Adding code coverage to your .NET tests

Editorial note (2026 revision). The original article is from 2019 and used .NET Core 2.2, the Ubuntu 16.04 agent, and Azure DevOps tasks that are now retired or deprecated. I’ve updated the samples to current task versions and added the approach recommended today (coverlet.collector). Even so, specific SDK and package versions age fast — pin whatever your project actually uses.

Day to day, a coverage number buys me a few things:

• Knowing how much code (as a percentage) I gained or lost in each iteration.
• Knowing which parts of the code have never been exercised.
• Knowing which parts of the code I can delete with some confidence.

A coverage metric helps you make decisions. Note the wording: it helps you decide, it isn’t a goal in itself. Ninety per cent coverage from tests that assert nothing is worth nothing.

My coverage tool of choice is Coverlet, which pulls the numbers out of my code. I then generate an HTML report with ReportGenerator, which also lets me attach the result to an Azure DevOps build.

The recommended option today: the collector

The easiest way in is to add the coverlet.collector package to your test project. Current dotnet new xunit, nunit and mstest templates already include it:

dotnet add tests/MiAppMolona.Tests package coverlet.collector

From there, collecting coverage is a single flag:

dotnet test --collect:"XPlat Code Coverage"

That drops a coverage.cobertura.xml into tests/MiAppMolona.Tests/TestResults/<guid>/.

The original post’s option: coverlet.msbuild

Back in 2019 I used the MSBuild integration. It’s still valid and gives you finer control over exclusions and thresholds. Add it by editing your test library’s .csproj:

<ItemGroup>
  <PackageReference Include="coverlet.msbuild" Version="6.0.4">
    <PrivateAssets>all</PrivateAssets>
    <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
  </PackageReference>
</ItemGroup>

And run it like this:

dotnet test tests/MiAppMolona.Tests/MiAppMolona.Tests.csproj -p:CollectCoverage=true -p:CoverletOutputFormat=cobertura "-p:Exclude=[xunit.*]*" --logger trx

Note the quotes around -p:Exclude. Brackets and asterisks get eaten by the shell (PowerShell and bash in particular), and without quoting the filter either arrives empty or the command fails outright.

The local report

ReportGenerator is a global tool, so you install it once per machine:

dotnet tool install -g dotnet-reportgenerator-globaltool

Then point it at the coverage XML:

reportgenerator -reports:tests/MiAppMolona.Tests/**/coverage.cobertura.xml -targetdir:./CodeCoverage -reporttypes:Html

You get a very readable HTML report in the CodeCoverage folder.

The Azure DevOps pipeline

Here’s the updated equivalent of the YAML from the original post:

name: Mi Build Molona

trigger:
- main

pool:
  vmImage: 'ubuntu-latest'

variables:
  buildConfiguration: 'Release'

steps:
- task: UseDotNet@2
  displayName: 'Install the .NET SDK'
  inputs:
    packageType: sdk
    version: '8.0.x'

- task: DotNetCoreCLI@2
  displayName: 'Building the solution in $(buildConfiguration)'
  inputs:
    command: build
    projects: '**/*.csproj'
    arguments: '--configuration $(buildConfiguration)'

- script: >
    dotnet test tests/MiAppMolona.Tests/MiAppMolona.Tests.csproj
    --configuration $(buildConfiguration)
    -p:CollectCoverage=true
    -p:CoverletOutputFormat=cobertura
    "-p:Exclude=[xunit.*]*"
    --logger trx
  displayName: 'Running the unit tests'

- task: PublishTestResults@2
  displayName: 'Publishing unit test results'
  condition: succeededOrFailed()
  inputs:
    testRunner: VSTest
    testResultsFiles: '**/*.trx'

- script: |
    dotnet tool install -g dotnet-reportgenerator-globaltool
    reportgenerator \
      -reports:$(Build.SourcesDirectory)/tests/**/coverage.cobertura.xml \
      -targetdir:$(Build.SourcesDirectory)/CodeCoverage \
      -reporttypes:'Cobertura;HtmlInline_AzurePipelines'
  displayName: 'Creating the code coverage report'

- task: PublishCodeCoverageResults@2
  displayName: 'Publishing the code coverage report'
  inputs:
    summaryFileLocation: '$(Build.SourcesDirectory)/CodeCoverage/Cobertura.xml'
    pathToSources: '$(Build.SourcesDirectory)'

What changed from the original, and why:

• DotNetCoreInstaller@0 is retired; UseDotNet@2 replaces it. While I was there I fixed a classic slip in the original YAML: the displayName said 2.2.103 while the installed version was 2.2.105.
• The Ubuntu 16.04 image no longer exists on hosted agents. ubuntu-latest keeps itself current.
• PublishCodeCoverageResults@1 is deprecated. Version 2 detects the format and needs neither codeCoverageTool nor reportDirectory.
• I ask ReportGenerator for a Cobertura report alongside the HTML one, so I can publish a single consolidated file instead of several scattered ones.
• I added condition: succeededOrFailed() to the test publish step: a failing test run is exactly when you most want the report.

That gives you a pipeline that builds, runs the tests and produces a coverage report. It doesn’t publish artifacts, but it works perfectly well as a pull request validation build.

Run it on Azure DevOps and the report shows up in its own tab, right next to the test results 😎.