[jonathanpeppers]

Allow <ReadLibraryProjectImportsCache/> to run in process during multithreaded MSBuild builds instead of requiring a separate task host. Resolve the cache file through TaskEnvironment, while preserving empty/missing cache behavior, output items, metadata, and logging.

Add small relative-path and missing-cache tests to the existing MultiThreadedMigrationTests fixture. Leave existing build tests and shared XML helpers unchanged.

Validation

• .\dotnet-local.cmd build Microsoft.Android.slnx -t:BuildDotNet -c Release -nr:false -v:minimal -bl:sdk-build-final.binlog - passed, 92 warnings and zero errors. Used private .NET 11 RC2 SDK/runtime pins and a compatible workload manifest; temporary source settings were restored.
• .\dotnet-local.cmd build src\Xamarin.Android.Build.Tasks\Tests\Xamarin.Android.Build.Tests\Xamarin.Android.Build.Tests.csproj -c Release --no-restore -p:BuildProjectReferences=false -nr:false -v:quiet -bl:migration-tests-build.binlog - passed, zero warnings and errors.
• .\dotnet-local.cmd test bin\TestRelease\net10.0\Xamarin.Android.Build.Tests.dll --filter FullyQualifiedName~MultiThreadedMigrationTests --logger "trx;LogFileName=cache-reader-migration.trx" --results-directory TestResults -v:minimal - all four tests passed.
• Ordinary and strict -m:2 -mt -nr:false -warnaserror:MSB4286,MSB4287,MSB4288 task checks passed. Two overlapping calls ran in one process without task hosts. All seven output groups, including relative identities, metadata, and project-relative FullPath, matched across two project directories. Error cases retained their expected failures. Binlogs were inspected with binlogtool.

Related issue: N/A.

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Pull Request
title and
description
should follow the
commit-messages.md workflow documentation, and in particular should include:

• Useful description of why the change is necessary.
• Links to issues fixed
• Unit tests

[Copilot]

🟢 Approval recommended

The focused migration has no identified blocking issues, and reported validation covers regression tests and strict multithreaded execution.

No open issues 🎉

What changed in this PR

Enables ReadLibraryProjectImportsCache to run in-process during multithreaded MSBuild builds while preserving existing cache outputs and missing-cache behavior.

Changes:

• Migrates the task to MultithreadedAndroidTask and adds the direct multithreading attribute.
• Resolves cache paths through TaskEnvironment.
• Adds tests for project-relative cache paths and empty or missing cache files.

File	Description
src/​Xamarin.Android.Build.Tasks/​Tests/​Xamarin.Android.Build.Tests/​Tasks/​MultiThreadedMigrationTests.cs	Adds relative-path and missing-cache regression tests.
src/​Xamarin.Android.Build.Tasks/​Tasks/​ReadLibraryProjectImportsCache.cs	Enables multithreaded execution and project-relative cache access.

_{Review effort: Balanced}

[jonathanpeppers]

@dalexsoto review

[dalexsoto]

Reviewed the complete 2-file task/test change and the relevant task base, MSBuild environment/routing, production consumers and source tests. ReadLibraryProjectImportsCache uses the project environment while preserving all seven outputs and existing consumer contracts. No high-confidence blockers remain in the bounded static review.