unit-and-snapshot-testing

name	unit-and-snapshot-testing
description	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.

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
      ...