[work-in-progress][API] RuntimeFeature.IsMultithreadingSupported by pavelsavara · Pull Request #124603 · dotnet/runtime

pavelsavara · 2026-02-19T16:30:03Z

Fixes #77541
Continuation of #123329

Copilot

Pull request overview

This pull request introduces a new public API RuntimeFeature.IsMultithreadingSupported to replace the internal Thread.IsSingleThreaded property. This is part of API proposal #77541 to provide a public feature flag for determining if multithreading is supported on the current platform (particularly relevant for single-threaded WebAssembly environments).

Changes:

Introduces public RuntimeFeature.IsMultithreadingSupported property and internal ThrowIfMultithreadingIsNotSupported() method
Removes internal Thread.IsSingleThreaded and Thread.ThrowIfSingleThreaded()
Renames PlatformDetection.IsThreadingSupported → IsMultithreadingSupported and IsNotWasmThreadingSupported → IsNotMultithreadingSupported
Updates hundreds of test files to use the new naming convention
Configures the feature switch in MSBuild targets for WebAssembly

Reviewed changes

Copilot reviewed 272 out of 272 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
System.Runtime.cs	Adds public API surface for IsMultithreadingSupported
RuntimeFeature.cs	Implements new property and internal throw helper
Thread.cs	Removes old IsSingleThreaded API
PlatformDetection.cs (multiple)	Updates to use RuntimeFeature instead of environment variable
Environment.cs	Uses new API for ProcessorCount logic
Microsoft.NET.Sdk.WebAssembly.Browser.targets	Configures feature switch for WASM
tests.browser.targets	Removes IsBrowserThreadingSupported environment variable
200+ test files	Updates ConditionalFact/Theory attributes to use new naming

Copilot

Pull request overview

Copilot reviewed 273 out of 273 changed files in this pull request and generated no new comments.

jkotas · 2026-02-19T22:25:12Z

IsThreadingSupported -> IsMultithreadingSupported

Can you do 1 boring prep PR that has just this rename and nothing else, and do the rest in this PR? It is hard to find any interesting changes in 250+ changed files.

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeFeature.cs

Copilot

Pull request overview

Copilot reviewed 273 out of 273 changed files in this pull request and generated no new comments.

pavelsavara · 2026-02-20T08:09:38Z

Can you do 1 boring prep PR that has just this rename and nothing else, and do the rest in this PR? It is hard to find any interesting changes in 250+ changed files.

For now this is a draft on which I'm trying to make it compile. There is change of namespace and the new field doesn't exist in Net10/11.0.0-preview.1.26104.118 or lower (for which we build some tests).

Once I have passing build, I would try to make it easier to review. Perhaps this could be rebased as 3 commits ?

Copilot

Pull request overview

Copilot reviewed 292 out of 293 changed files in this pull request and generated 3 comments.

src/libraries/Common/tests/TestUtilities/TestUtilities.csproj

src/tests/Common/CoreCLRTestLibrary/PlatformDetection.cs

src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs

Copilot

Pull request overview

Copilot reviewed 292 out of 293 changed files in this pull request and generated 5 comments.

src/mono/wasm/threads.md

src/tests/Common/CoreCLRTestLibrary/PlatformDetection.cs

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeFeature.cs

...nuget/Microsoft.NET.Sdk.WebAssembly.Pack/build/Microsoft.NET.Sdk.WebAssembly.Browser.targets

src/mono/wasm/threads.md

Copilot

Pull request overview

Copilot reviewed 292 out of 293 changed files in this pull request and generated 3 comments.

src/libraries/System.Runtime/tests/System.Globalization.Tests/CultureInfo/CultureInfoNames.cs

src/libraries/System.Net.WebSockets.Client/tests/SendReceiveTest.cs

...ging.Console/tests/Microsoft.Extensions.Logging.Console.Tests/SimpleConsoleFormatterTests.cs

Copilot

Pull request overview

Copilot reviewed 34 out of 34 changed files in this pull request and generated 2 comments.

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeFeature.cs

Copilot

Pull request overview

Copilot reviewed 34 out of 34 changed files in this pull request and generated 2 comments.

Copilot · 2026-02-23T13:18:02Z

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeFeature.cs

+        /// <summary>
+        /// Gets a value that indicates whether the runtime supports multithreading, including
+        /// creating threads and using blocking synchronization primitives. This property
+        /// returns <see langword="false"/> on platforms that do not support multithreading,
+        /// such as browser and WASI.
+        /// </summary>
+        [UnsupportedOSPlatformGuard("browser")]
+        [UnsupportedOSPlatformGuard("wasi")]
+        [FeatureSwitchDefinition("System.Runtime.CompilerServices.RuntimeFeature.IsMultithreadingSupported")]
+        public static bool IsMultithreadingSupported { get; }
+            = AppContext.TryGetSwitch("System.Runtime.CompilerServices.RuntimeFeature.IsMultithreadingSupported", out bool isMultithreadingSupported) ? isMultithreadingSupported : true;
+
+        internal static void ThrowIfMultithreadingIsNotSupported()
+        {
+            if (!IsMultithreadingSupported)
+            {
+                throw new PlatformNotSupportedException();
+            }


IsMultithreadingSupported defaults to true when the AppContext switch isn't set, but the XML doc and platform guards state it returns false on unsupported platforms like browser/WASI. I couldn't find any non-browser wiring for this switch (only Browser.targets sets it), so on WASI (and any single-threaded host missing the config) this will incorrectly report true and may allow thread/blocking paths to run. Consider defaulting based on platform/feature defines (e.g., #if FEATURE_SINGLE_THREADED => false, otherwise check OperatingSystem.IsBrowser()/IsWasi() or ensure the host sets the switch for WASI as well).

Copilot · 2026-02-23T13:18:02Z

...tests/Loader/classloader/TypeInitialization/CircularCctors/CircularCctorTwoThreadsThreeCC.cs

+    [SkipOnPlatform(TestPlatforms.Browser, "Requires threads")]
    [Fact]


This test creates/joins threads; if WASI is also single-threaded (as implied by the new RuntimeFeature guards elsewhere), it should be skipped there as well. Consider skipping on TestPlatforms.Wasi (or using a multithreading-supported condition) to avoid runtime failures on WASI test runs.

Copilot

Pull request overview

Copilot reviewed 34 out of 34 changed files in this pull request and generated no new comments.

jkotas · 2026-02-23T16:04:46Z

src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs

-        public static bool IsMultithreadingSupported => (!IsWasi && !IsBrowser) || IsWasmThreadingSupported;
+
+        // TODO-WASM: https://github.com/dotnet/runtime/issues/124748
+        // this is compiled with 11.0.0-preview.1.26104.118\ref


Why is this compiled against stale references?

I would expect that the libraries tests are always built against the live references. Otherwise, we would not be able to add new public APIs and their tests in the same PR.

I don't know

This file compiles for both live .NET and .NET Standard. Maybe that's the problem?

Other places in this file deal with it by #if NET and implementing fallback for .NET Standard under #if !NET.

I saw nuget path in the binlog, I think it was not .NET Standard problem. I can try again ...

src/libraries/System.Private.CoreLib/src/System/Threading/SpinWait.cs

…Wait.cs Co-authored-by: Jan Kotas <jkotas@microsoft.com>

Copilot

Pull request overview

Copilot reviewed 34 out of 34 changed files in this pull request and generated no new comments.

pavelsavara added this to the 11.0.0 milestone Feb 19, 2026

pavelsavara self-assigned this Feb 19, 2026

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pavelsavara added area-System.Threading os-browser Browser variant of arch-wasm labels Feb 19, 2026

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pavelsavara force-pushed the IsMultithreadingSupported branch from 26ff2f4 to 7b0c965 Compare February 20, 2026 12:18

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build-analysis bot mentioned this pull request Feb 20, 2026

Wasm build terminated with exit code 137 #108299

Open

booring namespace

7e1a2fb

pavelsavara mentioned this pull request Feb 23, 2026

Adopt RuntimeFeature.IsMultithreadingSupported for PlatformDetection #124748

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pavelsavara added 2 commits February 23, 2026 12:03

tests

22c030f

interesting bits

9b8972d

pavelsavara force-pushed the IsMultithreadingSupported branch from 8904240 to 9b8972d Compare February 23, 2026 11:06

IsNotWasmThreadingSupported back

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pavelsavara added 2 commits February 23, 2026 13:24

add UnsupportedOSPlatformGuard

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fix

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pavelsavara added 2 commits February 23, 2026 15:07

more

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wasi

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fix

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Update src/libraries/System.Private.CoreLib/src/System/Threading/Spin…

90a4976

…Wait.cs Co-authored-by: Jan Kotas <jkotas@microsoft.com>

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		[SkipOnPlatform(TestPlatforms.Browser, "Requires threads")]
		[Fact]

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pavelsavara commented Feb 19, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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pavelsavara commented Feb 19, 2026 •

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