Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps by Copilot · Pull Request #130603 · dotnet/runtime

Copilot · 2026-07-13T08:10:08Z

main PR

Description

System.STAThreadAttribute was unnecessarily retained in trimmed single-threaded Mono apps (browser/WASI).

This PR now addresses both sides of the behavior:

src/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml keeps System.STAThreadAttribute rooted only when System.Runtime.CompilerServices.RuntimeFeature.IsMultithreadingSupported is true.
src/mono/mono/metadata/object.c was updated to tolerate the attribute type being absent by using a try-load path when checking for [STAThread] on entrypoint methods.

This preserves existing behavior on multithreaded Mono platforms while allowing single-threaded browser/WASI apps to trim the attribute safely.

Customer Impact

Without this fix, trimmed single-threaded Mono apps can retain unnecessary metadata, and WASI startup could fail when the attribute is trimmed.

Regression

No known product regression is introduced. The native startup path now handles missing System.STAThreadAttribute gracefully instead of assuming it is always present.

Testing

Baseline build: ./build.sh mono+libs
Targeted runtime build: ./build.sh mono.runtime -c Release
Targeted WASI/Mono build: ./build.sh -os wasi -subset mono+libs -c Debug
Parallel validation (code review + security scan) completed.

Risk

Low to medium. The change is narrowly scoped to STA attribute lookup in Mono startup and linker rooting configuration. Multithreaded platforms continue to root the type; single-threaded platforms can trim it, and runtime lookup now safely handles absence.

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>

MichalStrehovsky · 2026-07-13T09:04:08Z

How are we ending up needing these attribute in the first place? When I make a new console app (on Windows even!), and PublishTrimmed the app, this attribute trims just fine. Does Blazor place the attribute on Main for some reason? Or roots some other code that needs it? I think this is the core part that should be addressed.

pavelsavara · 2026-07-13T09:26:36Z

How are we ending up needing these attribute in the first place?

Great question, thanks

<type fullname="System.STAThreadAttribute" /> in

https://github.com/dotnet/runtime/blame/0ded4d28b4990cc4bdb6f1e44bafca3f22aa6eda/src/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml#L338

@copilot removing root cause is better alternative. Investigate if this type rooting is still necessary on other Mono/mobile platforms. If not, remove it from ILLink.Descriptors.xml

If it's still need, make it only explicitly used for those (MT) Mono platforms.

…tributes change Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>

Copilot · 2026-07-13T09:34:45Z

How are we ending up needing these attribute in the first place? When I make a new console app (on Windows even!), and PublishTrimmed the app, this attribute trims just fine. Does Blazor place the att...

Good catch — the attribute trims fine on CoreCLR because nothing roots it. On Mono it's the descriptor that keeps it alive: src/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml unconditionally roots System.STAThreadAttribute because the native runtime (object.c: prepare_thread_to_exec_main) looks it up via mono_class_load_from_name (which aborts if the type is missing) to set apartment state when launching an entry point through mono_runtime_run_main. Blazor WASM doesn't reach that path (the entry point is invoked directly), so the root was pure dead weight there. I've switched to fixing the root cause — see the updated commit.

Note

This comment was generated by GitHub Copilot.

How are we ending up needing these attribute in the first place?

Great question, thanks
...

Done in the latest commit. I confirmed the root is still needed on multithreaded Mono platforms (desktop/iOS/Android launch the entry point via mono_runtime_run_main → prepare_thread_to_exec_main, which calls mono_class_load_from_name for System.STAThreadAttribute and aborts if it's missing). Browser/wasi invoke the entry point directly and never hit that path, and apartment state is meaningless there.

So rather than removing the root outright, I gated it in ILLink.Descriptors.xml on System.Runtime.CompilerServices.RuntimeFeature.IsMultithreadingSupported (featurevalue="true" featuredefault="true") — kept for MT platforms, trimmed on single-threaded browser/wasi where that switch is false. I also reverted the earlier ILLink.LinkAttributes.Shared.xml change in favor of this root-cause fix.

Note

This comment was generated by GitHub Copilot.

pavelsavara · 2026-07-13T13:55:17Z

/ba-g CI failure is #130618

Copilot

Pull request overview

This PR attempts to reduce trimming roots for Mono single-threaded WebAssembly targets by making System.STAThreadAttribute no longer unconditionally rooted in Mono’s ILLink descriptors, instead gating its retention on RuntimeFeature.IsMultithreadingSupported.

Changes:

Removes the unconditional System.STAThreadAttribute root from the main System.Private.CoreLib descriptor list.
Re-adds System.STAThreadAttribute as a feature-gated root conditioned on System.Runtime.CompilerServices.RuntimeFeature.IsMultithreadingSupported == true.

Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>

MichalStrehovsky · 2026-07-13T22:08:34Z

+	<!-- STAThreadAttribute is read by the native runtime (object.c: prepare_thread_to_exec_main)
+	     when launching an entry point via mono_runtime_run_main to set the apartment state.
+	     On single-threaded runtimes this attribute can be trimmed: apartment state is meaningless
+	     there and the native lookup tolerates the type being absent. -->
+	<assembly fullname="System.Private.CoreLib" feature="System.Runtime.CompilerServices.RuntimeFeature.IsMultithreadingSupported" featurevalue="true" featuredefault="true">
+		<type fullname="System.STAThreadAttribute" />
+	</assembly>


If the runtime tolerates it now, why keep it?

For the windows which are not single-threaded. To have the apartment state set correctly.

-> CoInitializeEx()

runtime/src/mono/mono/metadata/threads.c

Lines 3905 to 3922 in 1baa904

mono_thread_init_apartment_state (void)

{

#ifdef HOST_WIN32

MonoInternalThread* thread = mono_thread_internal_current ();

/* Positive return value indicates success, either

* S_OK if this is first CoInitialize call, or

* S_FALSE if CoInitialize already called, but with same

* threading model. A negative value indicates failure,

* probably due to trying to change the threading model.

*/

if (CoInitializeEx(NULL, (thread->apartment_state == ThreadApartmentState_STA)

? COINIT_APARTMENTTHREADED

: COINIT_MULTITHREADED) < 0) {

thread->apartment_state = ThreadApartmentState_Unknown;

}

#endif

}

Or you are saying that the C# code would keep the attribute if it was needed ? And that we don't need to root is just for the VM sake ?

The only test I can find is src\mono\mono\tests\bug-79215.2.il which is never executed, it's a dead weight.
Since we are not running any workload on Windows/Mono I can take a risk. I'm not willing to add tests for this.

@steveisok @lewing thoughts ?

Copilot

Pull request overview

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

 			<!-- exception.c: mono_get_exception_stack_overflow / mono_exception_from_name -->
 			<method signature="System.Void .ctor()" />
 		</type>

-		<!-- object.c: mono_runtime_exec_main -->
-		<type fullname="System.STAThreadAttribute" />
-
 		<!-- domain.c: mono_defaults.string_class -->


 	MonoInternalThread* thread = mono_thread_internal_current ();
 	MonoCustomAttrInfo* cinfo;
+	MonoClass *sta_thread_attribute_class = mono_class_try_get_sta_thread_attribute_class ();
 	gboolean has_stathread_attribute;


Initial plan

9b35a53

Copilot AI self-assigned this Jul 13, 2026

Copilot AI review requested due to automatic review settings July 13, 2026 08:10

Copilot AI assigned pavelsavara Jul 13, 2026

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Copilot started work on behalf of pavelsavara July 13, 2026 08:10 View session

Copilot AI linked an issue Jul 13, 2026 that may be closed by this pull request

STAThreadAttribute is unnecessarily being left in a Blazor WASM default app #47861

Open

Trim STAThread/MTAThread attributes when multithreading is unsupported

5bc3ea6

Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot July 13, 2026 08:14

dotnet-policy-service Bot added the linkable-framework Issues associated with delivering a linker friendly framework label Jul 13, 2026

Copilot AI changed the title ~~[WIP] Remove STAThreadAttribute from Blazor WASM app~~ Trim STAThread/MTAThread attributes when multithreading is unsupported Jul 13, 2026

Copilot finished work on behalf of pavelsavara July 13, 2026 08:16

Copilot AI requested a review from pavelsavara July 13, 2026 08:16

pavelsavara added arch-wasm WebAssembly architecture size-reduction Issues impacting final app size primary for size sensitive workloads labels Jul 13, 2026

pavelsavara temporarily deployed to copilot-pat-pool July 13, 2026 08:41 — with GitHub Actions Inactive

pavelsavara added the area-System.Threading label Jul 13, 2026

pavelsavara temporarily deployed to copilot-pat-pool July 13, 2026 08:42 — with GitHub Actions Inactive

Copilot started work on behalf of pavelsavara July 13, 2026 09:27 View session

Gate Mono STAThreadAttribute rooting on multithreading; revert LinkAt…

de13be9

…tributes change Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot July 13, 2026 09:34

Copilot AI changed the title ~~Trim STAThread/MTAThread attributes when multithreading is unsupported~~ Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps Jul 13, 2026

Copilot finished work on behalf of pavelsavara July 13, 2026 09:36

build-analysis Bot mentioned this pull request Jul 13, 2026

Unable to pull image from mcr.microsoft.com #117164

Open

pavelsavara marked this pull request as ready for review July 13, 2026 13:54

pavelsavara requested a review from steveisok as a code owner July 13, 2026 13:54

Copilot AI review requested due to automatic review settings July 13, 2026 13:54

pavelsavara requested a review from vitek-karas as a code owner July 13, 2026 13:54

Copilot started reviewing on behalf of pavelsavara July 13, 2026 13:54 View session

Copilot AI reviewed Jul 13, 2026

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Comment thread src/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml Outdated

Copilot started work on behalf of pavelsavara July 13, 2026 15:15 View session

Handle missing STAThreadAttribute in Mono/WASI startup path

a714677

Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot July 13, 2026 15:40

Copilot AI requested a review from thaystg as a code owner July 13, 2026 15:40

Copilot finished work on behalf of pavelsavara July 13, 2026 15:41

build-analysis Bot mentioned this pull request Jul 13, 2026

Wasm.Build.NativeRebuild.Tests.ReferenceNewAssemblyRebuildTest.ReferenceNewAssembly failure: Expected changed file: driver-gen.c #130540

Closed

MichalStrehovsky reviewed Jul 13, 2026

View reviewed changes

remove STAThreadAttribute root

8af0562

Copilot AI review requested due to automatic review settings July 14, 2026 09:06

pavelsavara temporarily deployed to copilot-pat-pool July 14, 2026 09:06 — with GitHub Actions Inactive

Merge branch 'main' into copilot/remove-stathreadattribute-blazor-wasm

98e56c1

Copilot started reviewing on behalf of pavelsavara July 14, 2026 09:07 View session

pavelsavara temporarily deployed to copilot-pat-pool July 14, 2026 09:08 — with GitHub Actions Inactive

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Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps#130603

Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps#130603
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	mono_thread_init_apartment_state (void)
	{
	#ifdef HOST_WIN32
	MonoInternalThread* thread = mono_thread_internal_current ();

	/* Positive return value indicates success, either
	* S_OK if this is first CoInitialize call, or
	* S_FALSE if CoInitialize already called, but with same
	* threading model. A negative value indicates failure,
	* probably due to trying to change the threading model.
	*/
	if (CoInitializeEx(NULL, (thread->apartment_state == ThreadApartmentState_STA)
	? COINIT_APARTMENTTHREADED
	: COINIT_MULTITHREADED) < 0) {
	thread->apartment_state = ThreadApartmentState_Unknown;
	}
	#endif
	}

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Description

Customer Impact

Regression

Testing

Risk

Package authoring no longer needed in .NET 9

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