Re-enable net481 code coverage once coverlet .NET Framework instrumentation is fixed

[Shyam-Gupta]

Code coverage collection is currently disabled for the net481 target because coverlet 8.0.0 crashes when instrumenting .NET Framework assemblies. This issue tracks re-enabling it once the upstream bugs are fixed.

Issue

Debug/x64 PR builds fail at test execution (not compilation), with every test in the net481 leg throwing:

System.TypeInitializationException : The type initializer for
'Coverlet.Core.Instrumentation.Tracker.Microsoft.Private.Windows.Polyfills.Tests_d6550c7c-fd38-4022-a909-2613cf55c4ed'
threw an exception.
---- System.IO.FileNotFoundException : Could not load file or assembly
'System.Runtime, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
or one of its dependencies. The system cannot find the file specified.

Only this leg is affected because eng/pipelines/build-PR.yml enables Coverage solely for public Debug x64 builds.

Root cause

Commit f3422f9 (#14371) bumped CoverletMSBuildPackageVersion from 6.0.0 to 8.0.0 in eng/Versions.props.

Coverlet 8.0.0 selects its task assembly by MSBuild host, not target framework (buildMultiTargeting/coverlet.msbuild.props):

<CoverletToolsPath Condition="'$(CoverletToolsPath)' == '' and '$(MSBuildRuntimeType)' == 'Core'">$(MSBuildThisFileDirectory)..\tasks\net8.0\</CoverletToolsPath>

Arcade builds with dotnet build, so the net8.0 flavor is loaded. Its coverlet.core.dll references System.Runtime 8.0.0.0 (the netstandard2.0 flavor, and 6.0.0, reference netstandard 2.0.0.0). The instrumenter copies its tracker template IL out of the loaded coverlet.core.dll into the assembly under test, carrying that reference along — so the tracker injected into our net481 test assembly references an assembly the .NET Framework CLR cannot resolve, and its static constructor throws on the first recorded hit.

This is an acknowledged upstream regression, not an intentional change: coverlet-coverage/coverlet#1818 (identical repro on net481/x64, labeled bug, fixed in coverlet 8.0.1).

Temporary fix

In Directory.Build.props, inside the '$(Coverage)' == 'true' group:

<CollectCoverage Condition="'$(TargetFramework)' == 'net481'">false</CollectCoverage>

Tests still build and run; only .NET Framework coverage collection is skipped. There is no real loss of signal — that leg was producing a red build and no valid coverage data.

Affected projects (the only ones that are both IsTestProject=true and target net481):

Project	Effect
Microsoft.Private.Windows.Polyfills.Tests	No longer contributes coverage (its $(NetCurrent) leg already had CollectCoverage=false)
System.Private.Windows.Core.Tests	net481 leg not instrumented; .NET leg unchanged

Change needed once coverlet fixes this

Do not simply bump the package version — every release after 6.x is still broken for .NET Framework. All references below are in the coverlet-coverage/coverlet repo:

Version	.NET Framework behaviour
6.0.x	Works (tracker references netstandard 2.0.0.0)
8.0.0	This crash (coverlet-coverage/coverlet#1818)
8.0.1	Crash fixed, but no coverage reported at all (coverlet-coverage/coverlet#1842)
10.0.0	Fixes coverlet#1842; the fix for silent zero coverage since 8.0.0 (coverlet-coverage/coverlet#1985) is still unreleased
10.0.1	New regression: TypeInitializationException: Could not load type 'System.Collections.Concurrent.ConcurrentBag\1'onnet481` (coverlet-coverage/coverlet#2009); fix (coverlet-coverage/coverlet#2010) unreleased

Wait for coverlet-coverage/coverlet#1985 and coverlet-coverage/coverlet#2010 to ship in a release, then:

1. Update CoverletMSBuildPackageVersion in eng/Versions.props.
2. Remove the CollectCoverage override and its explanatory comment from Directory.Build.props.
3. Verify a net481 coverage file is actually produced under artifacts\bin\<project>\Debug\net481\coverage\ — the failure mode in coverlet-coverage/coverlet#1842 is silent, so a green build alone does not confirm success.