor record auto-implemented properties.
Default is false.
DoesNotReturnAttribute Methods marked with these attributes are known not to return, statements following them will be excluded from coverage.
Default is none.
DeterministicReport Generates deterministic report in context of deterministic build. Take a look at documentation for further information.
Default is false.
ExcludeAssembliesWithoutSources Specifies whether to exclude assemblies without source. Options are either MissingAll, MissingAny or None.
Default is MissingAll.

How to specify these options via runsettings?

<?xml version="1.0" encoding="utf-8" ?>
<RunSettings>
  <DataCollectionRunSettings>
    <DataCollectors>
      <DataCollector friendlyName="XPlat code coverage">
        <Configuration>
          <Format>json,cobertura,lcov,teamcity,opencover</Format>
          <Exclude>[coverlet.*.tests?]*,[*]Coverlet.Core*</Exclude> <!-- [Assembly-Filter]Type-Filter -->
          <Include>[coverlet.*]*,[*]Coverlet.Core*</Include> <!-- [Assembly-Filter]Type-Filter -->
          <ExcludeByAttribute>Obsolete,GeneratedCodeAttribute,CompilerGeneratedAttribute</ExcludeByAttribute>
          <ExcludeByFile>**/dir1/class1.cs,**/dir2/*.cs,**/dir3/**/*.cs,</ExcludeByFile> <!-- Globbing filter -->
          <IncludeDirectory>../dir1/,../dir2/,</IncludeDirectory>
          <SingleHit>false</SingleHit>
          <UseSourceLink>true</UseSourceLink>
          <IncludeTestAssembly>true</IncludeTestAssembly>
          <SkipAutoProps>true</SkipAutoProps>
          <DeterministicReport>false</DeterministicReport>
          <ExcludeAssembliesWithoutSources>MissingAll,MissingAny,None</ExcludeAssembliesWithoutSources>
        </Configuration>
      </DataCollector>
    </DataCollectors>
  </DataCollectionRunSettings>
</RunSettings>

Filtering details are present on msbuild guide.

This runsettings file can easily be provided using command line option as given :

  • dotnet test --collect:"XPlat Code Coverage" --settings coverlet.runsettings
  • dotnet vstest C:\project\bin\Debug\netcoreapp3.0\publish\testdll.dll --collect:"XPlat Code Coverage" --settings coverlet.runsettings

Take a look at our HelloWorld sample.

Passing runsettings arguments through commandline

You can avoid passing a runsettings file to dotnet test driver by using the xml flat syntax in the command line.

For instance if you want to set the Format element as a runsettings option you can use this syntax:

dotnet test --collect:"XPlat Code Coverage" -- DataCollectionRunSettings.DataCollectors.DataCollector.Configuration.Format=json,cobertura,lcov,teamcity,opencover

Take a look here for further information:https://github.com/microsoft/vstest/blob/main/docs/RunSettingsArguments.md

How it works

Coverlet integration is implemented with the help of datacollectors. When we specify --collect:"XPlat Code Coverage" VSTest platform tries to load coverlet collectors inside coverlet.collector.dll

  1. Out-of-proc Datacollector: The outproc collector run in a separate process(datacollector.exe/datacollector.dll) than the process in which tests are being executed(testhost*.exe/testhost.dll). This datacollector is responsible for calling into Coverlet APIs for instrumenting dlls, collecting coverage results and sending the coverage output file back to test platform.

  2. In-proc Datacollector: The in-proc collector is loaded in the testhost process executing the tests. This collector will be needed to remove the dependency on the process exit handler to flush the hit files and avoid to hit this serious known issue

Architecture

coverlet.collector uses a controller-style integration where VSTest remains the orchestrator and Coverlet hooks into collector lifecycle callbacks.

Dependencies and functionality

Component Dependencies Functionality
coverlet.collector VSTest Data Collector APIs (Microsoft.TestPlatform.ObjectModel), coverlet.core Coordinates pre/post test collection, instruments targets, collects hit files, publishes coverage as test attachments
coverlet.core Mono.Cecil, filtering/reporting infrastructure IL instrumentation, hit tracking, coverage aggregation, report generation 
flowchart LR
    VS["vstest.console / dotnet test"] --> OOP["Out-of-proc Data Collector\ncoverlet.collector"]
    VS --> TH["testhost"]
    TH --> IP["In-proc Data Collector\ncoverlet.collector"]
    OOP --> CORE["coverlet.core"]
    IP --> CORE
    CORE --> REP["coverage reports"]
    OOP --> ATT["VSTest attachments\nTestResults/<guid>"]
Loading

Limitations and constraints

  • Output files are attached under TestResults/<guid> (platform-controlled, non-deterministic folder names).
  • Feature parity with other modes is partial (for example, threshold validation and built-in merge are not provided by this integration mode).
  • Coverage collection depends on VSTest data-collector lifecycle and testhost behavior.
  • Requires compatible Microsoft.NET.Test.Sdk and collector setup.

Known Issues

For a comprehensive list of known issues check the detailed documentation KnownIssues.md