Using FluentValidation for Model Validation in ASP.NET Core

ASP.NET Core provides built‑in model validation via attributes in System.ComponentModel.DataAnnotations , but its default validation is limited to controller actions and can be intrusive. FluentValidation offers a more powerful, non‑intrusive alternative that works in any scenario.

First, install the FluentValidation.AspNetCore NuGet package. Define the model to be validated, for example:

public class Student
{
    public int Id { get; set; }
    public int Age { get; set; }
    public string Name { get; set; }
}

Create a validator that inherits from AbstractValidator<Student> :

public class StudentValidator : AbstractValidator<Student>
{
    public StudentValidator()
    {
        RuleFor(x => x.Age).InclusiveBetween(10, 50);
        RuleFor(x => x.Name).NotEmpty().MaximumLength(5);
    }
}

Register the validator in Startup.ConfigureServices :

services.AddControllers()
        .AddFluentValidation(conf =>
        {
            conf.RegisterValidatorsFromAssemblyContaining<StudentValidator>();
            conf.RunDefaultMvcValidationAfterFluentValidationExecutes = false;
        });

In a controller, add an action that receives the model:

[HttpPost]
public IActionResult Add([FromBody] Student student)
{
    return Ok(student);
}

When validation fails, ASP.NET Core returns a 400 response with a JSON payload describing the errors. FluentValidation allows customizing these messages using placeholders, WithMessage , WithName , and conditional rules such as When , Unless , and ApplyConditionTo.CurrentValidator .

Advanced features include chaining multiple validators, setting CascadeMode to stop further checks after a failure, defining dependent rules, asynchronous validation with MustAsync , value transformation with Transform , failure callbacks via OnFailure , and overriding PreValidate for pre‑validation logic.

The article demonstrates both controller‑based and non‑controller scenarios, showing how to inject IValidator<Student> into services and invoke Validate or ValidateAsync manually.