Managed Code After Thread State Destroyed on .NET 9 and .NET 10

Curtis Chau

Updated: June 10, 2026

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  # Managed Code After Thread State Destroyed on .NET 9 and .NET 10 On .NET 9.0.5 and later (including .NET 10), an application using IronPDF exits with a fatal assertion error and exit code 127. ```txt Attempt to execute managed code after the .NET runtime thread state has been destroyed. ``` Microsoft introduced stricter FLS (Fiber Local Storage) thread cleanup in .NET 9.0.5. The OS controls the ordering of FLS callback execution, which can trigger CLR teardown before IronPDF's Chrome renderer finishes its own shutdown sequence. This issue is Windows-specific; Linux and macOS are not affected. .NET 6, 7, and 8 exit cleanly. IronPDF's PDF generation completes successfully before the assertion fires. Microsoft has filed the issue as "Future" with no planned fix date. ## Options ### Option 1: Downgrade to .NET 8 (Recommended) .NET 6.0.36 and .NET 8.0.26 exit cleanly. Downgrade the target framework if clean process exit is required. ### Option 2: Use test result files instead of exit codes in CI/CD The PDF work completes successfully before the assertion fires. Publish test results from result files rather than relying on exit code: Azure DevOps: ```yaml - script: dotnet test --logger "trx" || true - task: PublishTestResults@2 inputs: testResultsFiles: '**/*.trx' ``` GitHub Actions: ```yaml - run: dotnet test --logger "trx" || true - uses: dorny/test-reporter@v1 with: path: '**/*.trx' reporter: dotnet-trx ``` ### Option 3: Filter exit code in shell ```bash dotnet run 2>stderr.log; EXIT=$? if grep -q "thread state has been destroyed" stderr.log; then exit 0; else exit $EXIT; fi ``` Known non-working mitigations: `Installation.SkipShutdown = true`, `SetErrorMode`, `Environment.Exit(0)`, and manual GC collection before exit.

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On .NET 9.0.5 and later (including .NET 10), an application using IronPDF exits with a fatal assertion error and exit code 127.

Attempt to execute managed code after the .NET runtime thread state has been destroyed.

Attempt to execute managed code after the .NET runtime thread state has been destroyed.

Microsoft introduced stricter FLS (Fiber Local Storage) thread cleanup in .NET 9.0.5. The OS controls the ordering of FLS callback execution, which can trigger CLR teardown before IronPDF's Chrome renderer finishes its own shutdown sequence. This issue is Windows-specific; Linux and macOS are not affected. .NET 6, 7, and 8 exit cleanly. IronPDF's PDF generation completes successfully before the assertion fires. Microsoft has filed the issue as "Future" with no planned fix date.

Options

Option 1: Downgrade to .NET 8 (Recommended)

.NET 6.0.36 and .NET 8.0.26 exit cleanly. Downgrade the target framework if clean process exit is required.

Option 2: Use test result files instead of exit codes in CI/CD

The PDF work completes successfully before the assertion fires. Publish test results from result files rather than relying on exit code:

Azure DevOps:

- script: dotnet test --logger "trx" || true
- task: PublishTestResults@2
  inputs:
    testResultsFiles: '**/*.trx'

- script: dotnet test --logger "trx" || true
- task: PublishTestResults@2
  inputs:
    testResultsFiles: '**/*.trx'

GitHub Actions:

- run: dotnet test --logger "trx" || true
- uses: dorny/test-reporter@v1
  with:
    path: '**/*.trx'
    reporter: dotnet-trx

- run: dotnet test --logger "trx" || true
- uses: dorny/test-reporter@v1
  with:
    path: '**/*.trx'
    reporter: dotnet-trx

Option 3: Filter exit code in shell

dotnet run 2>stderr.log; EXIT=$?
if grep -q "thread state has been destroyed" stderr.log; then exit 0; else exit $EXIT; fi

dotnet run 2>stderr.log; EXIT=$?
if grep -q "thread state has been destroyed" stderr.log; then exit 0; else exit $EXIT; fi

Known non-working mitigations: Installation.SkipShutdown = true, SetErrorMode, Environment.Exit(0), and manual GC collection before exit.

Curtis Chau

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Technical Writer

Curtis Chau holds a Bachelor’s degree in Computer Science (Carleton University) and specializes in front-end development with expertise in Node.js, TypeScript, JavaScript, and React. Passionate about crafting intuitive and aesthetically pleasing user interfaces, Curtis enjoys working with modern frameworks and creating well-structured, visually appealing manuals.

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