csharp-refactoring git:20260915.3b8d365A

Immutable. This exact content is served forever at /api/v1/blob/303366ff05cae0a0.

---
name: csharp-refactoring
description: "Performs safe, behavior-preserving refactoring of C#/.NET code, verified with build, tests, and analyzers. USE FOR: rename or move a symbol/type/file; extract a method/type/interface; inline a wrapper/method/local; merge or consolidate near-identical classes or duplicate helpers; split or modernize C# code; generated/partial declarations; public, serialized, friend-assembly, or multi-targeted contracts; and mixed requests where a feature, bug fix, package/framework upgrade, public nullability change, or other behavior/contract change is presented as a refactor and must be separated or declined. DO NOT USE FOR: ordinary feature or bug-fix requests not framed as refactoring; upgrades after reclassification (use dotnet-upgrade); new tests; or formatting-only passes (use dotnet format)."
license: MIT
---

# C# Refactoring (behavior-preserving)

A refactor changes **structure**, never observable **behavior**. Do the edit with binding-aware tools,
then confirm behavior held with a build + the relevant tests. Keep the effort proportional to the change:
a one-line local rename does not need the ceremony a public multi-targeted change does.

## Mandatory gate: classify before validation or editing

Read only enough repository context to classify the request. Do this before restoring, building, or
making an edit. If the entire requested operation changes behavior or cannot preserve the relevant
public/source contract, the correct result of this skill is a decisive handoff, not an implementation
attempt:

1. State: `Not a behavior-preserving refactor: <specific reason>.`
2. State: `No files changed.`
3. State: `Next workflow: <workflow>.` Then stop. Do not add manual implementation steps, alternatives,
   an offer to proceed without the workflow, or a follow-up question — even when that workflow is unavailable.

| Requested as a "refactor" | Classification and action |
|---|---|
| Framework or NuGet version change | **Upgrade.** Do not edit or validate the upgrade here; hand off to `dotnet-upgrade`. |
| New capability, flag, endpoint, tier, or behavior | **Feature.** Do not implement it here; hand off to the repository's feature workflow. |
| Threshold, rate, output, or bug-result change | **Behavior change.** Defer it and hand off to the repository's bug-fix or behavior-change workflow; still complete any clearly separable structural operation. |
| Tighten or loosen a shipped/public nullable annotation | **Source-contract change.** Leave the declaration and API record unchanged; hand off to the repository's API-contract workflow. |

The three-line stop response applies only when the entire request is outside behavior-preserving
refactoring. For a mixed request, perform only a clearly separable structural operation and explicitly
defer the behavior/contract change. Never state `No files changed.` after completing that structural
operation, and never modify tests to make an unauthorized behavior change appear preserved.

## Work only in the current repository

Resolve the repository root first (`git rev-parse --show-toplevel`) and resolve any prompt-provided
relative solution/project path inside that root. Search and edit only that workspace. Never use
filesystem-wide search or select a similarly named clone, temporary directory, build output, or
another worktree because a file also exists there. If the named path is absent from the current
repository, stop and report that mismatch instead of guessing another workspace.

## Rename / move by bindings, not text

The #1 way a "rename" silently corrupts code is editing textual matches (comments, strings, unrelated
overloads) instead of real **bindings**. Find every binding reference first, then edit semantically. Use
the strongest tool available: an IDE/Roslyn workspace refactoring, then the C# LSP the
[`dotnet` plugin declares](https://github.com/dotnet/skills/blob/main/plugins/dotnet/lsp.json)
(`findReferences`, `goToDefinition`, `incomingCalls`, `rename` code action), then analyzer code-fixes /
Roslynator, then compiler-validated edits (edit the true bindings, rebuild, let the compiler flag misses).
Plain find/replace only when scope is provably tiny and every hit is verified. Include **every** `partial`
declaration, and edit the generator input, never generated (`*.g.cs`) output.

For the operation → Roslyn-provider mapping and representative PRs, see
[references/operation-catalog.md](references/operation-catalog.md).

## Consolidate toward the existing source of truth

When de-duplicating, preserve the ownership direction stated by the code or request. If `B` duplicates
an implementation already owned by `A`, keep `A` canonical and make `B` delegate to it; do not invert
the dependency merely because either direction compiles. Preserve public compatibility wrappers when
the duplicate surface is shipped, and migrate only in-repo callers that are safe to move.

## Decisions that change the edit

Use the first matching row instead of applying the requested operation mechanically:

| Situation | Do | Never |
|---|---|---|
| Inline an internal, unshipped pass-through wrapper | Migrate every binding reference to the target, remove the wrapper, then compile to catch misses. | Keep dead indirection "for compatibility" when no compatibility boundary exists. |
| Inline or remove a shipped/public wrapper | Migrate ordinary in-repo callers, but retain an `[Obsolete]` forwarding entry point unless the request explicitly authorizes a breaking change. | Delete a shipped API merely because all current source callers were migrated. |
| Rename a member reached by a string, reflection, DI, or configuration | Rename binding-based callers; preserve the observed external name with a forwarding shim or metadata, and exercise the old-name path. | Rewrite an external/configured name just to make the new source name consistent. |
| Extract duplicated logic whose callers pass different values | Extract the algorithm and pass each caller's existing inputs through unchanged. | Collapse distinct inputs, evaluation order, rounding, or side effects into one caller's version. |
| Rename code compiled under `#if` or multiple TFMs | Update every source branch and validate each target framework explicitly. | Treat a green default-target build as evidence for unbuilt branches. |
| Merge near-identical types | Parameterize only the values that differ, migrate every construction site, and preserve each old value exactly. If the old types are internal/unshipped and the request says to merge into one type, delete their declarations. | Retain unnecessary aliases, static holders, factories, or wrapper types that leave the requested merge incomplete; introduce a new hierarchy or behavior. |

## Preserve contracts beyond C# call sites

Compilation proves binding compatibility, not every external contract. Before renaming or moving a
type/member, check whether its name or metadata is observed by serialization, reflection, dependency
injection, configuration binding, source generators, P/Invoke, or `dynamic`.

| Boundary | Required decision |
|---|---|
| Serialized/configuration name | Preserve the external name with the repository's existing mechanism (for example, `JsonPropertyName`) while migrating C# callers; run a focused round-trip or payload test. |
| Public nullable annotation | The mandatory classification gate applies: leave it unchanged and hand off as a source-contract change. |
| Uncovered reflection or runtime lookup | Do not guess that a compile-clean rename is safe. Preserve the observed name or stop and report the unverified runtime boundary. |

## Verify proportionally

Confirm behavior is preserved after the edit — scaled to blast radius, not a fixed ceremony:

- **Local / private** (method-local or `private` member, one file, single target framework, no public
  surface, no `partial`/generated/`#if`): build once and run the **relevant** tests once after the edit.
  If the tree is already known-green, don't burn a second full "before" baseline — rely on the post-edit
  gate. Let the compiler catch missed references.
- **Cross-boundary** (public/shipped symbol, multi-targeted project, `#if`/platform branches, or
  `partial`/generated code): establish a baseline, then run an explicit build and the relevant tests for
  **each** target framework after the edit (a test command's implicit build is not separate build evidence;
  a green default build can hide a break on another TFM), and run the hazards check below.

Use the repo's own build/test workflow when it documents one (`README`/`CONTRIBUTING`, `build.*`, `eng/`,
`global.json`, `.github/workflows`); its instructions win over any generic command. Otherwise:

```bash
dotnet build   # 0 errors
dotnet test    # stays green; same pass count as before
git restore .  # if the gate fails, revert THIS step and reassess
```

One operation per step; never mix a refactor and a behavior change in the same step. On red, stop and report the failure; repair only your edit without discarding unrelated worktree changes.

## Final response contract

Keep the handoff concise and evidence-based:

- **Refactor:** name the structural operation and the symbols/files changed.
- **Preserved:** name the behavior or compatibility boundary and the mechanism that preserved it.
- **Validation:** report the exact commands and observed result; never claim success after a failed restore,
  build, target framework, or test run.
- **Deferred:** for a mixed request, name the behavior/contract change intentionally left undone and its
  correct next workflow. Omit this line when nothing was deferred.

## Cross-boundary hazards (only when it touches a boundary)

If — and only if — the change touches a **public** symbol, a **multi-targeted** project, or
`partial`/generated code, some breaks won't show up as a failing test. Search the repo for the surface
that governs the symbol (don't assume): the public-API gate (`PublicAPI.Shipped/Unshipped.txt` for
PublicApiAnalyzers, and/or `ApiCompat`/`<EnablePackageValidation>` — not interchangeable),
`<TargetFrameworks>`/`#if` branches, and `InternalsVisibleTo`. Moving a public type to another assembly
needs `[TypeForwardedTo]` in the original assembly; a move within one assembly does not. A public
*rename* needs an `[Obsolete]` shim, not a forwarder. For a provably local/private change, skip these
checks.

## Stop and ask when

- The baseline is already red (you can't prove you preserved behavior).
- A public/shipped API would change without a forwarder/shim or explicit authorization for a breaking change.
- Equivalence depends on runtime behavior tests don't cover (reflection, DI, serialization, `dynamic`,
  P/Invoke) — flag it.