Add App Host Support for MSBuild by YuliiaKovalova · Pull Request #13175 · dotnet/msbuild

YuliiaKovalova · 2026-02-02T13:10:09Z

Part 1 of: #12995

Context

This PR implements App Host support for MSBuild, enabling MSBuild to run as a native executable (MSBuild.exe or MSBuild on Unix) instead of dotnet + MSBuild.dll

Changes Made

Added DotnetHostEnvironmentHelper class to manage DOTNET_ROOT environment variables

When MSBuild runs as an app host, child processes (worker nodes and task hosts) need DOTNET_ROOT set to locate the .NET runtime
These variables are cleared after startup so they don't leak to tools executed by child processes
Only clears DOTNET_ROOT variants if they weren't present in the original build process environment

Node Launcher Updates

Updated NodeLauncher to pass environment overrides when launching worker nodes
NodeLaunchData record struct with EnvironmentOverrides support
Implemented BuildEnvironmentBlock for Windows process creation with custom environment
NodeProviderOutOfProc and NodeProviderOutOfProcTaskHost now provide DOTNET_ROOT overrides

Notes

Backward compatible: Falls back to dotnet MSBuild.dll if app host doesn't exist (older SDKs)
Uses ProcessStartInfo.Environment for thread-safe, scoped environment overrides
The SDK can continue using dotnet MSBuild.dll or opt to use the new app host—this is not a forced change

YuliiaKovalova · 2026-02-03T15:38:53Z

@copilot could you please move all $(LatestDotNetCoreForMSBuild) related changes to a separate PR?

Copilot · 2026-02-03T15:39:04Z

@YuliiaKovalova I've opened a new pull request, #13189, to work on those changes. Once the pull request is ready, I'll request review from you.

src/Shared/DotnetHostEnvironmentHelper.cs

…t_support

…otnet/msbuild into dev/ykovalova/app_host_support

src/MSBuild/MSBuild.csproj

…otnet/msbuild into dev/ykovalova/app_host_support

YuliiaKovalova · 2026-02-06T15:11:28Z

src/MSBuild/CommandLine/CommandLineParser.cs

-                exeName += msbuildExtn;
-            }
-
            fullCommandLine = $"'{string.Join(" ", commandLineArgs)}'";


it looks like it's needed with app host support, because for NET now it attaches:

MSBuild.exe.dll

src/Build/Resources/Constants.cs

YuliiaKovalova · 2026-02-11T17:18:32Z

src/MSBuild/MSBuild.csproj

+
+  <Target Name="CopyDllConfigToAppHostConfig" AfterTargets="_CopyAppConfigFile" Condition="'$(TargetFrameworkIdentifier)' != '.NETFramework'">    
+    <Copy SourceFiles="$(OutputPath)$(AssemblyName).dll.config" DestinationFiles="$(OutputPath)$(AssemblyName)$(_AppHostExtension).config" SkipUnchangedFiles="true" Condition="Exists('$(OutputPath)$(AssemblyName).dll.config')" />
+    <Copy SourceFiles="$(OutputPath)$(AssemblyName).dll.config" DestinationFiles="$(ArtifactsDir)bin\Microsoft.Build.Engine.UnitTests\$(Configuration)\$(TargetFramework)\$(AssemblyName)$(_AppHostExtension).config" SkipUnchangedFiles="true" Condition="Exists('$(OutputPath)$(AssemblyName).dll.config')" />


@rainersigwald ptal

YuliiaKovalova · 2026-02-11T17:23:49Z

src/Build.UnitTests/NetTaskHost_E2E_Tests.cs

+            using TestEnvironment env = TestEnvironment.Create(_output, setupDotnetHostPath: true);
+            var coreDirectory = Path.Combine(RunnerUtilities.BootstrapRootPath, "core");
+            env.SetEnvironmentVariable("PATH", $"{coreDirectory}{Path.PathSeparator}{Environment.GetEnvironmentVariable("PATH")}");
+            env.SetEnvironmentVariable("DOTNET_ROOT", coreDirectory);


@rainersigwald ptal

Copilot

Pull request overview

This PR adds initial “app host” support so MSBuild can run as a native executable (MSBuild[.exe]) and ensures spawned nodes/task hosts can locate the .NET runtime by applying scoped DOTNET_ROOT* environment overrides (and clearing bootstrap-only variables afterward).

Changes:

Introduces DotnetHostEnvironmentHelper and threads environment overrides through node/task-host launching (new NodeLaunchData path).
Updates build environment detection and test infrastructure to prefer/use the MSBuild app host where available.
Expands/adjusts test coverage for app host behavior and updates resources/packaging to include app-host config where needed.

Reviewed changes

Copilot reviewed 63 out of 64 changed files in this pull request and generated 27 comments.

Show a summary per file

File	Description
src/Utilities.UnitTests/ToolLocationHelper_Tests.cs	Updates tests to use `Constants.MSBuildExecutableName` instead of hardcoded names.
src/UnitTests.Shared/TestEnvironment.cs	Adds `setupDotnetHostPath` option to configure `DOTNET_HOST_PATH` for app-host-related tests.
src/UnitTests.Shared/RunnerUtilities.cs	Simplifies MSBuild invocation to run the resolved MSBuild entrypoint directly; updates bootstrap execution path.
src/UnitTests.Shared/BootstrapLocationAttribute.cs	Clarifies docs about resolving MSBuild entrypoint across OS/runtime.
src/Tasks.UnitTests/RoslynCodeTaskFactory_Tests.cs	Ensures tests set up `DOTNET_HOST_PATH` when out-of-proc is used.
src/Tasks.UnitTests/CreateItem_Tests.cs	Enables `setupDotnetHostPath` for out-of-proc TaskHost tests.
src/Shared/UnitTests/TestAssemblyInfo.cs	Uses constants for `DOTNET_HOST_PATH` env var name.
src/Shared/INodePacket.cs	Bumps packet version to 3 to account for new app-host-related behavior.
src/Shared/FrameworkLocationHelper.cs	Uses `Constants.MSBuildExecutableName` when validating framework install contents.
src/Shared/EnvironmentUtilities.cs	Tweaks `ProcessPath` caching implementation (and related comment).
src/Shared/BuildEnvironmentHelper.cs	Updates environment discovery and “running in MSBuild.exe” detection for app host scenarios.
src/MSBuildTaskHost/MSBuildTaskHost.csproj	Links `Constants.cs` to keep TaskHost consistent with new constants usage.
src/MSBuild/XMake.cs	Adjusts args handling to use native process path on .NET.
src/MSBuild/OutOfProcTaskHostNode.cs	Clears bootstrap-only `DOTNET_ROOT*` variables after applying build environment in task host.
src/MSBuild/MSBuild.csproj	Adds copying of `.dll.config` to apphost `.config`; conditions XSD inclusion for .NET Framework only.
src/MSBuild/CommandLine/CommandLineParser.cs	Removes extension-appending logic; relies on `CurrentMSBuildExePath` directly.
src/MSBuild.UnitTests/XMake_Tests.cs	Updates tests to run MSBuild directly and use `Constants.MSBuildExecutableName`.
src/Framework/TaskHostParameters.cs	Renames MSBuild path property to reflect executable/apphost usage and updates merge logic.
src/Framework/DotnetHostEnvironmentHelper.cs	New helper to create/apply env overrides and clear bootstrap-only `DOTNET_ROOT*`.
src/Framework/BinaryTranslator.cs	Updates serialization variable naming for `TaskHostParameters` MSBuild path.
src/Build/Resources/xlf/Strings.zh-Hant.xlf	Adds new resource id; updates some existing targets’ state/content.
src/Build/Resources/xlf/Strings.zh-Hans.xlf	Adds new resource id; updates some existing targets’ state/content.
src/Build/Resources/xlf/Strings.tr.xlf	Adds new resource id; updates some existing targets’ state/content.
src/Build/Resources/xlf/Strings.ru.xlf	Adds new resource id; updates some existing targets’ state/content.
src/Build/Resources/xlf/Strings.pt-BR.xlf	Adds new resource id; updates some existing targets’ state/content.
src/Build/Resources/xlf/Strings.pl.xlf	Adds new resource id; updates some existing targets’ state/content.
src/Build/Resources/xlf/Strings.ko.xlf	Adds new resource id; updates some existing targets’ state/content.
src/Build/Resources/xlf/Strings.ja.xlf	Adds new resource id; updates some existing targets’ state/content.
src/Build/Resources/xlf/Strings.it.xlf	Adds new resource id; updates some existing targets’ state/content.
src/Build/Resources/xlf/Strings.fr.xlf	Adds new resource id; updates some existing targets’ state/content.
src/Build/Resources/xlf/Strings.es.xlf	Adds new resource id; updates some existing targets’ state/content.
src/Build/Resources/xlf/Strings.de.xlf	Adds new resource id; updates some existing targets’ state/content.
src/Build/Resources/xlf/Strings.cs.xlf	Adds new resource id; updates some existing targets’ state/content.
src/Build/Resources/Strings.resx	Adds `DotnetHostPathNotSet` resource.
src/Build/Instance/TaskFactories/TaskHostTask.cs	Improves error reporting on abort and generalizes exception handling.
src/Build/Instance/TaskFactories/AssemblyTaskFactory.cs	Uses updated TaskHostParameters MSBuild path property.
src/Build/Evaluation/IntrinsicFunctions.cs	Uses updated TaskHostParameters MSBuild path property.
src/Build/BackEnd/Node/OutOfProcNode.cs	Clears bootstrap-only `DOTNET_ROOT*` after applying build environment in worker node.
src/Build/BackEnd/Node/NativeMethods.cs	Adds `CREATE_UNICODE_ENVIRONMENT` constant for Windows process creation.
src/Build/BackEnd/Components/SdkResolution/SdkResolverLoader.cs	Refines fallback logic based on whether running in MSBuild app host.
src/Build/BackEnd/Components/Communications/RarNodeLauncher.cs	Uses `NodeLaunchData` overload to launch RAR node.
src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcTaskHost.cs	Adds app-host-or-dotnet fallback and environment override support for task hosts.
src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcBase.cs	Threads `NodeLaunchData` through node acquisition; updates error details and doc comment.
src/Build/BackEnd/Components/Communications/NodeProviderOutOfProc.cs	Uses `NodeLaunchData` and applies DOTNET_ROOT overrides when launching worker nodes.
src/Build/BackEnd/Components/Communications/NodeLauncher.cs	Adds Windows environment-block construction and per-process environment overrides.
src/Build/BackEnd/Components/Communications/INodeLauncher.cs	Introduces `NodeLaunchData` record struct and updates launcher API.
src/Build/BackEnd/Components/Communications/DetouredNodeLauncher.cs	Adapts detoured launching to `NodeLaunchData` and applies environment overrides.
src/Build/BackEnd/Client/MSBuildClient.cs	Updates server launch to use `NodeLaunchData`.
src/Build.UnitTests/TestComparers/TaskRegistryComparers.cs	Updates comparer to use new TaskHostParameters MSBuild path property.
src/Build.UnitTests/TaskHostFactoryLifecycle_E2E_Tests.cs	Enables `setupDotnetHostPath` when exercising TaskHostFactory flows.
src/Build.UnitTests/ProjectCache/ProjectCacheTests.cs	Removes now-unneeded internal using.
src/Build.UnitTests/NetTaskHost_E2E_Tests.cs	Adds explicit tests for app-host usage and fallback behavior.
src/Build.UnitTests/Instance/TaskItem_Tests.cs	Ensures `DOTNET_HOST_PATH` is set for TaskHostFactory-related escaping tests.
src/Build.UnitTests/EscapingInProjects_Tests.cs	Ensures `DOTNET_HOST_PATH` is set for task-host scenarios.
src/Build.UnitTests/Definition/ToolsetRegistryReader_Tests.cs	Adds `Microsoft.Build.Framework` using for constants.
src/Build.UnitTests/Definition/ToolsetReader_Tests.cs	Adds `Microsoft.Build.Framework` using for constants.
src/Build.UnitTests/BuildEnvironmentHelper_Tests.cs	Updates tests to expect `Constants.MSBuildExecutableName`.
src/Build.UnitTests/BackEnd/TaskRouter_IntegrationTests.cs	Enables `setupDotnetHostPath` for integration tests that spawn nodes.
src/Build.UnitTests/BackEnd/TaskHostFactory_Tests.cs	Enables `setupDotnetHostPath` for TaskHostFactory tests.
src/Build.UnitTests/BackEnd/SdkResolverLoader_Tests.cs	Updates commentary to match new app-host-based fallback criteria.
src/Build.UnitTests/BackEnd/BuildManager_Tests.cs	Wraps one test in `TestEnvironment` setup for app-host correctness.
src/Build.UnitTests/BackEnd/AppHostSupport_Tests.cs	New unit tests for DOTNET_ROOT override/clearing behavior.
eng/MicrobuildTest.ps1	Removes legacy Microbuild validation script.
azure-pipelines/.vsts-dotnet-build-jobs.yml	Trims whitespace in a script block (format-only change).

src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcBase.cs

src/Framework/DotnetHostEnvironmentHelper.cs

src/Build.UnitTests/BackEnd/BuildManager_Tests.cs

src/Build/Resources/xlf/Strings.de.xlf

src/Build/Resources/xlf/Strings.fr.xlf

src/Framework/DotnetHostEnvironmentHelper.cs

src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcBase.cs

src/Build.UnitTests/BackEnd/BuildManager_Tests.cs

src/Build.UnitTests/EscapingInProjects_Tests.cs

.vsts-dotnet-ci.yml

eng/Signing.props

rainersigwald · 2026-02-12T18:55:41Z

src/UnitTests.Shared/TestEnvironment.cs

        /// </param>
+        /// <param name="setupDotnetHostPath">
+        /// When true, configures .NET environment variable DOTNET_HOST_PATH to point to the bootstrap core directory.
+        /// It's necessary for the tests that rely on app host execution and need to find the correct .NET runtime.


Does the mainline support stuff (using the .NET SDK and getting the pointer that way) not work in our tests? Should it?

nope, it doesn't work when we point to bootstrapped app host during the test execution :(

src/UnitTests.Shared/BootstrapLocationAttribute.cs

src/UnitTests.Shared/RunnerUtilities.cs

.vsts-dotnet-ci.yml

rainersigwald · 2026-02-12T19:16:00Z

src/Tasks.UnitTests/RoslynCodeTaskFactory_Tests.cs

        public void InlineTaskWithAssemblyPlatformAgnostic(bool forceOutOfProc)
        {
-            using (TestEnvironment env = TestEnvironment.Create())
+            using (TestEnvironment env = TestEnvironment.Create(setupDotnetHostPath: true))


I don't think I understand why we need the setupDotnetHostPath: true here. That feels wrong and bad; can we avoid it?

it needs it due to this change -

Previously, in net it used dotnet.exe +msbuild.dll resolved , now it knows only path to app host

should that method learn to pass the necessary environment, instead?

rainersigwald · 2026-02-12T19:45:29Z

src/Framework/DotnetHostEnvironmentHelper.cs

+        /// </summary>
+        /// <param name="dotnetHostPath">Path to the dotnet executable.</param>
+        /// <returns>Dictionary of environment variable overrides, or null if dotnetHostPath is empty.</returns>
+        internal static IDictionary<string, string>? CreateDotnetRootEnvironmentOverrides(string? dotnetHostPath = null)


How often is this called? Worth stashing the last value so we don't create a bunch of transient dictionaries?

added caching

src/Framework/DotnetHostEnvironmentHelper.cs

src/Build.UnitTests/TaskHostFactoryLifecycle_E2E_Tests.cs

src/Build.UnitTests/EscapingInProjects_Tests.cs

rainersigwald · 2026-02-13T22:01:32Z

src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcBase.cs

                msbuildLocation = Constants.MSBuildExecutableName;
            }

            var expectedProcessName = Path.GetFileNameWithoutExtension(CurrentHost.GetCurrentHost() ?? msbuildLocation);


This needs to be updated. As is right now, node reuse isn't working. To test, build a multi-project solution with -m -nr:true and watch the MSBuild.exe process accumulate.

YuliiaKovalova added 5 commits February 2, 2026 14:04

add apphost support

531d264

add e2e tests

2e623d0

fix the tests

072cb7b

avoid target framework hardcoding in codebase

d023200

exclude adding source build

96159c0

YuliiaKovalova self-assigned this Feb 2, 2026

Copilot AI mentioned this pull request Feb 3, 2026

Add $(LatestDotNetCoreForMSBuild) infrastructure for centralized framework targeting #13189

Merged

YuliiaKovalova added 2 commits February 3, 2026 17:45

fix the test

d2d33a6

revert the condition

95f4da2

YuliiaKovalova force-pushed the dev/ykovalova/app_host_support branch from 90e06c7 to 95f4da2 Compare February 3, 2026 16:46

fix the tests

68880eb

kasperk81 reviewed Feb 4, 2026

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YuliiaKovalova and others added 8 commits February 4, 2026 10:42

Merge branch 'main' into dev/ykovalova/app_host_support

ad34e84

Merge remote-tracking branch 'origin/main' into dev/ykovalova/app_hos…

16ac07f

…t_support

Merge branch 'dev/ykovalova/app_host_support' of https://github.com/d…

5b0b950

…otnet/msbuild into dev/ykovalova/app_host_support

fix review commet

0fd74ed

fix the issue

1e5e8c3

undo changes in tests

fc15e94

undo more change

d2004c9

fix the tests

0d3288f

YuliiaKovalova commented Feb 4, 2026

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YuliiaKovalova and others added 6 commits February 4, 2026 17:03

more test fixes

011735e

more test fixes

4daad92

fix a typo

fb8eb1e

Update cibuild_bootstrapped_msbuild.ps1

72bbf09

update build env helper

e9a28f6

Merge branch 'dev/ykovalova/app_host_support' of https://github.com/d…

e517aa0

…otnet/msbuild into dev/ykovalova/app_host_support

YuliiaKovalova commented Feb 6, 2026

View reviewed changes

fix the tests

60c2d79

YuliiaKovalova force-pushed the dev/ykovalova/app_host_support branch 3 times, most recently from 7e349a2 to fd3905f Compare February 10, 2026 20:11

fix the tests and avoid hardcoded app name

cbe4fff

YuliiaKovalova force-pushed the dev/ykovalova/app_host_support branch 2 times, most recently from 15673a9 to 3641a4b Compare February 11, 2026 13:14

YuliiaKovalova commented Feb 11, 2026

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YuliiaKovalova commented Feb 11, 2026

View reviewed changes

more changes around tests and logic

b9d8d49

YuliiaKovalova force-pushed the dev/ykovalova/app_host_support branch from b351ea2 to b9d8d49 Compare February 11, 2026 17:55

merge

07e999a

YuliiaKovalova marked this pull request as ready for review February 11, 2026 18:42

YuliiaKovalova requested a review from a team as a code owner February 11, 2026 18:42

Copilot AI review requested due to automatic review settings February 11, 2026 18:42

Copilot started reviewing on behalf of YuliiaKovalova February 11, 2026 18:42 View session

Copilot AI reviewed Feb 11, 2026

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fix the last tests in BuildManager

0d7153f

YuliiaKovalova commented Feb 11, 2026

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YuliiaKovalova force-pushed the dev/ykovalova/app_host_support branch from 44df75a to cada425 Compare February 12, 2026 14:28

fix the comments

7433663

YuliiaKovalova force-pushed the dev/ykovalova/app_host_support branch from 2e0df7c to 7433663 Compare February 12, 2026 17:49

merge

ffbbf48

rainersigwald reviewed Feb 12, 2026

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YuliiaKovalova added 4 commits February 13, 2026 09:48

fix review comment

8b1070e

fix nullability

c992bc9

try to revert condition

6f7ce52

undo extra changes in tests (?)

34b7f33

rainersigwald reviewed Feb 13, 2026

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Context

Changes Made

Added DotnetHostEnvironmentHelper class to manage DOTNET_ROOT environment variables

Node Launcher Updates

Notes

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YuliiaKovalova commented Feb 2, 2026 •

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