dotnet-fluent-validation

# FluentValidation Rules Generator

## Overview

FluentValidation provides a fluent interface for building strongly-typed validation rules:

- **Declarative rules** - Readable, maintainable validation logic
- **Separation of concerns** - Validation separate from domain
- **Integration with MediatR** - Automatic validation via pipeline behavior
- **Custom validators** - Reusable validation components

## Quick Reference

| Validator Type | Purpose | Example |
|----------------|---------|---------|
| Built-in | Common validations | `NotEmpty()`, `MaximumLength()` |
| Custom | Reusable rules | `Must(BeValidEmail)` |
| Async | Database checks | `MustAsync(BeUniqueEmail)` |
| Child | Nested objects | `SetValidator(new AddressValidator())` |
| Collection | List items | `RuleForEach(x => x.Items)` |

---

## Validator Structure

```
/Application/{Feature}/
├── Create{Entity}/
│   ├── Create{Entity}Command.cs
│   └── Create{Entity}CommandValidator.cs    # Or inline in Command.cs
├── Update{Entity}/
│   └── Update{Entity}Command.cs             # Validator inline
└── Validators/
    ├── EmailValidator.cs                     # Reusable validators
    └── PhoneNumberValidator.cs
```

---

## Template: Basic Command Validator

```csharp
// src/{name}.application/{Feature}/Create{Entity}/Create{Entity}CommandValidator.cs
using FluentValidation;

namespace {name}.application.{feature}.Create{Entity};

public sealed class Create{Entity}CommandValidator : AbstractValidator<Create{Entity}Command>
{
    public Create{Entity}CommandValidator()
    {
        // ═══════════════════════════════════════════════════════════════
        // STRING VALIDATIONS
        // ═══════════════════════════════════════════════════════════════
        
        RuleFor(x => x.Name)
            .NotEmpty()
                .WithMessage("{Entity} name is required")
            .MaximumLength(100)
                .WithMessage("{Entity} name must not exceed 100 characters")
            .MinimumLength(2)
                .WithMessage("{Entity} name must be at least 2 characters");

        RuleFor(x => x.Description)
            .MaximumLength(500)
                .WithMessage("Description must not exceed 500 characters")
            .When(x => !string.IsNullOrEmpty(x.Description));

        // ═══════════════════════════════════════════════════════════════
        // GUID VALIDATIONS
        // ═══════════════════════════════════════════════════════════════
        
        RuleFor(x => x.OrganizationId)
            .NotEmpty()
                .WithMessage("Organization ID is required")
            .NotEqual(Guid.Empty)
                .WithMessage("Organization ID cannot be empty GUID");

        // ═══════════════════════════════════════════════════════════════
        // OPTIONAL FOREIGN KEY
        // ═══════════════════════════════════════════════════════════════
        
        RuleFor(x => x.ParentId)
            .NotEqual(Guid.Empty)
                .WithMessage("Parent ID cannot be empty GUID")
            .When(x => x.ParentId.HasValue);
    }
}
```

---

## Template: Inline Validator (Preferred Pattern)

```csharp
// src/{name}.application/{Feature}/Create{Entity}/Create{Entity}Command.cs
using FluentValidation;
using {name}.application.abstractions.messaging;
using {name}.domain.abstractions;

namespace {name}.application.{feature}.Create{Entity};

// ═══════════════════════════════════════════════════════════════
// COMMAND
// ═══════════════════════════════════════════════════════════════
public sealed record Create{Entity}Command(
    string Name,
    string? Description,
    Guid OrganizationId,
    string Email,
    decimal Amount,
    List<CreateItemRequest> Items) : ICommand<Guid>;

public sealed class CreateItemRequest
{
    public required string Name { get; init; }
    public int Quantity { get; init; }
}

// ═══════════════════════════════════════════════════════════════
// VALIDATOR (internal, same file)
// ═══════════════════════════════════════════════════════════════
internal sealed class Create{Entity}CommandValidator : AbstractValidator<Create{Entity}Command>
{
    public Create{Entity}CommandValidator()
    {
        RuleFor(x => x.Name)
            .NotEmpty()
            .MaximumLength(100);

        RuleFor(x => x.Email)
            .NotEmpty()
            .EmailAddress()
                .WithMessage("A valid email address is required");

        RuleFor(x => x.Amount)
            .GreaterThan(0)
                .WithMessage("Amount must be positive")
            .LessThanOrEqualTo(1_000_000)
                .WithMessage("Amount cannot exceed 1,000,000");

        RuleFor(x => x.Items)
            .NotEmpty()
                .WithMessage("At least one item is required")
            .Must(items => items.Count <= 100)
                .WithMessage("Cannot have more than 100 items");

        RuleForEach(x => x.Items)
            .ChildRules(item =>
            {
                item.RuleFor(i => i.Name)
                    .NotEmpty()
                    .MaximumLength(200);

                item.RuleFor(i => i.Quantity)
                    .GreaterThan(0)
                    .LessThanOrEqualTo(10000);
            });
    }
}

// ═══════════════════════════════════════════════════════════════
// HANDLER
// ═══════════════════════════════════════════════════════════════
internal sealed class Create{Entity}CommandHandler 
    : ICommandHandler<Create{Entity}Command, Guid>
{
    // ... implementation
}
```

---

## Template: Async Validator with Database Check

```csharp
// src/{name}.application/{Feature}/Create{Entity}/Create{Entity}CommandValidator.cs
using FluentValidation;
using {name}.domain.{aggregate};

namespace {name}.application.{feature}.Create{Entity};

internal sealed class Create{Entity}CommandValidator : AbstractValidator<Create{Entity}Command>
{
    private readonly I{Entity}Repository _{entity}Repository;
    private readonly IOrganizationRepository _organizationRepository;

    public