name: software-csharp-backend description: "Applies C# and .NET backend standards. Use when shaping API boundaries, data access, resilience, observability, or security defaults." compatibility: Portable core. Works on Claude Code and Codex. version: "1.1" last_validated: 2026-07-11

C# Backend Engineering

Quick Reference

Route other tasks:

• NUnit fixture design, WireMock/Testcontainers setup, or flake reduction → $qa-testing-nunit
• nuke/Build.cs, CI target sequencing, or artifact publication → $ops-nuke-cicd
• Legacy ILogger or Serilog rewrite automation → $dev-structured-logs

When to Use This Skill

• Building or reviewing C# / .NET backend services
• Choosing between ASP.NET Core API styles (controller, minimal API, background worker)
• Implementing data access with EF Core, Dapper, or MongoDB
• Adding resilience, observability, or security baselines to .NET services
• Refactoring .NET backend code or detecting common anti-patterns

When NOT to Use This Skill

• General backend patterns (Node.js, Python, Go, Rust) → software-backend
• NUnit fixture design and test infrastructure → qa-testing-nunit
• NUKE pipeline targets and CI/CD → ops-nuke-cicd
• ILogger/Serilog migration automation → dev-structured-logs
• System architecture beyond a single service → software-architecture-design
• Security audits and threat modeling → software-security-appsec

Workflow

1. Classify the requested change (new feature, refactor, bug fix, review).
2. Choose runtime shape before implementation details.

Load references/scenario-guides.md and, for HTTP services, references/aspnet-core-api-patterns.md.

1. Apply language and coding standards.

Load references/csharp-language-practices.md and references/dotnet-coding-standards.md.

1. Confirm architecture and boundaries before editing internals.

Load references/backend-architecture-principles.md and references/modular-architecture-principles.md.

1. Choose persistence and consistency strategy from query/write shape.

Load references/data-access-patterns.md; if EF Core is selected, load references/efcore-persistence-patterns.md.

1. Add resilience behavior for outbound I/O and long-running work.

Load references/reliability-and-resilience.md and references/resilience-policy-defaults.md.

1. Define tests by risk and boundary.

Load references/testing-practices.md.

1. Add logs, traces, metrics, health probes, and operability defaults.

Load references/observability-standards.md, and for API/runtime deployment defaults load references/runtime-ops-checklist.md. If the service is distributed or cloud-native by design, also load references/scenario-guides.md for the Aspire-oriented profile.

1. Validate auth, validation, and secrets handling.

Load references/security-baseline.md.

1. Run feedback loop validation for changed behavior.

For NUKE-based repositories, run BuildAll, LocalUnitTest, ApiTest (when relevant), and TestAll; use $ops-nuke-cicd for pipeline-target edits.

1. Run final review against anti-pattern checklist.

Load references/code-review-checklist.md.

ASCII Flow

C# backend task
  -> Identify API, worker, data access, or service boundary
  -> Confirm .NET version, hosting model, DI, and persistence choices
  -> Design validation, auth, errors, retries, and observability
  -> Implement bounded slice with unit and integration tests
  -> Check EF, async, cancellation, and deployment traps
  -> Run focused verification and capture follow-up risks

Decision Tree

• If the issue is naming, nullability, exception usage, or async flow, read references/csharp-language-practices.md.
• If the issue is project layout, DI, configuration, or layering, read references/dotnet-coding-standards.md.
• If the issue is service boundaries or clean architecture drift, read references/backend-architecture-principles.md.
• If the issue is module boundaries, composition hosts, or modular-monolith tradeoffs, read references/modular-architecture-principles.md.
• If the issue is API style, middleware ordering, ProblemDetails, OpenAPI, request timeouts, rate limiting, health/readiness, or graceful shutdown, read references/aspnet-core-api-patterns.md.
• If the issue is SQL/Mongo/EF access, pagination, transactions, idempotency, or N+1, read references/data-access-patterns.md.
• If EF Core is selected for relational persistence, also read references/efcore-persistence-patterns.md.
• If the issue is system profile specific (high-throughput API, event-driven worker, multi-tenant service, distributed/Aspire host), read references/scenario-guides.md.
• If the issue is retry/timeout/circuit behavior, cancellation propagation, standard resilience handler behavior, or worker reliability, read references/reliability-and-resilience.md and references/resilience-policy-defaults.md.
• If the issue is container/runtime readiness, startup config validation, HybridCache defaults, or deployment reliability gates, read references/runtime-ops-checklist.md.
• If the issue is target framework constraints, package/API compatibility, multi-targeting, or migration between .NET Framework/netstandard2.0 and modern .NET, read references/version-compatibility-notes.md.
• If the issue is flaky tests, fake time control, or weak coverage strategy, read references/testing-practices.md.
• If the issue is logs/traces/metrics/health checks, read references/observability-standards.md.
• If the issue is input validation, auth boundaries, secret handling, or secure defaults, read references/security-baseline.md.
• If the task is reviewing a PR for backend quality risks, read references/code-review-checklist.md.
• If the task is designing or fixing build-test pipeline loop behavior, use $ops-nuke-cicd.
• If the task needs deep HTTP controller + CQRS error-contract design, keep it in this skill and align handler/result patterns to the repository's existing conventions rather than introducing new framework requirements by default.

Do / Avoid

Known Traps

• Mixing minimal APIs, controllers, and bespoke endpoint frameworks without a clear contract strategy or error-shape policy.
• Letting cancellation stop at the controller boundary while downstream HTTP, EF Core, queue, or cache calls continue running.
• Treating EF Core defaults as safe under load: lazy loading, implicit tracking, and missing query shaping frequently create hidden cost.
• Using retries around non-idempotent handlers, transaction scopes, or third-party calls without dedupe or timeout coordination.
• Shipping background workers without graceful shutdown, lease or lock ownership, or poison-message handling.
• Binding configuration directly into services without options validation, startup failure checks, or explicit secret handling.
• Choosing Native AOT for a service that depends on EF Core, reflection-based JSON, or dynamic plugin loading — EF Core is not fully AOT-compatible today, and unbounded reflection breaks at publish/trim time. Verify each dependency's AOT/trim compatibility before committing, or keep the service on the standard runtime.
• Instantiating new HttpClient() per call or per request instead of resolving it through IHttpClientFactory/AddHttpClient<T> — this exhausts sockets under load (new HttpClient() disposed per call) or goes DNS-stale (one long-lived static instance) if done by hand.
• Leaving Server GC (the ASP.NET Core default) unexamined in a memory-constrained or high-density container deployment — Server GC sizes heaps to the visible processor count, which can over-allocate memory on small/shared containers. Use DOTNET_GCHeapHardLimit/DATAS (Dynamic Adaptation to Application Sizes — opt-in on .NET 8, default-on from .NET 9) or switch to Workstation GC for high-density, low-CPU hosting; keep Server GC for dedicated, CPU-bound API pods.

Common Anti-Patterns

• Building clean architecture layers that are mostly pass-through wrappers with no boundary or policy value.
• Returning exceptions for expected domain outcomes instead of explicit result or error contracts.
• Sharing repository abstractions everywhere even when the real need is a focused query handler or aggregate persistence boundary.
• Hiding cross-cutting behavior inside ad hoc helpers instead of using middleware, filters, options, or resilience handlers deliberately.
• Using integration tests as the only safety net while unit seams, fake time, and deterministic failure cases stay untested.

Navigation

References

• C# Language Practices
• Dotnet Coding Standards
• Backend Architecture Principles
• Modular Architecture Principles
• ASP.NET Core API Patterns
• Data Access Patterns
• EF Core Persistence Patterns
• Scenario Guides
• Reliability and Resilience
• Resilience Policy Defaults
• Testing Practices
• Observability Standards
• Runtime Ops Checklist
• Version Compatibility Notes
• Security Baseline
• Code Review Checklist
• Skill Sources: curated primary sources (last verified 2026-07-11) plus modular-architecture supporting references.

Templates

• API Host Template
• Service Class Template
• Options Configuration Template
• Resilient HTTP Client Template
• Dapper Query Handler Template
• Mongo Repository Template
• Test Data Builder Template
• Pull Request Checklist Template

Related Skills

• software-backend → General backend patterns and multi-language guidance
• software-architecture-design → System-level design and decomposition
• software-security-appsec → Security audits and threat modeling
• software-code-review → Review workflow and judgment patterns
• qa-testing-nunit → NUnit fixture design and test infrastructure
• ops-nuke-cicd → NUKE build pipeline and CI/CD targets
• dev-structured-logs → Structured logging migration

Freshness Protocol

When users ask version-sensitive questions about .NET, C#, or ASP.NET Core, verify current information before answering.

Trigger Conditions

• "What's the latest .NET LTS version?"
• "Should I use EF Core or Dapper?"
• "What's new in ASP.NET Core / C#?"
• "Is [pattern/library] still recommended for .NET?"

How to Freshness-Check

1. Start from data/sources.json (official docs, release notes, support policies).
2. Run a targeted web search for the specific .NET component.
3. Prefer official Microsoft docs over blogs for versions and support windows.

What to Report

• Current landscape: what is stable and widely used now
• Emerging trends: what is gaining traction (and why)
• Deprecated/declining: what is falling out of favor (and why)
• Recommendation: default choice + 1-2 alternatives, with trade-offs

Fact-Checking

• Known bugs, regressions, framework/compiler/runtime footguns, and version-specific crash or workaround guidance must be verified against current primary web sources before being treated as current fact.
• Use web search/web fetch to verify current external facts, versions, pricing, deadlines, regulations, or platform behavior before final answers.
• Prefer primary sources; report source links and dates for volatile information.
• If web access is unavailable, state the limitation and mark guidance as unverified.

Learnings Loop

Before applying this skill on a non-trivial task, read learnings.consolidated.md in this directory (and learnings.md if present).

After applying it, if you encountered a pattern worth remembering, a mistake worth preventing, or a domain fact that surprised you, append one dated bullet to learnings.md via agents-skills-feedback-loop/scripts/append_learning.py. Do not modify SKILL.md itself.