# C#

> Write robust C# avoiding null traps, async deadlocks, and LINQ pitfalls.

- Skill: `luokai0/c-2` (Agent Skill, multi-file: 9 files)
- Install (CLI): `npx skillmds@latest add luokai0/c-2`
- Raw SKILL.md: https://api.skillmd.com/api/skills/luokai0/c-2/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: luokai0 (https://skillmd.com/u/luokai0)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/luokai0/c-2

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## Quick Reference

| Topic | File |
|-------|------|
| Null reference, nullable types | `nulls.md` |
| Async/await, deadlocks | `async.md` |
| Deferred execution, closures | `linq.md` |
| Value vs reference, boxing | `types.md` |
| Iteration, equality | `collections.md` |
| IDisposable, using, finalizers | `dispose.md` |

## Critical Rules

- `?.` and `??` prevent NRE but `!` overrides warnings — still crashes if null
- `.Result` or `.Wait()` on UI thread — deadlock, use `await` or `ConfigureAwait(false)`
- LINQ is lazy — `query.Where(...)` doesn't execute until iteration
- Multiple enumeration of IEnumerable — may re-query database, call `.ToList()` first
- Closure captures variable, not value — loop variable in lambda captures last value
- Struct in async method — copied, modifications lost after await
- String comparison culture — `StringComparison.Ordinal` for code, `CurrentCulture` for UI
- `GetHashCode()` must be stable — mutable fields break dictionary lookup
- Modifying collection while iterating — throws, use `.ToList()` to iterate copy
- `decimal` for money — `float`/`double` have precision loss
- `readonly struct` prevents defensive copies — use for performance
- `sealed` prevents inheritance — enables devirtualization optimization