fitness-functions

# Fitness Functions

## When to Use This Skill

Use this skill when you need to:

- Enforce architectural boundaries between modules
- Test that dependencies follow prescribed rules
- Validate layer constraints (e.g., no UI → Domain)
- Create performance fitness functions
- Generate architecture test code
- Audit existing architecture for violations

**Keywords:** fitness functions, architecture tests, NetArchTest, ArchUnitNET, dependency rules, layer constraints, architectural boundaries, module isolation, architecture validation, performance tests

## What Are Fitness Functions?

Fitness functions are automated tests that validate architectural characteristics. They provide objective, repeatable verification that the system maintains desired properties as it evolves.

### Types of Fitness Functions

| Type | Validates | Example |
| --- | --- | --- |
| **Dependency** | Component relationships | "Domain cannot depend on Infrastructure" |
| **Layer** | Vertical slice rules | "Controllers only call Application layer" |
| **Naming** | Convention compliance | "Handlers must end with 'Handler'" |
| **Performance** | Runtime characteristics | "API response < 200ms at p95" |
| **Cyclomatic** | Code complexity | "No method > 10 cyclomatic complexity" |

## Quick Start

### 1. Install Required Package

```bash
# For NetArchTest (simpler, recommended for most cases)
dotnet add package NetArchTest.Rules

# For ArchUnitNET (more powerful, Java-like syntax)
dotnet add package ArchUnitNET
dotnet add package ArchUnitNET.xUnit  # or .NUnit
```

### 2. Create Test Project

```bash
dotnet new xunit -n YourSolution.ArchitectureTests
dotnet add YourSolution.ArchitectureTests reference src/YourSolution.Domain
dotnet add YourSolution.ArchitectureTests reference src/YourSolution.Application
dotnet add YourSolution.ArchitectureTests reference src/YourSolution.Infrastructure
```

### 3. Write First Test

```csharp
public class DependencyTests
{
    [Fact]
    public void Domain_ShouldNotDep