Code Navigator

Downloader — code navigator & style guide

Goal: spend the fewest tokens to land in the right file and produce code that matches the repo. For deep architecture, read src/CLAUDE.md (don't re-derive it). This skill is the fast index + the non-obvious style rules .editorconfig enforces.

1. Jump straight to the owning file

Don't grep the tree first — map the concept, open the file, read only the relevant region.

Cross-cutting concepts and their home: cancellation→Stopped rule and retry loop → DownloadService.cs; redirect/cookie-challenge → SocketClient.FetchResponseHeaders; unknown Content-Length progress → AbstractDownloadService.RaiseProgressChangedEvents.

2. Cheap recon, in order

1. This skill + src/CLAUDE.md first (already summarize structure, hazards, issue history).
2. Need a symbol? grep -rn "MethodName" src/Downloader scoped to the library, not the whole repo.
3. Read targeted line ranges, not whole files, once the table above points you at one.
4. Tests: there is almost always an existing test mirroring what you're changing — find it by name (grep -rn "Redirect\|Resume\|Truncate" src/Downloader.Test) and copy its arrange/act/assert shape.
5. Don't re-read a file you just edited to "verify" — Edit/Write fail loudly on mismatch.

3. House C# style (enforced by src/.editorconfig)

Match these exactly; they differ from common defaults:

• No var. Always use explicit types — HttpResponseMessage response = ..., not var. (csharp_style_var_* = false.)
• File-scoped namespaces: namespace Downloader;. Usings outside the namespace, System groups not sorted first, groups not separated.
• 4-space indent, CRLF, no final newline.
• Allman braces for types/methods/properties/accessors/control blocks; always brace even one-line if. Object/collection initializers stay K&R: SocketsHttpHandler handler = new() {.
• Expression-bodied properties/accessors/indexers/lambdas = yes; methods/constructors = no.
• Prefer pattern matching & switch expressions, null-propagation/coalescing, target-typed new(), collection expressions [], range/index operators, is null checks.
• readonly fields where possible; predefined types (string/int) over BCL names.
• Modifier order: public private protected internal static ... readonly ... async.
• Discard unused: _ = .... Static local functions where applicable.
• Primary constructors are used (e.g. class ChunkDownloader(...), controllers) — keep that idiom.

4. AOT & correctness rules that bite (don't regress)

• Reflection-free / AOT-safe. Classify exceptions by type/status only — never Exception.Source (empty under AOT, issue #226). System.Text.Json source-gen only.
• Don't trust a clean EOF — verify Chunk.Position == Chunk.Length; a short stream is a retry, not success (issue #231).
• Cancellation → Stopped even when the thrown exception isn't a cancel type; check IsCancelled first (issue #225).
• Unknown Content-Length: never report 100%/percentage until the size is finalized (issue #230).
• Redirects: clear ResponseHeaders before re-probing a redirect target; same-URL 307 challenges resolve only with a CookieContainer. No HTTP client defeats a JS challenge or an expired signed link — inspect the live response (curl -I, -r 0-0) before assuming a code bug.

5. Verifying changes

• Tests target net10.0: dotnet test src/Downloader.Test/ --filter "FullyQualifiedName~<Name>".
• The DummyHttpServer auto-starts in-process (dynamic port) on first DummyFileHelper reference — no manual server start needed.
• Add new server behavior as a DummyFileController endpoint + a DummyFileHelper.Get*Url builder, then an integration test mirroring an existing one.