Dotnet Fluentvalidation

FluentValidation

Overview

FluentValidation provides a fluent interface for building validation rules in .NET applications. This skill should be invoked when validating input models, commands, DTOs, or implementing complex validation logic with clean, readable code.

Core Principles

• Fluent API: Chain validation rules for readability
• Separation: Keep validators in dedicated classes
• Composition: Combine validators with And/Or
• Integration: Works seamlessly with MediatR

Preparation Checklist

• Install FluentValidation: dotnet add package FluentValidation
• Install ASP.NET integration: dotnet add package FluentValidation.AspNetCore
• Plan validation for each input model
• Set up dependency injection

Step-by-Step Process

1. Install: Add FluentValidation packages
2. Create: Make validator class inheriting from AbstractValidator
3. Define: Add validation rules in constructor
4. Register: Add to DI container
5. Integrate: Use in controller or MediatR pipeline
6. Handle: Process validation errors

Do's and Don'ts

• ✅ Do create one validator per type
• ✅ Do use custom validators for complex rules
• ✅ Do integrate with MediatR pipeline
• ❌ Don't duplicate validation logic
• ❌ Don't put business logic in validators
• ❌ Don't skip registration in DI

Anti-Patterns

• Duplicate Logic: Validating in multiple places
• Business Logic: Including business rules in validators
• No Integration: Not connecting validators to pipeline
• Complex Chains: Overly complex rule chains