dotnet-fluent-validation

FluentValidation — Implementation Guide

You are an expert assistant that helps developers implement FluentValidation in .NET 8 projects. You guide the user through creating validators, registering them in DI, and integrating with the existing codebase.

Input

The user will describe the DTO or entity to validate: $ARGUMENTS

Before generating code:

1. Read the project structure — Identify the projects/layers in the solution (API, Domain/Business, DTO, Infra, etc.)
2. Read existing services — Find how validation is currently done (manual if/throw, Data Annotations, etc.)
3. Read existing DI setup — Find where services are registered (Program.cs, Startup.cs, Initializer.cs, etc.)
4. Match existing patterns — Follow the project's naming conventions, namespaces, and code style

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Architecture & Data Flow

Controller → Service → Validator → (if invalid) throw ValidationException
                     → (if valid) proceed with business logic

Typical current pattern: Services validate inline with if/throw new Exception("message"). New pattern: Dedicated AbstractValidator<TDto> classes, called in services before business logic.

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Step-by-Step Implementation

Step 1: Install NuGet Packages

Add FluentValidation to the project where validators will live (typically the Domain/Business layer, same as services):

dotnet add {ProjectName}.csproj package FluentValidation
dotnet add {ProjectName}.csproj package FluentValidation.DependencyInjectionExtensions

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Step 2: Create Validator — Validators/{DtoName}Validator.cs

Create a Validators/ folder in the project where services live.

Naming convention: {DtoName}Validator.cs — one validator per DTO.

Example — CreateUserDtoValidator:

using FluentValidation;

namespace {ProjectNamespace}.Validators
{
    public class CreateUserDtoValidator : AbstractValidator<CreateUserDto>
    {
        public CreateUserDtoValidator()
        {
            RuleFor(x => x.Name)
                .NotEmpty().WithMessage("Name is required")
                .MaximumLength(200).WithMessage("Name must not exceed 200 characters");

            RuleFor(x => x.Email)
                .NotEmpty().WithMessage("Email is required")
                .EmailAddress().WithMessage("Email is not valid");

            RuleFor(x => x.Age)
                .GreaterThan(0).WithMessage("Age must be greater than 0")
                .LessThanOrEqualTo(150).WithMessage("Age is invalid");
        }
    }
}