perf: emit source-generated test types after user code (Defender scan 5s → 0.2s at 10k tests)

[thomhurst]

Summary

On Windows, Microsoft Defender was spending ~5 seconds scanning every freshly built 10,000-test TUnit assembly, versus ~0.2 seconds for MSTest or xUnit assemblies of a similar size. Every build pays this cost, because CreateAppHost opens the DLL. The first test run after a build pays it again.

This is most of the gap between TUnit and the other frameworks in the build-time numbers from meziantou's framework benchmark.

Root cause

• The C# compiler writes namespaces to the assembly in reverse order of first declaration (it walks them with a stack).
• Generator output is always compiled after the project's own files, so TUnit.Generated is always declared last. That puts it first in the assembly.
• TUnit.Generated holds the per-class __TestSource static constructors. These are large linear methods, ~7 KB of IL per 100 tests.
• Defender analyses methods near the start of an assembly much more expensively.

I confirmed each step in isolation:

Experiment (10k tests)	Defender scan
TUnit assembly as built	4.5–5.5 s
Same assembly, generated types moved to the end with Cecil (no other change)	0.21 s
Generated code pasted into the project as plain source (compiles first, so it is emitted last)	0.22 s
Same code with only the __TestSource..cctor bodies stripped	0.18 s
Synthetic console app (no TUnit): 100 large array-initialising cctors emitted first	5.8 s
Same synthetic app, cctors emitted last	0.1 s
MSTest / xUnit 10k assemblies	0.2–0.3 s

Scan cost was measured as the first File.OpenRead of a fresh copy with a random suffix (so Defender cannot reuse a cached verdict).

Fix

TUnit.Core.props now adds TUnit.Core.GeneratedNamespace.cs (an empty namespace TUnit.Generated { }) as a Compile item. Package props are imported before the SDK's default **/*.cs glob, so this file compiles first. The compiler then writes the generated types at the end of the assembly. The generator output and runtime behaviour are unchanged.

• The item is only added for C# projects with source generation enabled. F# and VB projects are unaffected, and both still build.
• The file is marked Visible="false". TUnit.Core itself does not compile it.
• Repo test projects import the props from their project body, after the default glob. For them, a target in eng/TestLibrary.props moves the file to the front of the Compile items just before CoreCompile. The shipped package does not need or include this target.

Benchmarks

Local packages built from main vs this branch, using meziantou's harness (Windows 11, Defender real-time protection on, .NET 10 SDK 10.0.401, MTP runner). Cold build is the best of 3 runs.

Scenario	Tests	Cold build before	Cold build after
Bare	1,000	1.83 s	1.14 s
Bare	10,000	7.51 s	2.24 s
PerTestHooks	10,000	7.84 s	3.37 s
DataDriven	10,000	9.29 s	4.67 s

Edit a file, incremental build, then run the tests (Bare, 10k, 4 iterations each):

	Build	First run	Second run
Before	6.7–7.4 s	5.9–6.4 s	1.4–1.9 s
After	2.2–2.8 s	1.7–2.3 s	1.5–1.6 s

Warm run times did not change, as expected: Defender caches its verdict after the first scan.

Test plan

• tests/TUnit.TestProject builds; /*/*/BasicTests/* passes
• TUnit.TestProject.FSharp and TUnit.TestProject.VB.NET build
• Package layout: the .cs file ships next to TUnit.Core.props in build/ and every buildTransitive/<tfm>/
• Emitted type order checked from TypeDef metadata: TUnit.Generated.* types now follow the user's types, both in a consumer of the packed package and in tests/TUnit.TestProject. Before the in-repo fix, the repo test projects still emitted them first.
• TUnit.NugetTester GeneratedTypeOrderTests guards the packaged path. It passes on net10.0 and net472, and fails with the file removed.

Summary by CodeRabbit

• Bug Fixes
  • In consuming C# projects with TUnit source generation enabled, generated types now appear after user-defined types in the assembly.
  • Projects that disable TUnit source generation are unaffected.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-28T15:06:33.994902Z	a10c97f	PR opened

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📝 Walkthrough

Walkthrough

The change packages an empty TUnit.Generated namespace source file for consuming projects and configures its compile order. It also adds a test that checks generated types appear after user types in assembly metadata order.

Changes

Generated namespace compile integration

Layer / File(s)	Summary
Compile inclusion and packaging src/TUnit.Core/TUnit.Core.GeneratedNamespace.cs, src/TUnit.Core/TUnit.Core.csproj, src/TUnit.Core/TUnit.Core.props, eng/TestLibrary.props	Adds the empty namespace source file to consuming C# projects when source generation is enabled or unspecified. The core project excludes the file from its own compilation and packages it through two build paths. A target moves the file before other compile items when it is present.
Generated type ordering check tools/tunit-nuget-tester/TUnit.NugetTester/TUnit.NugetTester/GeneratedTypeOrderTests.cs	Adds a test that checks for generated and non-nested user types, then asserts that the first generated type’s metadata token follows the last user type’s.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Refactor

Merge Risk: ⚪ Minimal · up to 2bd49

The ordering source is included for standard C# consumers when source generation is enabled, and the test checks generated types follow user types. The earlier concern about the Language condition is resolved; no concrete merge-blocking risk remains.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 2bd49

The change affects how consuming test projects compile, but the reviewed changes do not introduce runtime privileges or data access. Some consumer build configurations remain unverified.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The independently affected scope is consuming C# builds that import the packaged asset; the visible effect is on compilation and assembly layout, rather than access to a shared runtime resource.

Trust Boundaries and Controls

• observed — The package-to-consumer transition is gated on C# and the source-generation setting. The inspected source and build target do not add an identity, credential, network, or authorization transition.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 2 files. (1 skipped: 1 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: emitting source-generated test types after user code to reduce Microsoft Defender scan time. The performance context supports the change with…

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 2 files. (1 skipped: 1 unsupported.)

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_{A rabbit checks the types in line,
Then hops through metadata, neat and fine.
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[github-actions]

Code review

Reviewed the change (src/TUnit.Core/TUnit.Core.GeneratedNamespace.cs, TUnit.Core.csproj, TUnit.Core.props). This is a small, well-scoped fix, and the PR description is excellent — the root-cause investigation (namespace emission order → Defender scan cost) and the benchmark table make the change easy to trust.

No functional issues found. A few observations, none blocking:

• Packaging is consistent with existing conventions. The new file follows the same TfmSpecificPackageFile + None/$(BuildPath)/$(BuildTransitivePath) pattern already used for TUnit.Core.props/.targets in the same csproj, and is correctly excluded from TUnit.Core's own compilation via <Compile Remove="TUnit.Core.GeneratedNamespace.cs" />.
• Namespace match verified. TUnit.Generated in the new file matches the namespace emitted by the source generators (checked InfrastructureGenerator.cs, DynamicTestsGenerator.cs, etc.), so this is targeting the right type.
• Minor edge case (not a bug): the EnableTUnitSourceGeneration != 'false' guard in TUnit.Core.props is evaluated against the property's value at props-import time, which happens before a consuming project's own .csproj body is evaluated. A project that opts into reflection mode via <EnableTUnitSourceGeneration>false</EnableTUnitSourceGeneration> in its own .csproj (rather than in a Directory.Build.props, which is imported early enough) won't actually suppress this Compile item — it'll still add the empty namespace declaration. This is harmless today (an empty namespace TUnit.Generated {} compiles fine either way and emits no members), but if this condition is ever load-bearing for something less inert, it's worth knowing it doesn't fully mirror the generator's own CompilerVisibleProperty evaluation timing.
• No new automated test covers the emission-order fix itself, but that's understandable given the nature of the bug (assembly-metadata layout affecting a third-party AV scanner) — the ILSpy verification and benchmark numbers in the description are a reasonable substitute, and this doesn't touch generator output or public API surface, so the usual snapshot/PublicAPI test requirements don't apply.

Nice find and a clean fix for a real-world pain point.

[greptile-apps]

Confidence Score: 5/5

[Medium risk] Reorders compiler input to change assembly metadata layout.

The PR appears safe to merge; no outstanding or new actionable finding remains.

Summary

The PR packages an early TUnit.Generated namespace declaration so generated test types are emitted after user types. A follow-up change reorders compile items for repository projects that import the props after the SDK glob. The packaged-consumer regression test addresses the previous type-order coverage finding.

• The new repository target leaves the compile-item set unchanged on its normal single execution.
• No actionable new issue was established.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[Early namespace declaration] --> B[User source files]
  B --> C[Source-generated test types]
  C --> D[Generated types emitted after user types]

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_{Reviews (3) · Last reviewed commit: "Put the TUnit.Generated namespace file f..."}

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Actionable comments posted: 1

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Inline comments:
Review comments at @src/TUnit.Core/TUnit.Core.props:
- Line 41: Update the ItemGroup condition in TUnit.Core.props to use the early
MSBuildProjectExtension discriminator for C# projects instead of Language, while
preserving the EnableTUnitSourceGeneration opt-out.

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[thomhurst]

Re: the Claude review note that the EnableTUnitSourceGeneration != 'false' guard is evaluated at props-import time. It isn't a problem. MSBuild evaluates every property before any item, so the ItemGroup condition sees the final value. I checked this on a packed-package consumer: <EnableTUnitSourceGeneration>false</EnableTUnitSourceGeneration> in the .csproj body removes the item from -getItem:Compile.

I kept the item in the .props rather than moving it to a .targets. Items added from a .targets land after the SDK's default **/*.cs glob, and the fix only works when this file comes first. The props also now explain this (562eb4d). Regression coverage is TUnit.NugetTester's GeneratedTypeOrderTests.