.NET breaking changes: the contract is wider than the file

When you edit .NET code, the observable contract is usually larger than the snippet in front of you. A change that compiles and keeps tests green can still break a downstream consumer, another target framework, a friend assembly, or regenerate away on the next build. This applies to a feature or a bug fix as much as a refactor, and to additions — a new public member, overload, or target framework — as much as removals: added surface is a permanent contract obligation, and added behavior on a multi-targeted or partial type must stay correct on every target and survive the next regeneration. (For the behavior-preserving refactoring process, use the csharp-refactoring skill; this skill is the compatibility knowledge it — and any other change — taps into.)

Inspect first — find which surfaces this repo actually has

The most valuable move is to look before you leap: find which hidden surfaces exist here, then check only those. Don't recite these or assume — the markers present dictate the plan; markers absent tell you a surface is not in play.

# Public-API gate (which one? they are NOT interchangeable)
git ls-files "**/PublicAPI.Shipped.txt" "**/PublicAPI.Unshipped.txt"                 # PublicApiAnalyzers
grep -rl "EnablePackageValidation\|ApiCompat" --include=*.props --include=*.targets --include=*.csproj .
# Multi-targeting and conditional code
grep -rl "<TargetFrameworks>" --include=*.csproj --include=*.props . ; grep -rn "#if " --include=*.cs .
# Generated / partial code
git ls-files "*.g.cs" "*.generated.cs" ; grep -rln "partial class\|partial record\|partial struct" --include=*.cs .
# Friend assemblies
grep -rn "InternalsVisibleTo" --include=*.cs --include=*.csproj --include=*.props .

Then read only the reference(s) for the surfaces you actually found. For every InternalsVisibleTo name, check whether that project is in the solution: a friend not built here is an external consumer of every internal it can see, exactly like a public caller, and a green solution build proves nothing about it.

Before editing, confirm the request's premise is true in the code. For "consolidate", "deduplicate", or "add X" requests, trace the actual calls: if one type already delegates to the other, or the member already exists, the work is done — change nothing and report the delegating call (see Stop and escalate).

The hidden surfaces (depth in references/)

Public API (1) is the default surface to check on any library edit; the other three are conditional — pursue them only when the inspect-first markers above show they are in play.

1. Public API. Renaming/moving/removing/re-signing a public member — or changing nullability, a generic constraint, or a trimming/AOT attribute — is a breaking change a green test run will not catch. Repos gate this two non-interchangeable ways: source-level (PublicApiAnalyzers + PublicAPI.*.txt, which you maintain) and binary/package-level (ApiCompat / <EnablePackageValidation>). Respect the gate that exists; if none exists, review the surface by hand and flag it — do not bolt on analyzer infrastructure as a side effect. Move a public type via a [TypeForwardedTo] forwarder; a rename needs an [Obsolete] shim, not a forwarder. See references/public-api.md.

2. Multi-targeting and #if. Code that multi-targets (<TargetFrameworks>) or compiles conditionally (#if NET8_0_OR_GREATER, platform/CoreCLR/Mono/NativeAOT branches) can build for the target your editor shows and break one it never invoked. Inspect every branch — including inactive ones — preserve intentional divergence, and re-gate each TFM/RID. See references/multi-targeting.md.

3. Source-generated and partial code. A type is often partial across several files, and part may be generated (source generators, Razor, *.g.cs). Include every partial declaration; treat generated files as derived and change the generator input/template unless the repo checks the output in as source. See references/source-generation.md.

4. InternalsVisibleTo. internal is not private across a solution: test and friend assemblies bind to internals (and, when strong-named, to a public key). An internal rename/move/removal can break a consumer with no reference in the declaring project. A friend built outside this repo cannot be updated, so keep the old internal name as an [Obsolete] forwarder. See references/internals-visible-to.md.

Decisions a green build will not make for you

Each row below compiles every caller and passes every test, yet breaks a consumer. Do not stop at a green build or test run. Use the safe shape instead.

When the safe shape departs from the literal request — an overload instead of the requested optional parameter, a forwarder kept instead of a deleted or renamed member, or no change at all — say so in your final answer in one or two sentences: what you did instead, and which consumer it would have broken (e.g. "already-compiled callers would throw MissingMethodException"). A silent substitution reads as ignoring the request.

Treat that substitution as one atomic deliverable: implement the safe shape, preserve the old contract and shipped baseline, test both old and new entry points, and explain the compatibility reason. After the last edit, inspect the diff or reopen the implementation file before claiming success. A green test run does not prove a failed patch applied when the unchanged fixture already passes those tests.

Do not use labels such as "for binary compatibility" as the whole explanation. Name the exact old entry point that remains, the consumer that still binds to it, and the concrete failure avoided. For example: "Kept Parse(string?, int) unchanged because already-compiled consumers still request that exact method and would throw MissingMethodException if it disappeared." For source-only breaks, name the affected syntax (raw: named arguments, derivation from a newly sealed class, or nullable callers) instead. This causal sentence is part of the implementation, not optional release-note polish.

Whenever you kept an old entry point or departed from the literal request, the final answer must contain this line, filled in — terse bullets such as "preserved for binary consumers" do not satisfy it:

Compatibility: Kept <exact old signature> unchanged because <who still binds to it> would <concrete failure> if it changed. <What you did instead of the literal request, and why.>

Before finishing a compatibility-preserving edit, make the delta easy to audit:

1. Name the new entry point or behavior and the exact old entry point that remains.
2. Name the in-repo caller, generated template, or friend assembly you migrated, plus any external consumer you could not migrate and therefore protected with a forwarder.
3. State which baseline stayed unchanged and which new surface was recorded.
4. Report the exact compatibility check that exercised the old entry point as well as the new one.

For a no-change outcome, lead with No files changed. and identify the decisive evidence before any general explanation: the existing delegation, shipped signature, parameter name, or nullability line.

When asked "is this a breaking change?" or to review a diff, make no edits unless asked and answer in this shape, scaled to the diff:

1. One verdict line first (e.g. "Do not merge as-is: N changes break consumers").
2. One numbered item per consumer-visible change: what changed, binary (already-compiled consumers fail), source (consumers fail to recompile), or behavioral (same signature, different result, possibly on only one target), who breaks, why CI stayed green (a shipped API line rewritten, a test edited to match, or a change the analyzer does not track, such as sealed or a private const), and the safe shape. State each as fact or not at all: a behavioral claim needs a concrete input whose old and new results differ; if unsure, probe both versions with a one-line program — framework parsers already tolerate much (e.g. bool.TryParse ignores surrounding whitespace), and a no-op rewrite is not a break.
3. One closing "Not breaking:" line covering every remaining hunk with its reason (internal-only edits, probed no-ops), instead of side notes inside the numbered items.

Cover only surfaces the diff touches; do not add trimming, AOT, or tooling commentary it does not raise.

Verify before reporting

Build and test every target framework (dotnet test -f <tfm> for each), not only the default. If the project sets EnablePackageValidation, run dotnet pack — package validation runs only after Pack, never on build. Report exactly which commands ran and their result.

Stop and escalate (do not silently proceed)

• The requested change is already in place — e.g. the "duplicated" code already delegates to one implementation, or the overload already exists. Say so and change nothing; do not invent a refactor to have something to show.
• A public/shipped API would change and you cannot verify compatibility — no shim/forwarder path and no PublicApiAnalyzers/ApiCompat/package-validation gate to catch a break.
• The edit changes an observable annotation (nullability, [DynamicallyAccessedMembers], [RequiresUnreferencedCode], generic constraints) on a public member.
• The change can be made consistent across some but not all target frameworks or platforms.
• The only way to apply it is editing generated output that the next build will overwrite.

When you change nothing (the request is already done or has no safe shape), a bare "no" looks like ignoring the request. Make the final answer show the work, in this order: "No files changed." and why; the evidence that decided it (the shipped PublicAPI.Shipped.txt line, or the call that already delegates, with its file); who would break, or where the single implementation already lives; what the user can do instead, if anything; and the validation you ran.

Reference files

Load a reference only for a surface the inspect-first step actually found:

• references/public-api.md — the two API gates and why they are not interchangeable, nullable/trimming/AOT as observable API, [Obsolete] shims and type-forwarders, suppression baselines.
• references/multi-targeting.md — TFMs, #if and platform symbols, RIDs, and how to inspect and re-gate every target.
• references/source-generation.md — partial types, generated output vs checked-in source, editing the generator input. (See also dotnet-msbuild/including-generated-files.)
• references/internals-visible-to.md — friend/test assemblies and strong-name keys.