Annotate Arcade MSBuild tasks with [MSBuildMultiThreadableTask] to avoid TaskHost marshalling in -mt builds

[ViktorHofer]

Summary

MSBuild's multi-threaded mode (-mt) runs all build nodes as threads inside a single process. Any task not marked with Microsoft.Build.Framework.MSBuildMultiThreadableTaskAttribute is routed to an out-of-proc sidecar TaskHost for isolation, so every invocation pays cross-process serialization of its inputs and outputs.

No task in this repository is currently annotated, so all ~136 Arcade tasks are marshalled out-of-proc in -mt builds. This measurably eats into the gains from -mt + MSBuild Server + node reuse.

Measured impact

A/B of the runtime inner repo build inside the dotnet/dotnet VMR (win-x64, same source SHA 2eebfbf, same SDK 11.0.100-rc.1.26420.103, MSBuild 18.11.0-1.26420.103). Command lines were identical except:

• A (baseline): /nr:False, no -mt -> MSBuild Server was requested but not used for this build: node reuse is disabled
• B: /nr:True -mt -> Reusing the running MSBuild Server node for this build

Wall time went 51.50 min -> 49.64 min, only 1.86 min (3.6%). Meanwhile:

metric	A (baseline)	B (-mt+server+nodereuse)
TaskHost invocations	849	14,125 (16.6x)
distinct caller processes	8	1 (all nodes are threads)
total node-busy time	13,622 s	12,189 s (-10.5%)
effective parallelism (of 8 nodes)	4.41x	4.09x
sum of non-nesting task time	4,026 s	4,124 s (+98 s)

Engine overhead genuinely dropped (MSBuild task inclusive 126,987s -> 106,276s), but ~100s of that was handed straight back as TaskHost marshalling. Arcade-owned tasks are the bulk of it:

task	A	B	ratio	invocations
ChooseBestP2PTargetFrameworkTask	1.5 s	17.6 s	11.8x	1,740
GetLicenseFilePath	0.4 s	15.0 s	36x	1,542
ChooseBestTargetFrameworksTask	0.4 s	12.6 s	32x	1,195
GeneratePackageReport	1.7 s	13.6 s	8.0x	11
ValidateLicense	0.4 s	10.8 s	30x	1,542
GetLastStablePackage	0.1 s	6.3 s	61x	130
GenerateResxSource	1.6 s	6.2 s	3.8x	606
GetMinimumNETStandard	0.3 s	3.0 s	10.3x	60
ApplyPreReleaseSuffix	0.03 s	2.5 s	77x	109
TranslateSource	1.8 s	3.1 s	1.7x	208

That is ~82 s of pure added overhead from 10 Arcade tasks in a single repo build, and it repeats for every repo in the VMR.

Proposed change

Audit each task and add [MSBuildMultiThreadableTask] where it is genuinely thread-safe.

Per MSBuildMultiThreadableTaskAttribute, a task qualifies only if it:

• does not modify global process state (env vars, current working directory, etc.), and
• does not depend on global process state, including relative path resolution.

Most of the P1 tasks above are pure item/string transforms, so they should qualify easily.

Implementation note

Arcade currently pins the MSBuild packages at 17.12.50 (eng/Version.Details.props, flowed from dotnet-msbuild), which predates the attribute. Step 1 is simply to flow up the MSBuild package versions to 18.x so MSBuildMultiThreadableTaskAttribute is available from Microsoft.Build.Framework directly:

<MicrosoftBuildPackageVersion>17.12.50</MicrosoftBuildPackageVersion>
<MicrosoftBuildFrameworkPackageVersion>17.12.50</MicrosoftBuildFrameworkPackageVersion>
<MicrosoftBuildTasksCorePackageVersion>17.12.50</MicrosoftBuildTasksCorePackageVersion>
<MicrosoftBuildUtilitiesCorePackageVersion>17.12.50</MicrosoftBuildUtilitiesCorePackageVersion>

The attribute is non-inheritable (Inherited = false), so it must be applied to each concrete task class - not to a shared base such as BuildTask / MSBuildTaskBase / XlfTask.

Task inventory

All concrete MSBuild task classes under src/ (excluding eng/common, abstract bases and test-only stubs). 136 tasks.

P1 - measured hot path (do these first)

• ChooseBestP2PTargetFrameworkTask - src\Microsoft.DotNet.Build.Tasks.TargetFramework\src\ChooseBestP2PTargetFrameworkTask.cs
• GetLicenseFilePath - src\Microsoft.DotNet.Arcade.Sdk\src\GetLicenseFilePath.cs
• ChooseBestTargetFrameworksTask - src\Microsoft.DotNet.Build.Tasks.TargetFramework\src\ChooseBestTargetFrameworksTask.cs
• GeneratePackageReport - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GeneratePackageReport.cs
• ValidateLicense - src\Microsoft.DotNet.Arcade.Sdk\src\ValidateLicense.cs
• GetLastStablePackage - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetLastStablePackage.cs
• GenerateResxSource - src\Microsoft.DotNet.Arcade.Sdk\src\GenerateResxSource.cs
• GetMinimumNETStandard - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetMinimumNETStandard.cs
• ApplyPreReleaseSuffix - src\Microsoft.DotNet.Build.Tasks.Packaging\src\ApplyPreReleaseSuffix.cs
• TranslateSource - src\Microsoft.DotNet.XliffTasks\Tasks\TranslateSource.cs

P2 / P3 - remaining tasks by package

Microsoft.DotNet.Arcade.Sdk (15)

• CalculateAssemblyAndFileVersions - src\Microsoft.DotNet.Arcade.Sdk\src\CalculateAssemblyAndFileVersions.cs
• CheckRequiredDotNetVersion - src\Microsoft.DotNet.Arcade.Sdk\src\CheckRequiredDotNetVersion.cs
• CompareVersions - src\Microsoft.DotNet.Arcade.Sdk\src\CompareVersions.cs
• DownloadFile - src\Microsoft.DotNet.Arcade.Sdk\src\DownloadFile.cs
• ExtractNgenMethodList - src\Microsoft.DotNet.Arcade.Sdk\src\ExtractNgenMethodList.cs
• GenerateChecksums - src\Microsoft.DotNet.Arcade.Sdk\src\GenerateChecksums.cs
• GenerateSourcePackageSourceLinkTargetsFile - src\Microsoft.DotNet.Arcade.Sdk\src\GenerateSourcePackageSourceLinkTargetsFile.cs
• GetAssemblyFullName - src\Microsoft.DotNet.Arcade.Sdk\src\GetAssemblyFullName.cs
• GroupItemsBy - src\Microsoft.DotNet.Arcade.Sdk\src\GroupItemsBy.cs
• InstallDotNetCore - src\Microsoft.DotNet.Arcade.Sdk\src\InstallDotNetCore.cs
• LocateDotNet - src\Microsoft.DotNet.Arcade.Sdk\src\LocateDotNet.cs
• SaveItems - src\Microsoft.DotNet.Arcade.Sdk\src\SaveItems.cs
• SetCorFlags - src\Microsoft.DotNet.Arcade.Sdk\src\SetCorFlags.cs
• SingleError - src\Microsoft.DotNet.Arcade.Sdk\src\SingleError.cs
• Unsign - src\Microsoft.DotNet.Arcade.Sdk\src\Unsign.cs

Microsoft.DotNet.Build.Tasks.Feed (12)

• ConfigureInputFeed - src\Microsoft.DotNet.Build.Tasks.Feed\src\ConfigureInputFeed.cs
• CreateAzureContainerIfNotExists - src\Microsoft.DotNet.Build.Tasks.Feed\src\common\CreateAzureContainerIfNotExists.cs
• CreateAzureDevOpsFeed - src\Microsoft.DotNet.Build.Tasks.Feed\src\CreateAzureDevOpsFeed.cs
• CreateNewAzureContainer - src\Microsoft.DotNet.Build.Tasks.Feed\src\common\CreateNewAzureContainer.cs
• LaunchDebugger - src\Microsoft.DotNet.Build.Tasks.Feed\src\LaunchDebugger.cs
• PublishArtifactsInManifest - src\Microsoft.DotNet.Build.Tasks.Feed\src\PublishArtifactsInManifest.cs
• PublishArtifactsInManifestV3 - src\Microsoft.DotNet.Build.Tasks.Feed\src\PublishArtifactsInManifestV3.cs
• PublishArtifactsInManifestV4 - src\Microsoft.DotNet.Build.Tasks.Feed\src\PublishArtifactsInManifestV4.cs
• PublishBuildToMaestro - src\Microsoft.DotNet.Build.Tasks.Feed\src\PublishBuildToMaestro.cs
• PublishSignedAssets - src\Microsoft.DotNet.Build.Tasks.Feed\src\PublishSignedAssets.cs
• PushToBuildStorage - src\Microsoft.DotNet.Build.Tasks.Feed\src\PushToBuildStorage.cs
• UploadToAzure - src\Microsoft.DotNet.Build.Tasks.Feed\src\common\UploadToAzure.cs

Microsoft.DotNet.Build.Tasks.FileCatalog (1)

• GenerateFileCatalog - src\Microsoft.DotNet.Build.Tasks.FileCatalog\GenerateFileCatalog.cs

Microsoft.DotNet.Build.Tasks.Installers (13)

• CreateChangelogFile - src\Microsoft.DotNet.Build.Tasks.Installers\src\CreateChangelogFile.cs
• CreateControlFile - src\Microsoft.DotNet.Build.Tasks.Installers\src\CreateControlFile.cs
• CreateDebPackage - src\Microsoft.DotNet.Build.Tasks.Installers\src\CreateDebPackage.cs
• CreateLightCommandPackageDrop - src\Microsoft.DotNet.Build.Tasks.Installers\src\CreateLightCommandPackageDrop.cs
• CreateMD5SumsFile - src\Microsoft.DotNet.Build.Tasks.Installers\src\CreateMD5SumsFile.cs
• CreateRpmPackage - src\Microsoft.DotNet.Build.Tasks.Installers\src\CreateRpmPackage.cs
• CreateWixBuildWixpack - src\Microsoft.DotNet.Build.Tasks.Installers\src\CreateWixBuildWixpack.cs
• ExecWithRetries - src\Microsoft.DotNet.Build.Tasks.Installers\src\ExecWithRetries.cs
• GenerateCurrentVersion - src\Microsoft.DotNet.Build.Tasks.Installers\src\GenerateCurrentVersion.cs
• GenerateGuidFromName - src\Microsoft.DotNet.Build.Tasks.Installers\src\GenerateGuidFromName.cs
• GenerateMacOSDistributionFile - src\Microsoft.DotNet.Build.Tasks.Installers\src\GenerateMacOSDistributionFile.cs
• GenerateMsiVersion - src\Microsoft.DotNet.Build.Tasks.Installers\src\GenerateMsiVersion.cs
• StabilizeWixFileId - src\Microsoft.DotNet.Build.Tasks.Installers\src\StabilizeWixFileId.cs

Microsoft.DotNet.Build.Tasks.Packaging (27)

• ApplyBaseLine - src\Microsoft.DotNet.Build.Tasks.Packaging\src\ApplyBaseLine.cs
• ApplyMetaPackages - src\Microsoft.DotNet.Build.Tasks.Packaging\src\ApplyMetaPackages.cs
• CreateTrimDependencyGroups - src\Microsoft.DotNet.Build.Tasks.Packaging\src\CreateTrimDependencyGroups.cs
• FilterUnknownPackages - src\Microsoft.DotNet.Build.Tasks.Packaging\src\FilterUnknownPackages.cs
• GenerateNuSpec - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GenerateNuSpec.cs
• GenerateRuntimeDependencies - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GenerateRuntimeDependencies.cs
• GetApplicableAssetsFromPackageReports - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetApplicableAssetsFromPackageReports.cs
• GetApplicableAssetsFromPackages - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetApplicableAssetsFromPackages.cs
• GetAssemblyReferences - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetAssemblyReferences.cs
• GetInboxFrameworks - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetInboxFrameworks.cs
• GetLayoutFiles - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetLayoutFiles.cs
• GetPackageDescription - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetPackageDescription.cs
• GetPackageDestination - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetPackageDestination.cs
• GetPackageFromModule - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetPackageFromModule.cs
• GetPackageVersion - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetPackageVersion.cs
• GetRuntimeJsonValues - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetRuntimeJsonValues.cs
• GetRuntimeTargets - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetRuntimeTargets.cs
• GetSupportedPackagesFromPackageReports - src\Microsoft.DotNet.Build.Tasks.Packaging\src\GetSupportedPackagesFromPackageReports.cs
• HarvestPackage - src\Microsoft.DotNet.Build.Tasks.Packaging\src\HarvestPackage.cs
• NuGetPack - src\Microsoft.DotNet.Build.Tasks.Packaging\src\NuGetPack.cs
• PromoteDependencies - src\Microsoft.DotNet.Build.Tasks.Packaging\src\PromoteDependencies.cs
• SplitDependenciesBySupport - src\Microsoft.DotNet.Build.Tasks.Packaging\src\SplitDependenciesBySupport.cs
• SplitReferences - src\Microsoft.DotNet.Build.Tasks.Packaging\src\SplitReferences.cs
• UpdatePackageIndex - src\Microsoft.DotNet.Build.Tasks.Packaging\src\UpdatePackageIndex.cs
• ValidateFrameworkPackage - src\Microsoft.DotNet.Build.Tasks.Packaging\src\ValidateFrameworkPackage.cs
• ValidateHarvestVersionIsLatestForRelease - src\Microsoft.DotNet.Build.Tasks.Packaging\src\ValidateHarvestVersionIsLatestForRelease.cs
• ValidatePackage - src\Microsoft.DotNet.Build.Tasks.Packaging\src\ValidatePackage.cs

Microsoft.DotNet.Build.Tasks.Templating (1)

• GenerateFileFromTemplate - src\Microsoft.DotNet.Build.Tasks.Templating\src\GenerateFileFromTemplate.cs

Microsoft.DotNet.Build.Tasks.VisualStudio (6)

• FinalizeInsertionVsixFile - src\Microsoft.DotNet.Build.Tasks.VisualStudio\Vsix\FinalizeInsertionVsixFile.cs
• FindLatestDrop - src\Microsoft.DotNet.Build.Tasks.VisualStudio\OptProf\FindLatestDrop.cs
• GenerateTrainingInputFiles - src\Microsoft.DotNet.Build.Tasks.VisualStudio\OptProf\GenerateTrainingInputFiles.cs
• GenerateTrainingPropsFile - src\Microsoft.DotNet.Build.Tasks.VisualStudio\OptProf\GenerateTrainingPropsFile.cs
• GetPkgDefAssemblyDependencyGuid - src\Microsoft.DotNet.Build.Tasks.VisualStudio\Vsix\GetPkgDefAssemblyDependencyGuid.cs
• GetRunSettingsSessionConfiguration - src\Microsoft.DotNet.Build.Tasks.VisualStudio\OptProf\GetRunSettingsSessionConfiguration.cs

Microsoft.DotNet.Build.Tasks.Workloads (4)

• CreateVisualStudioWorkload - src\Microsoft.DotNet.Build.Tasks.Workloads\src\CreateVisualStudioWorkload.wix.cs
• CreateVisualStudioWorkloadSet - src\Microsoft.DotNet.Build.Tasks.Workloads\src\CreateVisualStudioWorkloadSet.wix.cs
• HarvesterToolTask - src\Microsoft.DotNet.Build.Tasks.Workloads\src\Wix\HarvesterToolTask.cs
• WixToolTask - src\Microsoft.DotNet.Build.Tasks.Workloads\src\Wix\WixToolTask.cs

Microsoft.DotNet.CMake.Sdk (2)

• CreateCMakeFileApiQuery - src\Microsoft.DotNet.CMake.Sdk\src\CreateCMakeFileApiQuery.cs
• GetCMakeArtifactsFromFileApi - src\Microsoft.DotNet.CMake.Sdk\src\GetCMakeArtifactsFromFileApi.cs

Microsoft.DotNet.Deployment.Tasks.Links (2)

• CreateAkaMSLinks - src\Microsoft.DotNet.Deployment.Tasks.Links\src\CreateAkaMSLinks.cs
• DeleteAkaMSLinks - src\Microsoft.DotNet.Deployment.Tasks.Links\src\DeleteAkaMSLinks.cs

Microsoft.DotNet.GenAPI (1)

• GenAPITask - src\Microsoft.DotNet.GenAPI\GenAPITask.cs

Microsoft.DotNet.GenFacades (3)

• ClearAssemblyReferenceVersions - src\Microsoft.DotNet.GenFacades\ClearAssemblyReferenceVersions.cs
• GenPartialFacadeSource - src\Microsoft.DotNet.GenFacades\GenPartialFacadeSource.cs
• NotSupportedAssemblyGenerator - src\Microsoft.DotNet.GenFacades\NotSupportedAssemblyGenerator.cs

Microsoft.DotNet.Helix (16)

• CancelHelixJobs - src\Microsoft.DotNet.Helix\Sdk\CancelHelixJob.cs
• CheckAzurePipelinesTestResults - src\Microsoft.DotNet.Helix\Sdk\CheckAzurePipelinesTestResults.cs
• CheckHelixJobStatus - src\Microsoft.DotNet.Helix\Sdk\CheckHelixJobStatus.cs
• CreateMTPWorkItems - src\Microsoft.DotNet.Helix\Sdk\CreateMTPWorkItems.cs
• CreateTestsForWorkItems - src\Microsoft.DotNet.Helix\Sdk\CreateFailedTestsForFailedWorkItems.cs
• CreateXHarnessAndroidWorkItems - src\Microsoft.DotNet.Helix\Sdk\CreateXHarnessAndroidWorkItems.cs
• CreateXHarnessAppleWorkItems - src\Microsoft.DotNet.Helix\Sdk\CreateXHarnessAppleWorkItems.cs
• CreateXUnitWorkItems - src\Microsoft.DotNet.Helix\Sdk\CreateXUnitWorkItems.cs
• DownloadFromResultsContainer - src\Microsoft.DotNet.Helix\Sdk\DownloadFromResultsContainer.cs
• FindDotNetCliPackage - src\Microsoft.DotNet.Helix\Sdk\FindDotNetCliPackage.cs
• GetHelixWorkItems - src\Microsoft.DotNet.Helix\Sdk\GetHelixWorkItems.cs
• InstallDotNetTool - src\Microsoft.DotNet.Helix\Sdk\InstallDotNetTool.cs
• SendHelixJob - src\Microsoft.DotNet.Helix\Sdk\SendHelixJob.cs
• StartAzurePipelinesTestRun - src\Microsoft.DotNet.Helix\Sdk\StartAzurePipelinesTestRun.cs
• StopAzurePipelinesTestRun - src\Microsoft.DotNet.Helix\Sdk\StopAzurePipelinesTestRun.cs
• WaitForHelixJobCompletion - src\Microsoft.DotNet.Helix\Sdk\WaitForHelixJobCompletion.cs

Microsoft.DotNet.NuGetRepack (2)

• ReplacePackageParts - src\Microsoft.DotNet.NuGetRepack\tasks\src\ReplacePackageParts.cs
• UpdatePackageVersionTask - src\Microsoft.DotNet.NuGetRepack\tasks\src\UpdatePackageVersionTask.cs

Microsoft.DotNet.PackageTesting (3)

• GetCompatiblePackageTargetFrameworks - src\Microsoft.DotNet.PackageTesting\GetCompatiblePackageTargetFrameworks.cs
• VerifyClosure - src\Microsoft.DotNet.PackageTesting\VerifyClosure.cs
• VerifyTypes - src\Microsoft.DotNet.PackageTesting\VerifyTypes.cs

Microsoft.DotNet.SharedFramework.Sdk (5)

• CreateFrameworkListFile - src\Microsoft.DotNet.SharedFramework.Sdk\src\CreateFrameworkListFile.cs
• GeneratePlatformManifestEntriesFromFileList - src\Microsoft.DotNet.SharedFramework.Sdk\src\GeneratePlatformManifestEntriesFromFileList.cs
• GeneratePlatformManifestEntriesFromTemplate - src\Microsoft.DotNet.SharedFramework.Sdk\src\GeneratePlatformManifestEntriesFromTemplate.cs
• GenerateSharedFrameworkDepsFile - src\Microsoft.DotNet.SharedFramework.Sdk\src\GenerateSharedFrameworkDepsFile.cs
• ValidateFileVersions - src\Microsoft.DotNet.SharedFramework.Sdk\src\ValidateFileVersions.cs

Microsoft.DotNet.SignCheckTask (1)

• SignCheckTask - src\Microsoft.DotNet.SignCheckTask\SignCheckTask.cs

Microsoft.DotNet.SignTool (1)

• SignToolTask - src\Microsoft.DotNet.SignTool\src\SignToolTask.cs

Microsoft.DotNet.SourceBuild (4)

• ReadNuGetPackageInfos - src\Microsoft.DotNet.SourceBuild\tasks\src\ReadNuGetPackageInfos.cs
• WriteBuildOutputProps - src\Microsoft.DotNet.SourceBuild\tasks\src\WriteBuildOutputProps.cs
• WritePackageUsageData - src\Microsoft.DotNet.SourceBuild\tasks\src\UsageReport\WritePackageUsageData.cs
• WriteUsageReports - src\Microsoft.DotNet.SourceBuild\tasks\src\UsageReport\WriteUsageReports.cs

Microsoft.DotNet.SwaggerGenerator (1)

• GenerateSwaggerCode - src\Microsoft.DotNet.SwaggerGenerator\Microsoft.DotNet.SwaggerGenerator.MSBuild\GenerateSwaggerCode.cs

Microsoft.DotNet.XliffTasks (6)

• EnsureAllResourcesTranslated - src\Microsoft.DotNet.XliffTasks\Tasks\EnsureAllResourcesTranslated.cs
• GatherTranslatedSource - src\Microsoft.DotNet.XliffTasks\Tasks\GatherTranslatedSource.cs
• GatherXlf - src\Microsoft.DotNet.XliffTasks\Tasks\GatherXlf.cs
• SortXlf - src\Microsoft.DotNet.XliffTasks\Tasks\SortXlf.cs
• TransformTemplates - src\Microsoft.DotNet.XliffTasks\Tasks\TransformTemplates.cs
• UpdateXlf - src\Microsoft.DotNet.XliffTasks\Tasks\UpdateXlf.cs

/cc @dotnet/dnceng

[ViktorHofer]

Automated analysis + first migration wave

I ran every Arcade task through the rule set from src/TaskAnalyzer
(MSBuildTask0001/0002/0004 banned-API tables from BannedApiDefinitions.cs), plus static-mutable-state detection,
using a Roslyn syntax walker with transitive call-chain analysis (the equivalent of TransitiveUnsafeCall/MSBuildTask0005),
so violations reached indirectly through shared helper classes are caught too.

MSBuildTask0003 (relative file paths) was deliberately not treated as a blocker: MSBuild's own Copy, Exec, Unzip,
ZipDirectory, Touch and all ToolTask-derived tool tasks carry [MSBuildMultiThreadableTask], so heavy file/process I/O
is not by itself disqualifying. The practical bar is: no process-global state (env vars, cwd, Path.GetFullPath/GetTempPath,
Console.*, Process.Start) and no unsynchronized static mutable state.

Results

	Count
Tasks analyzed	136
Clean -> annotated in this wave	44
Blocked, need code changes	92

Prerequisite: MSBuild package bump

MSBuildMultiThreadableTaskAttribute was added in dotnet/msbuild#12625 and first
shipped in 18.3.x. Arcade was pinned at 17.12.50, so the attribute did not exist. Bumped to 18.4.0 in
eng/Version.Details.props + eng/Version.Details.xml (net472 + net10.0 libs, matching NetMinimum=net10.0).

This is runtime-safe on older MSBuild hosts: AssemblyTaskFactory only calls TaskRouter.NeedsTaskHostInMultiThreadedMode
when BuildParameters.MultiThreaded == true, so a 17.x host never enumerates the attribute.

Bonus fix: real race unblocking the two hottest tasks

TargetFrameworkResolver.CreateOrGet used an unsynchronized static readonly Dictionary with TryGetValue/Add.
Switched to ConcurrentDictionary.GetOrAdd. That unblocks ChooseBestP2PTargetFrameworkTask (1740 invocations, +16.1s under -mt)
and ChooseBestTargetFrameworksTask (1195 invocations, +12.2s) - together ~35% of the measured P1 TaskHost overhead.

Annotated in this wave (44)

Microsoft.DotNet.Arcade.Sdk (9)

• CheckRequiredDotNetVersion
• DownloadFile
• ExtractNgenMethodList
• GenerateChecksums
• GenerateResxSource
• GetAssemblyFullName
• GroupItemsBy
• SaveItems
• SingleError

Microsoft.DotNet.Build.Tasks.Feed (1)

• LaunchDebugger

Microsoft.DotNet.Build.Tasks.FileCatalog (1)

• GenerateFileCatalog

Microsoft.DotNet.Build.Tasks.Installers (10)

• CreateChangelogFile
• CreateControlFile
• CreateDebPackage
• CreateLightCommandPackageDrop
• CreateMD5SumsFile
• GenerateCurrentVersion
• GenerateGuidFromName
• GenerateMacOSDistributionFile
• GenerateMsiVersion
• StabilizeWixFileId

Microsoft.DotNet.Build.Tasks.Packaging (7)

• GetAssemblyReferences
• GetMinimumNETStandard
• GetPackageDestination
• GetPackageVersion
• GetRuntimeJsonValues
• GetRuntimeTargets
• SplitDependenciesBySupport

Microsoft.DotNet.Build.Tasks.TargetFramework (2)

• ChooseBestP2PTargetFrameworkTask
• ChooseBestTargetFrameworksTask

Microsoft.DotNet.Build.Tasks.VisualStudio (6)

• FinalizeInsertionVsixFile
• FindLatestDrop
• GenerateTrainingInputFiles
• GenerateTrainingPropsFile
• GetPkgDefAssemblyDependencyGuid
• GetRunSettingsSessionConfiguration

Microsoft.DotNet.CMake.Sdk (1)

• CreateCMakeFileApiQuery

Microsoft.DotNet.Deployment.Tasks.Links (2)

• CreateAkaMSLinks
• DeleteAkaMSLinks

Microsoft.DotNet.GenFacades (1)

• ClearAssemblyReferenceVersions

Microsoft.DotNet.SourceBuild (4)

• ReadNuGetPackageInfos
• WriteBuildOutputProps
• WritePackageUsageData
• WriteUsageReports

Blocked - require code changes (92)

Grouped by root cause. Note most of these are blocked by a shared helper, not by the task itself -
fixing 4 helpers unblocks the large majority.

A. LocateDotNet helper (Arcade.Sdk) - 8 tasks

Blocking APIs / state:

• Environment.GetEnvironmentVariable (use TaskEnvironment.GetEnvironmentVariable)
• Path.GetFullPath (use TaskEnvironment.GetAbsolutePath)

affected tasks

• CalculateAssemblyAndFileVersions _{Microsoft.DotNet.Arcade.Sdk}
• CompareVersions _{Microsoft.DotNet.Arcade.Sdk}
• GenerateSourcePackageSourceLinkTargetsFile _{Microsoft.DotNet.Arcade.Sdk}
• GetLicenseFilePath _{Microsoft.DotNet.Arcade.Sdk}
• LocateDotNet _{Microsoft.DotNet.Arcade.Sdk}
• SetCorFlags _{Microsoft.DotNet.Arcade.Sdk}
• Unsign _{Microsoft.DotNet.Arcade.Sdk}
• ValidateLicense _{Microsoft.DotNet.Arcade.Sdk}

B. PackageIndex helper (Build.Tasks.Packaging) - 23 tasks

Blocking APIs / state:

• Path.GetFullPath (use TaskEnvironment.GetAbsolutePath)
• static-mutable-field: static ConcurrentDictionary<string, PackageIndex> s_indexCache
• static-mutable-field: static JsonSerializer s_serializer
• static-mutable-field: static FrameworkReducer s_reducer
• static-readonly-mutable-type: static readonly string[] s_nullRidList
• static-mutable-field: static FrameworkAssemblyReferenceComparer frameworkAssemblyReferenceComparer

affected tasks

• ApplyBaseLine _{Microsoft.DotNet.Build.Tasks.Packaging}
• ApplyMetaPackages _{Microsoft.DotNet.Build.Tasks.Packaging}
• ApplyPreReleaseSuffix _{Microsoft.DotNet.Build.Tasks.Packaging}
• CreateTrimDependencyGroups _{Microsoft.DotNet.Build.Tasks.Packaging}
• FilterUnknownPackages _{Microsoft.DotNet.Build.Tasks.Packaging}
• GenerateNuSpec _{Microsoft.DotNet.Build.Tasks.Packaging}
• GeneratePackageReport _{Microsoft.DotNet.Build.Tasks.Packaging}
• GenerateRuntimeDependencies _{Microsoft.DotNet.Build.Tasks.Packaging}
• GetApplicableAssetsFromPackageReports _{Microsoft.DotNet.Build.Tasks.Packaging}
• GetApplicableAssetsFromPackages _{Microsoft.DotNet.Build.Tasks.Packaging}
• GetInboxFrameworks _{Microsoft.DotNet.Build.Tasks.Packaging}
• GetLastStablePackage _{Microsoft.DotNet.Build.Tasks.Packaging}
• GetLayoutFiles _{Microsoft.DotNet.Build.Tasks.Packaging}
• GetPackageFromModule _{Microsoft.DotNet.Build.Tasks.Packaging}
• GetSupportedPackagesFromPackageReports _{Microsoft.DotNet.Build.Tasks.Packaging}
• HarvestPackage _{Microsoft.DotNet.Build.Tasks.Packaging}
• NuGetPack _{Microsoft.DotNet.Build.Tasks.Packaging}
• PromoteDependencies _{Microsoft.DotNet.Build.Tasks.Packaging}
• SplitReferences _{Microsoft.DotNet.Build.Tasks.Packaging}
• UpdatePackageIndex _{Microsoft.DotNet.Build.Tasks.Packaging}
• ValidateFrameworkPackage _{Microsoft.DotNet.Build.Tasks.Packaging}
• ValidateHarvestVersionIsLatestForRelease _{Microsoft.DotNet.Build.Tasks.Packaging}
• ValidatePackage _{Microsoft.DotNet.Build.Tasks.Packaging}

C. Helix / AzureDevOps client stack - 15 tasks

Blocking APIs / state:

• Environment.GetEnvironmentVariable (use TaskEnvironment.GetEnvironmentVariable)
• static-mutable-field: static int s_firstRespondersHintShown
• static-mutable-field: static int s_queueStatsHeaderShown
• static-readonly-mutable-type: static readonly char[] s_slashes
• Path.GetTempPath (TMP/TEMP dependent)
• static-mutable-field: static HttpClient _client

affected tasks

• CancelHelixJobs _{Microsoft.DotNet.Helix}
• CheckAzurePipelinesTestResults _{Microsoft.DotNet.Helix}
• CheckHelixJobStatus _{Microsoft.DotNet.Helix}
• CreateMTPWorkItems _{Microsoft.DotNet.Helix}
• CreateTestsForWorkItems _{Microsoft.DotNet.Helix}
• CreateXHarnessAppleWorkItems _{Microsoft.DotNet.Helix}
• CreateXUnitWorkItems _{Microsoft.DotNet.Helix}
• DownloadFromResultsContainer _{Microsoft.DotNet.Helix}
• FindDotNetCliPackage _{Microsoft.DotNet.Helix}
• GetHelixWorkItems _{Microsoft.DotNet.Helix}
• InstallDotNetTool _{Microsoft.DotNet.Helix}
• SendHelixJob _{Microsoft.DotNet.Helix}
• StartAzurePipelinesTestRun _{Microsoft.DotNet.Helix}
• StopAzurePipelinesTestRun _{Microsoft.DotNet.Helix}
• WaitForHelixJobCompletion _{Microsoft.DotNet.Helix}

D. Feed / publishing stack - 10 tasks

Blocking APIs / state:

• Directory.GetCurrentDirectory (use TaskEnvironment.ProjectDirectory)
• Console.WriteLine
• Environment.GetEnvironmentVariable (use TaskEnvironment.GetEnvironmentVariable)
• Path.GetFullPath (use TaskEnvironment.GetAbsolutePath)
• static-mutable-field: static Regex FourPartVersionRegex
• static-readonly-mutable-type: static readonly Dictionary<string, string> CacheMappings

affected tasks

• ConfigureInputFeed _{Microsoft.DotNet.Build.Tasks.Feed}
• CreateAzureContainerIfNotExists _{Microsoft.DotNet.Build.Tasks.Feed}
• CreateAzureDevOpsFeed _{Microsoft.DotNet.Build.Tasks.Feed}
• CreateNewAzureContainer _{Microsoft.DotNet.Build.Tasks.Feed}
• PublishArtifactsInManifest _{Microsoft.DotNet.Build.Tasks.Feed}
• PublishArtifactsInManifestV3 _{Microsoft.DotNet.Build.Tasks.Feed}
• PublishArtifactsInManifestV4 _{Microsoft.DotNet.Build.Tasks.Feed}
• PublishBuildToMaestro _{Microsoft.DotNet.Build.Tasks.Feed}
• PublishSignedAssets _{Microsoft.DotNet.Build.Tasks.Feed}
• UploadToAzure _{Microsoft.DotNet.Build.Tasks.Feed}

E. Task-local issues - 36 tasks

Blocking APIs / state:

• Path.GetFullPath (use TaskEnvironment.GetAbsolutePath)
• static-readonly-mutable-type: static readonly char[] s_attrValueTrimChars
• static-readonly-mutable-type: static readonly char[] s_directorySeparatorChars
• static-mutable-field: static XmlSchemaSet s_schemaSet
• static-readonly-mutable-type: static readonly string[] RunnableSuffixes
• Path.GetTempPath (TMP/TEMP dependent)

affected tasks

• InstallDotNetCore _{Microsoft.DotNet.Arcade.Sdk}
• PushToBuildStorage _{Microsoft.DotNet.Build.Tasks.Feed}
• CreateRpmPackage _{Microsoft.DotNet.Build.Tasks.Installers}
• CreateWixBuildWixpack _{Microsoft.DotNet.Build.Tasks.Installers}
• ExecWithRetries _{Microsoft.DotNet.Build.Tasks.Installers}
• GetPackageDescription _{Microsoft.DotNet.Build.Tasks.Packaging}
• GenerateFileFromTemplate _{Microsoft.DotNet.Build.Tasks.Templating}
• CreateVisualStudioWorkload _{Microsoft.DotNet.Build.Tasks.Workloads}
• CreateVisualStudioWorkloadSet _{Microsoft.DotNet.Build.Tasks.Workloads}
• HarvesterToolTask _{Microsoft.DotNet.Build.Tasks.Workloads}
• WixToolTask _{Microsoft.DotNet.Build.Tasks.Workloads}
• GetCMakeArtifactsFromFileApi _{Microsoft.DotNet.CMake.Sdk}
• GenAPITask _{Microsoft.DotNet.GenAPI}
• GenPartialFacadeSource _{Microsoft.DotNet.GenFacades}
• NotSupportedAssemblyGenerator _{Microsoft.DotNet.GenFacades}
• CreateXHarnessAndroidWorkItems _{Microsoft.DotNet.Helix}
• ReplacePackageParts _{Microsoft.DotNet.NuGetRepack}
• UpdatePackageVersionTask _{Microsoft.DotNet.NuGetRepack}
• GetCompatiblePackageTargetFrameworks _{Microsoft.DotNet.PackageTesting}
• VerifyClosure _{Microsoft.DotNet.PackageTesting}
• VerifyTypes _{Microsoft.DotNet.PackageTesting}
• CreateFrameworkListFile _{Microsoft.DotNet.SharedFramework.Sdk}
• GeneratePlatformManifestEntriesFromFileList _{Microsoft.DotNet.SharedFramework.Sdk}
• GeneratePlatformManifestEntriesFromTemplate _{Microsoft.DotNet.SharedFramework.Sdk}
• GenerateSharedFrameworkDepsFile _{Microsoft.DotNet.SharedFramework.Sdk}
• ValidateFileVersions _{Microsoft.DotNet.SharedFramework.Sdk}
• SignCheckTask _{Microsoft.DotNet.SignCheckTask}
• SignToolTask _{Microsoft.DotNet.SignTool}
• GenerateSwaggerCode _{Microsoft.DotNet.SwaggerGenerator}
• EnsureAllResourcesTranslated _{Microsoft.DotNet.XliffTasks}
• GatherTranslatedSource _{Microsoft.DotNet.XliffTasks}
• GatherXlf _{Microsoft.DotNet.XliffTasks}
• SortXlf _{Microsoft.DotNet.XliffTasks}
• TransformTemplates _{Microsoft.DotNet.XliffTasks}
• TranslateSource _{Microsoft.DotNet.XliffTasks}
• UpdateXlf _{Microsoft.DotNet.XliffTasks}

Suggested next steps

1. Route LocateDotNet and PackageIndex through TaskEnvironment (unblocks bucket A + B in one change each).
2. Replace PackageIndex's static JsonSerializer s_serializer - Newtonsoft.Json.JsonSerializer is not thread-safe.
3. Helix/AzDO and Feed/publishing tasks (buckets C + D) run in publish legs, not the hot inner build - lowest priority.
4. Once TaskEnvironment is available in the pinned MSBuild version, re-run the analyzer and take a second wave.

[ViktorHofer]

Status: PR open — #17381

133 of 136 tasks migrated (98%). The remaining 3 are tracked below.

What changed vs. the original plan

A few things turned out differently than assumed when this issue was filed:

1. MSBuild 18.8.2 is required, not 18.4.0.
18.4.0 is the first version exposing [MSBuildMultiThreadableTask] and IMultiThreadableTask, but Microsoft.Build.Utilities.Core 18.4.0 does not yet have ToolTask.TaskEnvironment. That lands in 18.8.2.

2. Both the attribute and the interface are needed — they are separate signals.

Signal	Effect	Read by
[MSBuildMultiThreadableTask]	routing: skip the sidecar TaskHost	TaskRouter.NeedsTaskHostInMultiThreadedMode
IMultiThreadableTask	TaskEnvironment injection	TaskExecutionHost.cs

The spec implies TaskRouter consults the interface, but on main it is literally return !HasMultiThreadableTaskAttribute(taskType);. Filed as dotnet/msbuild#14779. Conversely, declaring the TaskEnvironment property without implementing the interface silently leaves it as TaskEnvironment.Fallback — an easy and invisible mistake, which I hit on 26 classes before catching it.

Also note Inherited = false on the attribute, so it must go on each concrete class; the interface is inherited, so a base class (ValidationTask, XlfTask) covers its derivations for the property.

3. MSBuildTask0003 is a correctness rule, not advisory. I initially treated the File.*/Directory.* diagnostics as noise. They are not: MSBuild's own Copy task routes every relative path through TaskEnvironment.GetAbsolutePath. In multi-threaded mode there is no per-project current directory, so any relative path resolved via Path.GetFullPath/File.* silently binds to the wrong directory.

4. The BuildTask/ILog base class was deleted. It only wrapped Microsoft.Build.Utilities.Task and TaskLoggingHelper, and standing between tasks and the MSBuild base class meant every derived task needed its own TaskEnvironment plumbing anyway. Removing it is the bulk of the diff.

Two things worth knowing before doing this elsewhere

• Incremental builds skip up-to-date projects, so the analyzer does not re-run. An incremental dotnet build will happily report "0 warnings" while dozens of diagnostics are outstanding. --no-incremental is mandatory for an authoritative count. This cost me several false "done" signals.
• The most effective technique is retyping shared helpers, not fixing call sites. The analyzer treats AbsolutePath- and ITaskItem-typed arguments as safe, so changing one helper's path parameter to AbsolutePath silences both the direct MSBuildTask0003 and every transitive MSBuildTask0005 chain running through it. Chasing individual call sites is much slower.
• Do not trust the code fixer. It wraps the flagged call's return value instead of its path argument when the call is nested as an argument (producing CS1503), emits an instance TaskEnvironment reference inside static methods (CS0120), and never wraps a second path argument (File.Copy(src, dest) only gets src). Filed as TaskAnalyzer: MSBuildTask0003 code fix produces non-compiling code for nested calls and static methods msbuild#14780.

Analyzer wiring gotcha

GlobalAnalyzerConfigFiles and AnalyzerConfigFiles do not work from a Directory.Build.targets-imported file — Microsoft.Managed.Core.targets folds them at evaluation time. Use EditorConfigFiles. Diagnose with dotnet build -v:normal and grep for /analyzerconfig:.

Bugs found and fixed along the way

• TargetFrameworkResolver.CreateOrGet used an unsynchronized static readonly Dictionary as a cache — a real data race under any concurrent access, not just -mt. Now a ConcurrentDictionary.
• PackageIndex shared a static Newtonsoft JsonSerializer instance, which is not thread-safe.
• NuGetUtility.GetAllVersionsForPackageId resolved against Directory.GetCurrentDirectory().

Remaining work (3 tasks)

Task	Package	Blocker
SignToolTask	Microsoft.DotNet.SignTool	BatchSignUtil, ZipData, Configuration, VerifySignatures, ContentUtil (29 files) resolve paths against the CWD; Telemetry reads env vars directly
CreateVisualStudioWorkload	Microsoft.DotNet.Build.Tasks.Workloads	WorkloadPackageBase, MsiBase, SwixProject, EmbeddedTemplates, Utils (62 files), same issue
CreateVisualStudioWorkloadSet	Microsoft.DotNet.Build.Tasks.Workloads	shares the above helpers

Each carries a TODO referencing this issue, with MSBuildTask0005 suppressed at the entry point. All three run once per build, so leaving them TaskHost-routed costs effectively nothing — this is deliberately deferred, not forgotten.

Also worth a follow-up: TaskEnvironment has no temp-directory member, so Path.GetTempPath() has to be suppressed (4 sites here). Might be worth raising with the MSBuild team.

[ViktorHofer]

CI findings: two bug classes the migration introduced

The first VMR validation run surfaced two distinct failure classes. Both are fixed in the latest push. Recording them here because they generalize to anyone doing this migration.

1. TaskEnvironment.GetAbsolutePath throws on null/empty

File.Exists(null) and Directory.Exists(null) return false. GetAbsolutePath(null) throws ArgumentNullException. So mechanically wrapping those two calls turns an optional, unset task property into a hard failure:

// before: false when the optional property was never set
if (File.Exists(ProdConBuildManifestFile))
// after: ArgumentNullException
if (File.Exists(TaskEnvironment.GetAbsolutePath(ProdConBuildManifestFile)))

This took out every source-build leg. File.Exists/Directory.Exists are the only two BCL path APIs with this tolerance — File.ReadAllText, Directory.GetFiles, File.Copy, new FileInfo, ZipFile.*, XDocument.Load all already threw on null, so the regression surface is exactly those two. I audited all 292 GetAbsolutePath call sites the migration added, cross-referenced each argument against [Required], and fixed the 6 fed by optional properties.

Worth flagging for everyone doing this migration: the failing stack came through MultiProcessTaskEnvironmentDriver on legs running without -mt. TaskEnvironment is injected in normal builds too, so this is a general correctness issue, not a multi-threading-only one.

2. Real concurrency bugs that the analyzer cannot see

Microsoft.Build.TaskAuthoring.Analyzer checks path/env/process API usage. It does not check thread-safety of shared mutable state, so a clean analyzer run is not evidence a task is thread-safe. Three genuine races got through:

• TargetFrameworkResolver — a single resolver is shared by every task using the same runtime graph. NuGet's ManagedCodeConventions and PatternSet populate internal non-concurrent caches while matching, so concurrent GetNearest calls threw Operations that change non-concurrent collections must have exclusive access out of ContentItemCollection.FindBestItemGroup. This hit ~15 runtime libraries projects at once. Fixed by serializing GetNearest (cheap relative to rebuilding the conventions per thread).
• GetPackageDescription.s_descriptionCache — unsynchronized static Dictionary read and written from Execute. Now a ConcurrentDictionary.
• GetCompatiblePackageTargetFrameworks — static List/Dictionary mutated by Initialize from Execute. Made instance state, which additionally fixes duplicate accumulation across repeated invocations on a reused node.

I then swept every assembly containing an annotated task for mutable static collections. The remaining hits are safe: PublishingConstants and the s_*Extensions arrays are immutable config, Language._languages is populated in a static ctor, EmbeddedTemplates is unreachable from the annotated Workloads tasks, and PackageIndex.s_indexCache is fine because the only task that mutates an index (UpdatePackageIndex) uses the uncached single-file Load overload.

Takeaway: annotating a task asserts thread-safety, and the analyzer only covers part of that. Static caches handing out shared instances need a manual pass.

[ViktorHofer]

Keeping Arcade migrated once the migration is done

Worth recording how regression protection should work, because the answer is deliberately not something Arcade builds itself.

The gap. All 11 shipping Microsoft.Build.TaskAuthoring.Analyzer rules (MSBuildTask0001-0011, verified by reflecting over SupportedDiagnostics) are either conditioned on [MSBuildMultiThreadableTask] already being present, or only flag unsafe API usage. Under msbuild_task_analyzer.scope = multithreadable_only — what Arcade uses today — an unannotated type is skipped entirely. Under scope = all, a new task's banned-API usage is reported, but a task that happens to use no banned API is never told to opt in.

So a task added after this migration lands is simply never mentioned. And the consequence is silent: TaskRouter.NeedsTaskHostInMultiThreadedMode routes unannotated tasks to an out-of-proc sidecar TaskHost, which is not an error and produces no warning. The build succeeds, just more slowly. For scale, in the dotnet/dotnet runtime inner-repo comparison that motivated this work, unannotated tasks drove TaskHost invocations from 849 to 14,125 and added ~98s of net task time.

Why not a local check. A reflection-based unit test over the built task assemblies asserting every concrete ITask carries the attribute would work, but it is the wrong place to solve it. Arcade ships the SDK that essentially every .NET repo consumes, and every one of those repos will need the identical guarantee once it migrates. Solving it here means the next several dozen repos each reinvent it, each with a different opinion about test tasks, abstract bases and generated code — and each reporting at test time rather than in the editor.

The fix belongs upstream, and is now proposed as dotnet/msbuild#14787: a new MSBuildTask0012, disabled by default, that flags concrete task types which do not opt into multithreaded execution, enabled through a third msbuild_task_analyzer.scope value (require_multithreadable) so a repo that has finished migrating flips one line in one .globalconfig. It also proposes a code fix that adds the attribute, the interface and the TaskEnvironment property, after which the existing rules take over and report whatever is genuinely unsafe.

Once that ships, Arcade's action is a one-line change to eng/MultiThreadableTaskAnalyzer.globalconfig — and, more usefully, Arcade is in a position to ship that setting to consuming repos through the SDK.

Until then the remaining exposure is small and bounded: eng/BuildTask.targets already wires the analyzer into all 23 build-task projects, so any new task that touches paths, the environment or process state is still reported. The uncovered case is narrow — a genuinely state-free new task that nobody remembers to annotate — and costs performance rather than correctness.

Also filed from this migration: dotnet/msbuild#14783, #14784, #14785 (analyzer gaps), and #14779, #14780 (routing doc mismatch and a broken code fix). All are now linked under the analyzer epic dotnet/msbuild#14772.