ASP.NET Core: Using fluent validations in your api controllers

April 21, 2020

If you need to validate the inputs to your api actions I really do recommend trying the FluentValidation.AspNetCore package. Its very easy to integrate and you can do quite powerful validations. As an example I created a Person controller.

[Route(api/[controller])]
[ApiController]
public class PersonController : ControllerBase
{

	[HttpPost]
	public async Task<ActionResult> Post([FromBody] PersonViewModel person)
	{
		//Do something
		return Ok();
	}
}

public class PersonViewModel
{
	public string Name { get; set; }
	public int Age { get; set; }
	public string Email { get; set; }
}

In this example I will put all my validation classes in the same executing assembly so I register it in the ConfigureServices method as:

services.AddControllers()
        .AddFluentValidation(fv =>
			fv.RegisterValidatorsFromAssembly(Assembly.GetExecutingAssembly())); 

Then I just need to create a class that implements the AbstractValidator interface.

public class PersonViewModelValidator : AbstractValidator<PersonViewModel>
{
	public PersonViewModelValidator()
	{
		RuleFor(x => x.Age).GreaterThan(18);
		RuleFor(x => x.Email).EmailAddress();
	}
}

This validator makes sure that the age is greater than 18 and the email address is a valid email. It has a lot of validation options so its quite possible to do quite complex validations.

If I was to post the following json to the action:

{	
	"name":"John", 
	"age":19, 
	"email":"someEmail"
}

I get back a JSON result:

{
	"type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
	"title": "One or more validation errors occurred.",
	"status": 400,
	"traceId": "|acba2675-4aa6f07682c5ca37.",
	"errors":{
		"Email":[
			"'Email' is not a valid email address."
		]
	}
}

What is nice is that you don't pollute your controllers will repeated validation code which makes them much cleaner.