FluentValidation C# (.NET): Methods, Examples…

What is FluentValidation and why is it needed?

FluentValidation is a popular library for .NET, designed for building strict and readable validation rules for business objects. Unlike built-in validation attributes (Data Annotations), FluentValidation offers an approach based on separating validation logic from the data model. This makes the code cleaner, more testable, and more flexible.

The library uses a "Fluent Interface", which allows building chains of validation rules using dot notation. FluentValidation supports complex scenarios: cascade validations, conditional rules, custom validators, working with lists and nested objects.

Why is FluentValidation needed?

Separation of concerns: Validation rules do not clutter the data model.
Readability: Rule chains are easy to read and maintain.
Flexibility: Easily create complex, conditional, and composite rules.
Testability: Validators can be tested like regular classes.
Integration: Integrates perfectly with ASP.NET Core, MVC, Web API, and other frameworks.

Installing FluentValidation

The library is installed via the NuGet package manager. Execute one of the following commands in the Package Manager Console or in the .NET CLI terminal:

// Installation via Package Manager ConsoleInstall-Package FluentValidation

// Installation via .NET CLIdotnet add package FluentValidation

For integration with ASP.NET Core, an additional package is required:

dotnet add package FluentValidation.AspNetCore

Quick start: code example

Suppose we have a user model:

public class User{    public int Id { get; set; }    public string Name { get; set; }    public string Email { get; set; }    public int Age { get; set; }    public string Password { get; set; }    public string ConfirmPassword { get; set; }}

Let's create a validator by inheriting from AbstractValidator<User>:

using FluentValidation;

public class UserValidator : AbstractValidator<User>{    public UserValidator()    {        RuleFor(user => user.Name)            .NotEmpty().WithMessage("Name is required")            .Length(2, 50).WithMessage("Name must be between 2 and 50 characters");

        RuleFor(user => user.Email)            .NotEmpty().WithMessage("Email is required")            .EmailAddress().WithMessage("Invalid email format");

        RuleFor(user => user.Age)            .InclusiveBetween(18, 120).WithMessage("Age must be between 18 and 120");

        RuleFor(user => user.Password)            .NotEmpty().WithMessage("Password is required")            .MinimumLength(6).WithMessage("Password must be at least 6 characters");

        RuleFor(user => user.ConfirmPassword)            .Equal(user => user.Password).WithMessage("Passwords do not match");    }}

Using the validator:

var validator = new UserValidator();var user = new User{    Name = "A",    Email = "invalid-email",    Age = 15,    Password = "123",    ConfirmPassword = "456"};

var result = validator.Validate(user);

if (!result.IsValid){    foreach (var error in result.Errors)    {        Console.WriteLine($"Property: {error.PropertyName}, Error: {error.ErrorMessage}");    }}