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coverlet.msbuild 10.0.1

Coverlet integration with MSBuild

In this mode, Coverlet doesn't require any additional setup other than including the coverlet.msbuild NuGet package in the unit test project. It integrates with the dotnet test infrastructure built into the .NET Core CLI and when enabled, will automatically generate coverage results after tests are run.

If a property takes multiple comma-separated values please note that you will have to add escaped quotes around the string like this: /p:Exclude=\"[coverlet.*]*,[*]Coverlet.Core*\", /p:Include=\"[coverlet.*]*,[*]Coverlet.Core*\", or /p:CoverletOutputFormat=\"json,opencover\".

To achieve same behavior above using powershell you need to use the verbatim argument marker --% like this:

dotnet test /p:CollectCoverage=true --% /p:CoverletOutputFormat=\"opencover,lcov\"

Code Coverage

Enabling code coverage is as simple as setting the CollectCoverage property to true

dotnet test /p:CollectCoverage=true

After the above command is run, a coverage.json file containing the results will be generated in the root directory of the test project. A summary of the results will also be displayed in the terminal.

Coverage Output

Coverlet can generate coverage results in multiple formats, which is specified using the CoverletOutputFormat property. For example, the following command emits coverage results in the opencover format:

dotnet test /p:CollectCoverage=true /p:CoverletOutputFormat=opencover

Supported Formats:

• json (default)
• lcov
• opencover
• cobertura
• teamcity

You can specify multiple output formats by separating them with a comma (,).

The output of the coverage result can be specified using the CoverletOutput property.

dotnet test /p:CollectCoverage=true /p:CoverletOutput='./result.json'

To specify a directory where all results will be written to (especially if using multiple formats), end the value with a /.

dotnet test /p:CollectCoverage=true /p:CoverletOutput='./results/'

Note: escape characters might produce some unexpected results.

The Coverlet MSBuild task sets the CoverletReport MSBuild item so that you can easily use the produced coverage reports. For example, using ReportGenerator to generate an html coverage report.

<Target Name="GenerateHtmlCoverageReport" AfterTargets="GenerateCoverageResultAfterTest">
 <ReportGenerator ReportFiles="@(CoverletReport)" TargetDirectory="../html-coverage-report" />
</Target>

TeamCity Output

Coverlet can output basic code coverage statistics using TeamCity service messages.

dotnet test /p:CollectCoverage=true /p:CoverletOutputFormat=teamcity

The currently supported TeamCity statistics are:

Merging Results

With Coverlet, you can combine the output of multiple coverage runs into a single result.

dotnet test /p:CollectCoverage=true /p:MergeWith='/path/to/result.json'

The value given to /p:MergeWith must be a path to Coverlet's own json result format. The results in result.json will be read, and added to the new results written to by Coverlet. .

Threshold

Coverlet allows you to specify a coverage threshold below which it fails the build. This allows you to enforce a minimum coverage percent on all changes to your project.

dotnet test /p:CollectCoverage=true /p:Threshold=80

The above command will automatically fail the build if the line, branch or method coverage of any of the instrumented modules falls below 80%. You can specify what type of coverage to apply the threshold value to using the ThresholdType property. For example to apply the threshold check to only line coverage:

dotnet test /p:CollectCoverage=true /p:Threshold=80 /p:ThresholdType=line

You can specify multiple values for ThresholdType by separating them with commas. Valid values include line, branch and method. You can do the same for Threshold as well.

dotnet test /p:CollectCoverage=true /p:Threshold=\"80,100,70\" /p:ThresholdType=\"line,branch,method\"

By default, Coverlet will validate the threshold value against the coverage result of each module. The /p:ThresholdStat option allows you to change this behavior and can have any of the following values:

• Minimum (Default): Ensures the coverage result of each module isn't less than the threshold
• Total: Ensures the total combined coverage result of all modules isn't less than the threshold
• Average: Ensures the average coverage result of all modules isn't less than the threshold

The following command will compare the threshold value with the overall total coverage of all modules:

dotnet test /p:CollectCoverage=true /p:Threshold=80 /p:ThresholdType=line /p:ThresholdStat=total

Excluding From Coverage

Attributes

You can ignore a method, an entire class or assembly from code coverage by creating and applying the ExcludeFromCodeCoverage attribute present in the System.Diagnostics.CodeAnalysis namespace.

You can also ignore additional attributes by using the ExcludeByAttribute property

• Use single or multiple attributes (separate by comma)
• Use attribute name, attribute full name or fully qualified name of the attribute type (Obsolete, ObsoleteAttribute, System.ObsoleteAttribute)

dotnet test /p:CollectCoverage=true /p:ExcludeByAttribute="Obsolete%2cGeneratedCodeAttribute"

Source Files

You can also ignore specific source files from code coverage using the ExcludeByFile property

• Use single or multiple paths (separate by comma)
• Use file path or directory path with globbing (e.g dir1/*.cs)

dotnet test /p:CollectCoverage=true /p:ExcludeByFile=\"**/dir1/class1.cs,**/dir2/*.cs,**/dir3/**/*.cs\"

Filters

Coverlet gives the ability to have fine grained control over what gets excluded using "filter expressions".

Syntax: /p:Exclude=[Assembly-Filter]Type-Filter

Wildcards

• * ⇒ matches zero or more characters
• ? ⇒ the prefixed character is optional

Examples

• /p:Exclude="[*]*" ⇒ Excludes all types in all assemblies (nothing is instrumented)
• /p:Exclude="[coverlet.*]Coverlet.Core.Coverage" ⇒ Excludes the Coverage class in the Coverlet.Core namespace belonging to any assembly that matches coverlet.* (e.g coverlet.core)
• /p:Exclude="[*]Coverlet.Core.Instrumentation.*" ⇒ Excludes all types belonging to Coverlet.Core.Instrumentation namespace in any assembly
• /p:Exclude="[coverlet.*.tests?]*" ⇒ Excludes all types in any assembly starting with coverlet. and ending with .test or .tests (the ? makes the s optional)
• /p:Exclude=\"[coverlet.*]*,[*]Coverlet.Core*\" ⇒ Excludes assemblies matching coverlet.* and excludes all types belonging to the Coverlet.Core namespace in any assembly

dotnet test /p:CollectCoverage=true /p:Exclude="[coverlet.*]Coverlet.Core.Coverage"

Coverlet goes a step in the other direction by also letting you explicitly set what can be included using the Include property.

Examples

• /p:Include="[*]*" ⇒ Includes all types in all assemblies (everything is instrumented)
• /p:Include="[coverlet.*]Coverlet.Core.Coverage" ⇒ Includes the Coverage class in the Coverlet.Core namespace belonging to any assembly that matches coverlet.* (e.g coverlet.core)
• /p:Include="[coverlet.*.tests?]*" ⇒ Includes all types in any assembly starting with coverlet. and ending with .test or .tests (the ? makes the s optional)

Both Exclude and Include properties can be used together but Exclude takes precedence. You can specify multiple filter expressions by separating them with a comma (,).

You can also include coverage of the test assembly itself by setting /p:IncludeTestAssembly to true.

Skip auto-implemented properties

Neither track nor record auto-implemented properties. Syntax: /p:SkipAutoProps=true

Instrument module without local sources file

Syntax: /p:InstrumentModulesWithoutLocalSources=true

Methods that do not return

Methods that do not return can be marked with attributes to cause statements after them to be excluded from coverage.

Attributes can be specified with the following syntax. Syntax: /p:DoesNotReturnAttribute="DoesNotReturnAttribute,OtherAttribute"

Note for Powershell / Azure DevOps users

To exclude or include multiple assemblies when using Powershell scripts or creating a .yaml file for an Azure DevOps build %2c should be used as a separator. Msbuild will translate this symbol to ,.

/p:Exclude="[*]*Examples?%2c[*]*Startup"

Azure DevOps builds do not require double quotes to be unescaped:

dotnet test --configuration $(buildConfiguration) --no-build /p:CollectCoverage=true /p:CoverletOutputFormat=cobertura /p:CoverletOutput=$(Build.SourcesDirectory)/TestResults/Coverage/ /p:Exclude="[MyAppName.DebugHost]*%2c[MyAppName.WebHost]*%2c[MyAppName.App]*"

Note for Linux users

There is an issue with MSBuild on Linux that affects the ability to escape quotes while specifying multiple comma-separated values. Linux MSBuild automatically translates \ to / in properties, tasks, etc. before using them, which means if you specified /p:CoverletOutputFormat=\"json,opencover\" in an MSBuild script, it will be converted to /p:CoverletOutputFormat=/"json,opencover/" before execution. This yields an error similar to the following:

MSBUILD : error MSB1006: Property is not valid. [/home/vsts/work/1/s/default.proj]
 Switch: opencover/

You'll see this if directly consuming Linux MSBuild or if using the Azure DevOps MSBuild task on a Linux agent.

The workaround is to use the .NET Core dotnet msbuild command instead of using MSBuild directly. The issue is not present in dotnet msbuild and the script will run with correctly escaped quotes.

SourceLink

Coverlet supports SourceLink custom debug information contained in PDBs. When you specify the /p:UseSourceLink=true property, Coverlet will generate results that contain the URL to the source files in your source control instead of local file paths.

Deterministic build

Take a look at for further information. To generate deterministic report the parameter is:

/p:DeterministicReport=true

Exclude assemblies without sources from coverage

The heuristic coverlet uses to determine if an assembly is a third-party dependency is based on the matching of the assembly`s source documents and the corresponding source files. This parameter has three different values to control the automatic assembly exclusion.

Here is an example of how to specify the parameter:

/p:ExcludeAssembliesWithoutSources="MissingAny"

Enable Restore of instrumented assembly

The DisableManagedInstrumentationRestore property controls whether Coverlet should restore (revert) an assembly to its original state after instrumentation. By default, this is set to false, meaning:

1. Coverlet instruments (modifies) the assembly to track code coverage
2. After coverage collection, it restores the assembly back to its original state

When set to true:

• The assembly remains in its instrumented state
• This can help avoid file access conflicts
• Useful for testing/debugging instrumentation without restoration

Example use case:

<PropertyGroup>
 
 <DisableManagedInstrumentationRestore>true</DisableManagedInstrumentationRestore>
</PropertyGroup>

This setting is particularly helpful when troubleshooting instrumentation issues or when dealing with file locking problems during coverage collection.

Architecture

coverlet.msbuild integrates through MSBuild targets/tasks and wraps test execution with instrumentation and report generation steps.

Dependencies and functionality

flowchart LR
 CLI["dotnet test"] --> MSB["MSBuild pipeline"]
 MSB --> PRE["InstrumentationTask\n(before test target)"]
 PRE --> CORE["coverlet.core"]
 MSB --> TEST["VSTest/TestHost execution"]
 TEST --> POST["CoverageResultTask\n(after test target)"]
 POST --> CORE
 CORE --> OUT["coverage.json/xml/etc."]
 POST --> THR["Threshold evaluation\n(build pass/fail)"]

Limitations and constraints

• This mode depends on MSBuild target ordering; custom targets can alter expected behavior.
• Instrumented binaries are modified on disk during test run and then restored (unless restore is explicitly disabled).
• Quoting/escaping of multi-value properties can vary by shell and OS (notably Linux MSBuild escaping behavior).
• If projects are rebuilt between instrumentation and execution, collected coverage can be invalid.