[Clint Rutkas (crutkas)]

Summary of the Pull Request

Fixes an intermittent PowerToys CI failure. The main build sometimes fails while MSIX packaging trims the Command Palette sample extension (SamplePagesExtension):

Microsoft.NET.ILLink.targets(150,5): error MSB6006: "dotnet.exe" exited with code 57005.
Microsoft.NET.ILLink.targets(103,5): error NETSDK1144: Optimizing assemblies for size failed.

The fix is one Release-only property in SamplePagesExtension.csproj: TrimmerRemoveSymbols=true. With it, ILLink doesn't rewrite debug symbols for this trim, so it never loads the native Windows PDB writer that crashes. The trimmed code doesn't change. The only difference is that the trimmed output has no PDBs.

This is backlog item BUILD-8 of the PowerToys speed & quality backlog.

PR Checklist

• Closes: N/A (backlog item BUILD-8, no GitHub issue)
• Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected (part of the speed & quality backlog)
• Tests: Added/updated and all pass (N/A: build-configuration change only, no product code. Validated with a local trimmed publish, repeated ILLink runs before and after the fix, and PR CI; see below)
• Localization: All end-user-facing strings can be localized (N/A: no strings)
• Dev docs: Added/updated (N/A)
• New binaries: Added on the required places (N/A: no new binaries)
  • JSON for signing for new binaries
  • WXS for installer for new binaries and localization folder
  • YML for CI pipeline for new test projects
  • YML for signed pipeline
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: N/A

Detailed Description of the Pull Request / Additional comments

Root cause

In the failing jobs, ILLink (dotnet.exe, Microsoft.NET.ILLink.Tasks 10.0.11) dies in its output step while it writes symbols:

Fatal error.
0xC0000005
   at Mono.Cecil.Pdb.ISymUnmanagedWriter2.Close()
   at Mono.Cecil.Pdb.SymWriter.Close()
   at Mono.Cecil.Pdb.NativePdbWriter.Write()
   at Mono.Linker.CustomSymbolWriter.Write()
   ...
   at Mono.Linker.Steps.OutputStep.WriteAssembly(Mono.Cecil.AssemblyDefinition, System.String)

• ILLink runs in every Release CI build. SamplePagesExtension.csproj sets GenerateAppxPackageOnBuild=true, and in Release it also sets PublishTrimmed=true. MSIX packaging during the solution build publishes the app with win-$(Platform).pubxml, so _RunILLink trims it. It's the only ILLink invocation in PR CI.
• ILLink rewrites the symbols. TrimmerRemoveSymbols defaults to false unless DebuggerSupport=false, so ILLink gets -b and rewrites the PDB of every assembly it trims.
• Those symbols are Windows PDBs. src\Common.Dotnet.props sets Release DebugType=pdbonly. That covers the sample and also Microsoft.CommandPalette.Extensions.Toolkit, a ProjectReference that's trimmed in the same ILLink run. Mono.Cecil can only write Windows PDBs through the native COM ISymUnmanagedWriter (the .NET Framework's DiaSymReader). The access violation happens inside that native Close(), so we can't fix it in this repo. We can only stop using that writer.

Affected builds and frequency

I used the public Azure DevOps REST API to scan every build still retained for the PowerToys CI pipeline (shine-oss/PowerToys, 2026-08-31 to 2026-09-30), including earlier attempts of retried jobs. Each failure below has the same stack, in the SamplePagesExtension _RunILLink step:

Build	Trigger	Queued (UTC)	Failed job
388605	main (batched CI)	2026-09-11	Build Release_x64
389658	PR #50843	2026-09-28	Build Release_arm64
389667	PR #50186	2026-09-28	Build Release_arm64
389871	PR #50507	2026-09-30	Build Release_x64
389888	PR #50884	2026-09-30	Build Release_x64

• Rate: 5 of 255 builds (~2%) and 5 of 486 x64/arm64 build jobs (~1%). In the last ~2 weeks (2026-09-14 to 2026-09-30) it was 4 of 196 builds (390 build jobs). PR builds only show up in this project from 2026-09-22, and 4 of those 125 PR builds (~3%) failed this way, all in the last three days. It hits both x64 and arm64 jobs. All five failures used ILLink 10.0.11, and every build I sampled from 2026-09-15 on used .NET SDK 10.0.303, so this isn't a new-toolset regression. The 30 scheduled "PowerToys CI (Caching)" builds had no hits.
• Cost: A hit fails the whole build job after ~30–34 minutes, and the job has to be re-run. A green build job takes ~53 minutes (median), and the test jobs wait for it. In 4 of the 5 builds, this crash was the job's only error. 389888 also had two unrelated unit-test failures in the same task.

Fix and why

Option	Result
(a) DebugType=portable for the sample (preferred, because it would keep symbols)	Not enough. The sample's PDB becomes portable, but the same ILLink run still rewrites Microsoft.CommandPalette.Extensions.Toolkit.pdb as a Windows PDB (verified locally, see Validation), so the native writer is still used. Switching the Toolkit to portable PDBs would change the symbol format of a shipping CmdPal/SDK binary, which is out of scope here.
(b) TrimmerRemoveSymbols=true (chosen)	ILLink runs without -b, so it doesn't read or write any symbols and never loads the native writer. The trimmed assemblies are otherwise identical.
(c) Don't package or trim the sample in CI	Not needed. It would also drop CI coverage of the trimmed, AOT-compatible sample that extension authors copy.

Effects of (b):

• The sample's trimmed publish output, and so its MSIX, has no PDBs (locally, the publish folder had none). The WinAppSDK MSIX targets skip _GenerateAppxSymbolPackage when the payload has no PDBs, so the sample no longer gets an .appxsym (confirmed in PR CI). The sample isn't shipped to users (it's not in the installer).
• The untrimmed build output keeps its PDB (<Platform>\Release\WinUI3Apps\CmdPalExtensions\SamplePagesExtension\SamplePagesExtension.pdb). That's the PDB CI's "Stage Unique Symbols" step already picks up.
• Debug builds don't trim, so the extension-author inner loop is unchanged. The property sits in the existing Release property group, and the repo-wide DebugType isn't touched.

Other projects checked

• ILLink in PR CI: I grepped every build task log of a green PR build (389905) for ILLink and ILC invocations. That covered x64 and arm64 (main build, BugReportTool, StylesReportTool, the seven "Publish … for Packaging" steps, DSC, and the installer) plus the "Build SDK" job. The only illink.dll invocation is SamplePagesExtension, once per platform, and nothing runs ILC.
• Other trimmed or AOT projects: Microsoft.CmdPal.Ext.PowerToys, Microsoft.CommandPalette.Extensions.Toolkit, UITestAutomation, PowerOCR, PowerDisplay (+CLI), and PowerAccent.UI set PublishAot or PublishTrimmed. PR CI only builds them and never publishes them, so neither ILLink nor ILC runs. CmdPal UI uses Native AOT only in the release pipeline (EnableCmdPalAOT=true). Native AOT trims with ILC, not ILLink/Cecil, so it doesn't use this writer. The release pipeline builds the same solution, so its SamplePagesExtension trim gets this fix too. No follow-ups needed.
• src\modules\cmdpal\ExtensionTemplate: Not built in CI (it's not in PowerToys.slnx, and verifyCommonProps.ps1 skips it). It doesn't import Common.Dotnet.props or set DebugType, so it uses the SDK default portable PDBs. The Toolkit it gets from NuGet ships without a PDB, so a trimmed template publish only uses Cecil's managed portable-PDB writer. No template change needed.

Validation Steps Performed

Local environment limits: The dev box is ARM64 with VS 18 and has no C++ toolset or Windows SDK, so there's no MakeAppx and MSIX packaging can't run locally. I exercised ILLink with a Release|x64 publish that skips packaging:

tools\build\build.ps1 -Platform x64 -Configuration Release /restore /t:Publish /p:GenerateAppxPackageOnBuild=false `
  /p:CsWinRTWindowsMetadata=<Microsoft.Windows.SDK.NET.Ref>\winmd `
  /p:CustomAfterMicrosoftCommonTargets=<local hook that swaps .vcxproj references for prebuilt winmd/dll>

The hook is local only and isn't part of this PR. Restores were anonymous (NUGET_PLUGIN_PATHS, NUGET_NETFX_PLUGIN_PATHS, and NUGET_NETCORE_PLUGIN_PATHS pointed at an empty folder), and no new packages were needed. Local ILLink is 10.0.12 (SDK 10.0.401); CI uses 10.0.11. To tell PDB formats apart, I checked the file header: a Windows PDB starts with Microsoft C/C++ MSF 7.00 and a portable PDB starts with BSJB.

1. Before (main): Publish exits 0. ILLink runs with -b and writes SamplePagesExtension.pdb and Microsoft.CommandPalette.Extensions.Toolkit.pdb as Windows PDBs, which means the native writer is used.
2. Option (a), not committed: SamplePagesExtension.pdb becomes portable (BSJB), but ILLink still writes Microsoft.CommandPalette.Extensions.Toolkit.pdb as a Windows PDB.
3. After (this PR): Publish exits 0 with the same two IL2104 warnings as main (both already in WarningsNotAsErrors). The ILLink task gets RemoveSymbols=True, so there's no -b, and it returns _ILLinkExitCode=0 without writing any PDBs. Compared with the "before" output, the trimmed SamplePagesExtension.dll (171 types, 743 methods, 251 fields) and Microsoft.CommandPalette.Extensions.Toolkit.dll (399 types, 1850 methods, 530 fields) have the same metadata counts and code size. The only change is the missing CodeView debug-directory entry (the pointer to the PDB).
4. Property evaluation: dotnet msbuild -getProperty gives TrimmerRemoveSymbols=true for Release|x64 and Release|ARM64 and leaves it empty for Debug, where PublishTrimmed=false.
5. Repeated trims: I replayed the exact ILLink command from the MSBuild log 20 times with the native arm64 dotnet and 20 times with the emulated x64 dotnet (closer to the x64 CI agents), before and after the fix.
   • Before: 40/40 passed, plus 10 more runs with DOTNET_GCgen0size=0x20000. Every run wrote two Windows PDBs through the native writer.
   • After: 40/40 passed, and no PDBs were written.
   • The crash didn't reproduce locally. That's consistent with a ~1% per-job rate, so PR CI is the real gate.
6. PR CI (389926): green on x64 and arm64. All jobs passed: Build Release_x64 (build + tests), Build Release_arm64, Build SDK, and both "Resolve affected UI test projects" jobs. In both main-build logs:
   • SamplePagesExtension's ILLink ran once (illink.dll -a "obj\x64\Release\SamplePagesExtension.dll" and obj\ARM64\..., ILLink 10.0.11) without -b. It has 253 arguments versus 254 in build 389905, which doesn't have this change, and the only one missing is -b.
   • There were no Fatal error, 0xC0000005, 57005, or NETSDK1144 markers. The task had 0 errors and the same two IL2104 warnings as 389905.
   • MSIX packaging still produced SamplePagesExtension_1.0.0.0_x64.msix and SamplePagesExtension_1.0.0.0_arm64.msix, and it skipped the sample's .appxsym as expected.
   • "Stage Unique Symbols" still links 202 PDBs per platform, the same as 389905, and that includes the untrimmed SamplePagesExtension.pdb.

[github-actions]

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Link the issue this PR fixes using a closing keyword such as Closes #123.

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