coverlet/Documentation/Examples/MSBuild/MergeWith/HowTo.md

Run from solution root sln

To merge report togheter you need to run separate test and merge in one json format file. Last command will join and create final needed format file.

dotnet test XUnitTestProject1\XUnitTestProject1.csproj /p:CollectCoverage=true  /p:CoverletOutput=../CoverageResults/
dotnet test XUnitTestProject2\XUnitTestProject2.csproj /p:CollectCoverage=true  /p:CoverletOutput=../CoverageResults/ /p:MergeWith="../CoverageResults/coverage.json"
dotnet test XUnitTestProject3\XUnitTestProject3.csproj /p:CollectCoverage=true  /p:CoverletOutput=../CoverageResults/ /p:MergeWith="../CoverageResults/coverage.json" /p:CoverletOutputFormat="opencover"

You can merge also running dotnet test and merge with single command from a solution file, but you need to ensure that tests will run sequentially(-m:1). This slow down testing but avoid invalid coverage result.

dotnet test /p:CollectCoverage=true  /p:CoverletOutput=../CoverageResults/ /p:MergeWith="../CoverageResults/coverage.json" /p:CoverletOutputFormat=\"opencover,json\" -m:1

N.B. You need to specify json format plus another format(the final one), because Coverlet can only merge proprietary format. At the end you can delete temporary coverage.json file.

You can also merge the coverage result and generate another valid format to export the content than opencover, like cobertura.

dotnet test /p:CollectCoverage=true  /p:CoverletOutput=../CoverageResults/ /p:MergeWith="../CoverageResults/coverage.json" /p:CoverletOutputFormat=\"cobertura,json\" -m:1