/solid-csharp

name: solid-csharp
description: "Use when writing or refactoring C#/.NET code, structuring Modules/[Feature]/ layers, or defining Contracts/ interfaces (SOLID, files < 100 lines)."
versions:
  csharp: "12"
  dotnet: "9"
user-invocable: true
references: references/principles.md, references/patterns.md
related-skills: solid-detection

<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

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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/`

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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]/`**

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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 |

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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 |

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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 |