C#/.NET Backend Patterns

Layout

• Vertical slices (feature folders: endpoint + handler + models per feature) for API-shaped services; full Clean Architecture layering only when the domain genuinely warrants it — record the choice as an ADR either way.
• Minimal APIs for small/medium surfaces; MVC controllers when filters/model-binding complexity earns it.

DI lifetimes (the classic traps)

• Scoped services must NEVER be captured by singletons (captive dependency — resolve via IServiceScopeFactory inside the singleton instead).
• HttpClient via IHttpClientFactory, never new HttpClient() per call (socket exhaustion).
• Register by interface at the boundary you intend to fake in tests; concrete elsewhere is fine.

Async all the way

• No .Result/.Wait()/GetAwaiter().GetResult() on async paths — deadlock and threadpool starvation fuel.
• CancellationToken flows from the endpoint through every service and EF call: await db.Orders.ToListAsync(ct). Endpoints accept it; libraries propagate it.
• ValueTask only where profiling justifies it; default is Task.

EF Core discipline

• Reads: AsNoTracking() by default; project to DTOs with Select — never return entities from queries that don't update.
• No lazy-loading proxies: load explicitly (Include) so every query's shape is visible — N+1 becomes impossible to miss.
• Migrations: one migration per change, reviewed like code; destructive column changes follow expand-contract (see db-optimization skill).
• Transactions explicit where multi-write consistency matters (ExecutionStrategy + TransactionScope-free patterns with BeginTransactionAsync).

Config — options pattern

• IOptions<T> with ValidateDataAnnotations().ValidateOnStart() — invalid config fails the boot, not the first request.
• Secrets from user-secrets/KeyVault/env — never appsettings committed values.

Error contract

• ProblemDetails (RFC 9457) via AddProblemDetails(); exceptions mapped centrally (exception handler middleware), typed domain exceptions → status codes. No try/catch per controller.
• Nullable reference types ON project-wide; warnings as errors for new code.

Testing shape

• xUnit; unit tests fake at the interface boundary.
• WebApplicationFactory<Program> for integration tests over the real pipeline (routing, filters, serialization) with a test database (Testcontainers where Docker exists).
• Snapshot the API surface where the contract matters (Verify/ApiApprovals) so drift is a failing test.