# Unit And Snapshot Testing

> Set of principles for unit and snapshot testing. Use when adding/fixing unit tests or when working with `Verify` snapshots. Do not load when working with integration or e2e tests. Use when this capability is needed.

- Skill: `tomevault-io/unit-and-snapshot-testing` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add tomevault-io/unit-and-snapshot-testing`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/unit-and-snapshot-testing/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- Author: tomevault-io (https://skillmd.com/u/tomevault-io)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/tomevault-io/unit-and-snapshot-testing

---


## Scope

- Unit tests live under `test/*UnitTests`.
- Follow these instructions when writing/updating **unit** tests. Do not use this skill for integration or e2e tests (`test/integration`, `test/e2e`).

## Principles

- Use `Verify` snapshot testing tool for generated output (source generated code, files, etc). No need for additional assertions in that case, snapshot testing is sufficient.
- For normal assertions, prefer small, domain-specific assertion helpers over chaining multiple `Assert.*` calls. Example: `DiagnosticAssert.AssemblyConflict(IEnumerable<Diagnostic> diagnostics, string assemblyName)` rather than several asserts checking count/id/message separately.
- Prefer a small number of representative scenarios; combine related assertions to keep snapshot sprawl low.
- Run targeted tests with `dotnet test --filter "FullyQualifiedName~<TestClass>.<TestMethod>"` (add `--no-build --no-restore` when appropriate).
- Treat snapshot artifacts as opaque: never open/read/inspect any `.received.txt` / `.verified.txt` files (or anything under test/AttributedDI.SourceGenerator.UnitTests/Snapshots/) and never include their contents in reasoning.
- Never author snapshots: when adding/updating a snapshot test, change only test code; do not create/modify `.verified.txt`. Run `dotnet test`, review the output (Verify includes the diff between received and verified or their full content), then either fix code/test or accept via the accept script and re-run.
- Accept snapshots only when the behavior change is intended and understood.
- Do not inspect or edit the accept scripts (treat them as black boxes).

## Accepting Snapshots

- One test: `.codex/skills/unit-and-snapshot-testing/scripts/accept-snapshot.ps1 <TestClassName> <TestMethodName>`
- All tests: `.codex/skills/unit-and-snapshot-testing/scripts/accept-all-snapshots.ps1`
- Re-run corresponding tests after accepting snapshots to ensure they pass.

## Recognizing a Verify Failure

Every time there is a mismatch between received/verified files following will be included in `dotnet test` output. Example:

```
VerifyException : Directory: /home/me/projects/AttributedDI/test/AttributedDI.SourceGenerator.UnitTests/Snapshots
  NotEqual:
    - Received: AddAttributedDiTests.GeneratesAddAttributedDiForEntryPoint.DotNet9_0.received.txt
      Verified: AddAttributedDiTests.GeneratesAddAttributedDiForEntryPoint.verified.txt

  FileContent:

  NotEqual:
      Received: AddAttributedDiTests.GeneratesAddAttributedDiForEntryPoint.DotNet9_0.received.txt
      ...
      Verified: AddAttributedDiTests.GeneratesAddAttributedDiForEntryPoint.verified.txt
      ...
```

---
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<!-- tomevault:4.0:skill_md:2026-04-11 -->