FluentValidation — Strongly Typed Validation Rules for .NET

Introduction

FluentValidation is a .NET library for building strongly typed validation rules using a fluent API. It separates validation logic from domain models into dedicated validator classes, producing clean and testable validation code.

What FluentValidation Does

• Defines validation rules with a readable fluent syntax
• Validates model properties with built-in rules (NotEmpty, MaxLength, GreaterThan, etc.)
• Supports conditional validation with When/Unless clauses
• Integrates with ASP.NET Core model validation pipeline
• Provides custom error messages and localization support

Architecture Overview

FluentValidation uses an AbstractValidator base class where each rule is built from a chain of validators attached to a property expression. At validation time, the validator iterates through all rules and collects failures into a ValidationResult. Rules can be composed, conditionally applied, or grouped into rule sets. The ASP.NET Core integration replaces the default DataAnnotations validator with FluentValidation validators resolved from DI.

Self-Hosting & Configuration

• Install via NuGet: dotnet add package FluentValidation
• For ASP.NET Core integration: dotnet add package FluentValidation.AspNetCore
• Register validators in DI: builder.Services.AddValidatorsFromAssemblyContaining<Program>()
• Create one validator class per model or DTO
• Compatible with .NET Standard 2.0+ and .NET 6 through .NET 9

Key Features

• Fluent API that reads like natural language validation rules
• Rule sets for validating different scenarios (create vs. update)
• Async validation support for rules that query databases or APIs
• Custom validators and property validators for complex logic
• Built-in severity levels (Error, Warning, Info) per rule

Comparison with Similar Tools

• Data Annotations — attribute-based validation built into .NET; simpler but less flexible and harder to test
• Vogen — value object generator with built-in validation; focuses on domain primitives, not DTOs
• Guard Clauses — precondition checks that throw exceptions; different purpose, not rule-based
• Humanizer + custom code — manual validation with friendly messages; no framework, more boilerplate
• MiniValidation — minimal validation for .NET minimal APIs; lighter, fewer features

FAQ

Q: Should I use FluentValidation or Data Annotations? A: FluentValidation is better for complex rules, conditional logic, and unit testing. Data Annotations work well for simple property constraints on small models.

Q: Can I use FluentValidation in a MediatR pipeline? A: Yes. Create an IPipelineBehavior that resolves the appropriate validator for each request type and runs it before the handler executes.

Q: Does FluentValidation support async rules? A: Yes. Use MustAsync or CustomAsync to define rules that perform asynchronous operations like database lookups.

Q: Can I localize error messages? A: Yes. FluentValidation supports resource files, custom message providers, and the WithMessage method accepts localization delegates.

Sources

• https://github.com/FluentValidation/FluentValidation
• https://docs.fluentvalidation.net/