Running .NET Core 3 XUnit code coverage in AppVeyor using OpenCover and Codecov

appveyor.yml

# Add this to your YML file
install:
 - cinst opencover.portable
 - cinst codecov

test: off

test_script:
 - ps: ./coverage.ps1

after_test:
 - codecov -f coverage.xml

coverage.ps1

$buildConfig = if ($ENV:CONFIGURATION -eq $null) { "Release" } else { $ENV:CONFIGURATION }

$buildFolder = if ($ENV:APPVEYOR_BUILD_FOLDER -eq $null) { $PSScriptRoot } else { $ENV:APPVEYOR_BUILD_FOLDER }

$openCover = 'C:\ProgramData\chocolatey\lib\opencover.portable\tools\OpenCover.Console.exe'

$target = '-target:C:\Program Files\dotnet\dotnet.exe'
$targetArgs = '-targetargs:"test -c:' + $buildConfig + ' --logger:trx;LogFileName=results.trx /p:DebugType=full"' 
$filter = '-filter:+[*]*-[*.Tests]*'
$output = '-output:' + $buildFolder + '\coverage.xml'
$register = if ($ENV:APPVEYOR -eq $true ) { '-register' } else { '-register:user' } # Magical parameter that breaks things

# Run OpenCover with all params
& $openCover $target $targetArgs $filter $register '-oldStyle' '-mergeoutput' $output