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

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

CopilotAI self-assigned this Jul 13, 2026
CopilotAI review requested due to automatic review settings July 13, 2026 08:10
CopilotAI removed the request for review from CopilotJuly 13, 2026 08:10
CopilotAI 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
Closed
@pavelsavara
Trim STAThread/MTAThread attributes when multithreading is unsupported
5bc3ea6
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
CopilotAI requested review from Copilot and removed request for CopilotJuly 13, 2026 08:14
@dotnet-policy-servicedotnet-policy-serviceBot added the linkable-framework Issues associated with delivering a linker friendly framework label Jul 13, 2026
CopilotAI changed the title [WIP] Remove STAThreadAttribute from Blazor WASM appTrim STAThread/MTAThread attributes when multithreading is unsupportedJul 13, 2026
CopilotAI requested a review from pavelsavaraJuly 13, 2026 08:16
@pavelsavarapavelsavara added arch-wasm WebAssembly architecture size-reduction Issues impacting final app size primary for size sensitive workloads labels Jul 13, 2026
@MichalStrehovsky

MichalStrehovsky commented Jul 13, 2026

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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

pavelsavara commented Jul 13, 2026

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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.

@pavelsavara
Gate Mono STAThreadAttribute rooting on multithreading; revert LinkAt…
de13be9
…tributes change
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
CopilotAI requested review from Copilot and removed request for CopilotJuly 13, 2026 09:34

CopilotAI commented Jul 13, 2026•
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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.

CopilotAI changed the title Trim STAThread/MTAThread attributes when multithreading is unsupportedTrim STAThreadAttribute in single-threaded (browser/wasi) Mono appsJul 13, 2026
@build-analysisbuild-analysisBot mentioned this pull request Jul 13, 2026
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pavelsavara marked this pull request as ready for review July 13, 2026 13:54
CopilotAI review requested due to automatic review settings July 13, 2026 13:54
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/ba-g CI failure is #130618

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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.

Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml Outdated
@pavelsavara
Handle missing STAThreadAttribute in Mono/WASI startup path
a714677
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
CopilotAI requested review from Copilot and removed request for CopilotJuly 13, 2026 15:40
CopilotAI requested a review from thaystg as a code ownerJuly 13, 2026 15:40
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MichalStrehovsky
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Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml Outdated
CopilotAI review requested due to automatic review settings July 14, 2026 09:06
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Pull request overview

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

Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Comment threadsrc/mono/mono/metadata/object.c
CopilotAI requested review from Copilot and removed request for CopilotJuly 14, 2026 09:11
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If you want to address copilot's startup concern, I can sign off again.

@pavelsavara

pavelsavara commented Jul 15, 2026

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/ba-g known issue #130616

@pavelsavara
pavelsavara merged commit fe97964 into mainJul 15, 2026
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STAThreadAttribute is unnecessarily being left in a Blazor WASM default app

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Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps by Copilot · Pull Request #130603 · dotnet/runtime · GitHub
Skip to content
Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps - #130603
Merged
pavelsavara merged 6 commits into
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copilot/remove-stathreadattribute-blazor-wasm
Jul 15, 2026
Merged
Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps#130603
pavelsavara merged 6 commits into
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copilot/remove-stathreadattribute-blazor-wasm
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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.
CopilotAI
self-assigned this
Jul 13, 2026
CopilotAI
review requested due to automatic review settings
July 13, 2026 08:10
CopilotAI
removed the request for review
from CopilotJuly 13, 2026 08:10
CopilotAI
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

Closed

@pavelsavara
Trim STAThread/MTAThread attributes when multithreading is unsupported
5bc3ea6
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
CopilotAI
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Issues associated with delivering a linker friendly framework
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CopilotAI
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[WIP] Remove STAThreadAttribute from Blazor WASM appTrim STAThread/MTAThread attributes when multithreading is unsupportedJul 13, 2026
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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
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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.
@pavelsavara
Gate Mono STAThreadAttribute rooting on multithreading; revert LinkAt…
de13be9
…tributes change
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
CopilotAI
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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.
CopilotAI
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Trim STAThread/MTAThread attributes when multithreading is unsupportedTrim STAThreadAttribute in single-threaded (browser/wasi) Mono appsJul 13, 2026
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/ba-g CI failure is #130618
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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.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Outdated

@pavelsavara
Handle missing STAThreadAttribute in Mono/WASI startup path
a714677
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
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Pull request overview
Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Comment threadsrc/mono/mono/metadata/object.c
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If you want to address copilot's startup concern, I can sign off again.
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/ba-g known issue #130616
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Successfully merging this pull request may close these issues.

STAThreadAttribute is unnecessarily being left in a Blazor WASM default app


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Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps by Copilot · Pull Request #130603 · dotnet/runtime · GitHub
Skip to content
Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps - #130603
Merged
pavelsavara merged 6 commits into
mainfrom 
copilot/remove-stathreadattribute-blazor-wasm
Jul 15, 2026
Merged
Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps#130603
pavelsavara merged 6 commits into
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copilot/remove-stathreadattribute-blazor-wasm
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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.
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STAThreadAttribute is unnecessarily being left in a Blazor WASM default app
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Trim STAThread/MTAThread attributes when multithreading is unsupported
5bc3ea6
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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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.
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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.
@pavelsavara
Gate Mono STAThreadAttribute rooting on multithreading; revert LinkAt…
de13be9
…tributes change
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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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.
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Trim STAThread/MTAThread attributes when multithreading is unsupportedTrim STAThreadAttribute in single-threaded (browser/wasi) Mono appsJul 13, 2026
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/ba-g CI failure is #130618
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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.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Outdated

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Handle missing STAThreadAttribute in Mono/WASI startup path
a714677
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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Pull request overview
Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Comment threadsrc/mono/mono/metadata/object.c
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If you want to address copilot's startup concern, I can sign off again.
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/ba-g known issue #130616
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STAThreadAttribute is unnecessarily being left in a Blazor WASM default app


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, 'i');
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Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps by Copilot · Pull Request #130603 · dotnet/runtime · GitHub
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Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps - #130603
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Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps#130603
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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.
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STAThreadAttribute is unnecessarily being left in a Blazor WASM default app
#47861

Closed

@pavelsavara
Trim STAThread/MTAThread attributes when multithreading is unsupported
5bc3ea6
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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[WIP] Remove STAThreadAttribute from Blazor WASM appTrim STAThread/MTAThread attributes when multithreading is unsupportedJul 13, 2026
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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.
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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.
@pavelsavara
Gate Mono STAThreadAttribute rooting on multithreading; revert LinkAt…
de13be9
…tributes change
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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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.
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Trim STAThread/MTAThread attributes when multithreading is unsupportedTrim STAThreadAttribute in single-threaded (browser/wasi) Mono appsJul 13, 2026
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/ba-g CI failure is #130618
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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.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Outdated

@pavelsavara
Handle missing STAThreadAttribute in Mono/WASI startup path
a714677
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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Pull request overview
Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Comment threadsrc/mono/mono/metadata/object.c
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If you want to address copilot's startup concern, I can sign off again.
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/ba-g known issue #130616
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STAThreadAttribute is unnecessarily being left in a Blazor WASM default app


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, 'i');
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var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',
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})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '
Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps by Copilot · Pull Request #130603 · dotnet/runtime · GitHub
Skip to content
Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps - #130603
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Jul 15, 2026
Merged
Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps#130603
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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.
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STAThreadAttribute is unnecessarily being left in a Blazor WASM default app
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Trim STAThread/MTAThread attributes when multithreading is unsupported
5bc3ea6
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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[WIP] Remove STAThreadAttribute from Blazor WASM appTrim STAThread/MTAThread attributes when multithreading is unsupportedJul 13, 2026
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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.
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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.
@pavelsavara
Gate Mono STAThreadAttribute rooting on multithreading; revert LinkAt…
de13be9
…tributes change
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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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.
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Trim STAThread/MTAThread attributes when multithreading is unsupportedTrim STAThreadAttribute in single-threaded (browser/wasi) Mono appsJul 13, 2026
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/ba-g CI failure is #130618
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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.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Outdated

@pavelsavara
Handle missing STAThreadAttribute in Mono/WASI startup path
a714677
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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Pull request overview
Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Comment threadsrc/mono/mono/metadata/object.c
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If you want to address copilot's startup concern, I can sign off again.
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/ba-g known issue #130616
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, 'i');
if (__m === '*' || __re.test(location.href)) {
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function tryTheater() {
var btn = document.querySelector('button[aria-label="Theater mode"], ytd-player #player button[title="Theater mode"]');
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btn.click();
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var lastUrl = location.href;
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if (location.href !== lastUrl) {
lastUrl = location.href;
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observer.observe(document.body, { childList: true, subtree: true });
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} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
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try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps by Copilot · Pull Request #130603 · dotnet/runtime · GitHub
Skip to content
Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps - #130603
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Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps#130603
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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.
CopilotAI
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CopilotAI
linked an issue
Jul 13, 2026
that may be
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STAThreadAttribute is unnecessarily being left in a Blazor WASM default app
#47861

Closed

@pavelsavara
Trim STAThread/MTAThread attributes when multithreading is unsupported
5bc3ea6
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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[WIP] Remove STAThreadAttribute from Blazor WASM appTrim STAThread/MTAThread attributes when multithreading is unsupportedJul 13, 2026
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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.
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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.
@pavelsavara
Gate Mono STAThreadAttribute rooting on multithreading; revert LinkAt…
de13be9
…tributes change
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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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.
CopilotAI
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Trim STAThread/MTAThread attributes when multithreading is unsupportedTrim STAThreadAttribute in single-threaded (browser/wasi) Mono appsJul 13, 2026
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/ba-g CI failure is #130618
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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.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Outdated

@pavelsavara
Handle missing STAThreadAttribute in Mono/WASI startup path
a714677
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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Pull request overview
Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Comment threadsrc/mono/mono/metadata/object.c
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If you want to address copilot's startup concern, I can sign off again.
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/ba-g known issue #130616
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STAThreadAttribute is unnecessarily being left in a Blazor WASM default app


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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Remove or un-stick sticky/fixed headers that block content
(function() {
function unstick() {
document.querySelectorAll('header, nav, [role="banner"], .header, .navbar, .sticky, .fixed-top, [style*="position: fixed"], [style*="position:sticky"]').forEach(function(el) {
if (el.style.position === 'fixed' || el.style.position === 'sticky' || getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {
el.style.position = 'static';
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})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps by Copilot · Pull Request #130603 · dotnet/runtime · GitHub
Skip to content
Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps - #130603
Merged
pavelsavara merged 6 commits into
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copilot/remove-stathreadattribute-blazor-wasm
Jul 15, 2026
Merged
Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps#130603
pavelsavara merged 6 commits into
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copilot/remove-stathreadattribute-blazor-wasm
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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.
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STAThreadAttribute is unnecessarily being left in a Blazor WASM default app
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Trim STAThread/MTAThread attributes when multithreading is unsupported
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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.
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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.
@pavelsavara
Gate Mono STAThreadAttribute rooting on multithreading; revert LinkAt…
de13be9
…tributes change
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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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.
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Trim STAThread/MTAThread attributes when multithreading is unsupportedTrim STAThreadAttribute in single-threaded (browser/wasi) Mono appsJul 13, 2026
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/ba-g CI failure is #130618
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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.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Outdated

@pavelsavara
Handle missing STAThreadAttribute in Mono/WASI startup path
a714677
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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Pull request overview
Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Comment threadsrc/mono/mono/metadata/object.c
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If you want to address copilot's startup concern, I can sign off again.
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/ba-g known issue #130616
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STAThreadAttribute is unnecessarily being left in a Blazor WASM default app


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Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps by Copilot · Pull Request #130603 · dotnet/runtime · GitHub
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Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps - #130603
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Trim STAThreadAttribute in single-threaded (browser/wasi) Mono apps#130603
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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.
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STAThreadAttribute is unnecessarily being left in a Blazor WASM default app
#47861

Closed

@pavelsavara
Trim STAThread/MTAThread attributes when multithreading is unsupported
5bc3ea6
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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[WIP] Remove STAThreadAttribute from Blazor WASM appTrim STAThread/MTAThread attributes when multithreading is unsupportedJul 13, 2026
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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.
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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.
@pavelsavara
Gate Mono STAThreadAttribute rooting on multithreading; revert LinkAt…
de13be9
…tributes change
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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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.
CopilotAI
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Trim STAThread/MTAThread attributes when multithreading is unsupportedTrim STAThreadAttribute in single-threaded (browser/wasi) Mono appsJul 13, 2026
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/ba-g CI failure is #130618
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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.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Outdated

@pavelsavara
Handle missing STAThreadAttribute in Mono/WASI startup path
a714677
Co-authored-by: pavelsavara <271576+pavelsavara@users.noreply.github.com>
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Pull request overview
Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.
Comment threadsrc/mono/System.Private.CoreLib/src/ILLink/ILLink.Descriptors.xml
Comment threadsrc/mono/mono/metadata/object.c
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If you want to address copilot's startup concern, I can sign off again.
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/ba-g known issue #130616
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STAThreadAttribute is unnecessarily being left in a Blazor WASM default app


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