Fluent Validation - How to use .NET library for building strongly-type validations rules in MVC Project (C#)

Fluent Validation is a validation library for .NET that uses a fluent interface to construct strongly-typed validation rules.

Steps.

1- Create a new project ASP .NET Web Application (.NET Framework) -> MVC project

2- Install the latest version through the Package Manager Console with the following command

Install-Package FluentValidation -Version 8.6.1

3-Create a new class Customer.

  public class Customer
  {
    public string Surname { get; set; }
    public string Forename { get; set; }
    public bool HasDiscount { get; set; }
    public decimal? Discount { get; set; }	
    public string Address { get; set; }
    public string Postcode { get; set; }
  
  }

4- Create a new validator for Customer.

 public class CustomerValidator : AbstractValidator<Customer>
    {
        public CustomerValidator()
        {
            RuleFor(x => x.Surname).NotEmpty();
            RuleFor(x => x.Forename).NotEmpty().WithMessage("Please specify a first name");
            RuleFor(x => x.Discount).NotEqual(0).When(x => x.HasDiscount);
            RuleFor(x => x.Address).Length(20, 250);
            RuleFor(x => x.Postcode).Must(BeAValidPostcode).WithMessage("Please specify a valid postcode");
        }

        private bool BeAValidPostcode(string postcode)
        {
            // custom postcode validating logic goes here
            return !string.IsNullOrEmpty(postcode) && postcode.Length <= 4;
        }
    }

So, add "using FluentValidations" and "using Customer model".

5- Create a new controller for Customer class and a Create method (HTTP GET, HTTP POST)
Use ValidationResult to validate the model and add validation errors in ModelState.

public class CustomerController : Controller
  {
    // GET: Customer
    public ActionResult Index()
    {
      return View();
    }

    public ActionResult Create()
    {

      return View();
    }

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Create(Models.Customer model)
    {			

      CustomerValidator validator = new CustomerValidator();

      ValidationResult validationResults = validator.Validate(model);
    
      if (!validationResults.IsValid)
      {
        foreach (var error in validationResults.Errors)
        {
          ModelState.AddModelError(error.PropertyName, error.ErrorMessage);

        }
      }

      if (ModelState.IsValid)
      {			

        return RedirectToAction("MyAction", "MyController");
      }

      return View("Create", model);

    }
  }

6- Create a new strongly-typed view for Customer.

7- Then, click on the Create button of our view, we will get the following result.

So, that's it for this short post. I hope it helps you learn a little about how to use strongly typed validation rules.

Thank you for reading!

See you next time.