# Solid Csharp

> Use when writing or refactoring C#/.NET code, structuring Modules/[Feature]/ layers, or defining Contracts/ interfaces (SOLID, files < 100 lines).

- Skill: `fusengine/solid-csharp` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add fusengine/solid-csharp`
- Raw SKILL.md: https://api.skillmd.com/api/skills/fusengine/solid-csharp/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: fusengine (https://skillmd.com/u/fusengine)
- Updated: 2026-09-10
- Page: https://skillmd.com/skills/fusengine/solid-csharp

---


<objective>
SOLID C# enforces a modular architecture for C# 12/.NET 9 projects: every feature lives under `Modules/[Feature]/` (Controllers, Services, Repositories, Contracts, Models) with shared code centralized in `Core/`, contracts are mandatory and live only in `Contracts/` directories, and interfaces stay small (1-3 members). File limits are strict (100 lines, split at 90; controllers under 50).

Before writing any new code it requires a DRY check -- grep the codebase and `Core/Services`/`Core/Contracts` for existing logic to reuse before creating something new. See `principles.md` for the 5 SOLID principles and `patterns.md` for directory layout, testing, and record usage.
</objective>

# SOLID C# - Modular Architecture

## Agent Workflow (MANDATORY)

Before ANY implementation, spawn 3 agents in parallel, one `Agent` call each with a `name`:

1. **fuse-ai-pilot:explore-codebase** - Analyze existing architecture
2. **fuse-ai-pilot:research-expert** - Verify .NET docs via Context7
3. **fuse-ai-pilot:sniper** - Post-implementation validation

---

## DRY - Reuse Before Creating (MANDATORY)

**Before writing ANY new code:**
1. **Grep the codebase** for similar interfaces, services, or logic
2. Check shared locations: `Core/Services/`, `Core/Contracts/`
3. If similar code exists -> extend/reuse instead of duplicate
4. If code will be used by 2+ features -> create it in `Core/`

---

## Architecture (Modules MANDATORY)

| Layer | Location | Max Lines |
|-------|----------|-----------|
| Controllers | `Modules/[Feature]/Controllers/` | 50 |
| Services | `Modules/[Feature]/Services/` | 100 |
| Repositories | `Modules/[Feature]/Repositories/` | 100 |
| Contracts | `Modules/[Feature]/Contracts/` | 30 |
| Models | `Modules/[Feature]/Models/` | 50 |
| Shared | `Core/{Services,Contracts,Models}/` | - |

**NEVER use flat structure - always `Modules/[Feature]/`**

---

## Critical Rules (MANDATORY)

| Rule | Value |
|------|-------|
| File limit | 100 lines (split at 90) |
| Controllers | < 50 lines, delegate to services |
| Interfaces | `Contracts/` directory ONLY |
| XML docs | Every public member documented |
| DI | Use `Microsoft.Extensions.DependencyInjection` |
| Small interfaces | 1-3 members max |
| Records | Prefer `record` for DTOs and value objects |

---

## Reference Guide

| Topic | Reference | When to consult |
|-------|-----------|-----------------|
| **SOLID Principles** | [principles.md](references/principles.md) | Quick reference for all 5 principles |
| **Patterns & Structure** | [patterns.md](references/patterns.md) | Directory layout, testing, records |

---

## Forbidden

| Anti-Pattern | Fix |
|--------------|-----|
| Files > 100 lines | Split at 90 |
| Interfaces in impl files | Move to `Contracts/` directory |
| Fat interfaces (4+ members) | Split into focused interfaces |
| Flat project structure | Use `Modules/[Feature]/` |
| `new` for dependencies | Use constructor injection + DI |
| Service Locator pattern | Use constructor injection |