Fluent Validation

Program.cs

static void Main(string[] args)
{
    var user = new User()
        {
            CreateInvoice = true,
            Nip = "123",
            Email = "daniel@plawgo.pl",
            Orders = new List<Order>()
            {
                new Order(){Product = "Laptop"},
                new Order(){Cost = 14}
            }
        }
        ;
 
    var validator = new UserValidator(new UserLogic());
 
    var validationResult = validator.Validate(user);
 
    if (validationResult.IsValid == false)
    {
        foreach (var error in validationResult.Errors)
        {
            Console.WriteLine("{0}: {1}", error.PropertyName, error.ErrorMessage);
        }
    }
}

User.cs

public class User
{
    public User()
    {
        Address = new Address();
    }
 
    [Display(Name = "Nazwa użytkownika")]
    public string Name { get; set; }
 
    public string Email { get; set; }
 
    public bool CreateInvoice { get; set; }
 
    public string Nip { get; set; }
 
    public Address Address { get; set; }
 
    public List<Order> Orders { get; set; }
}

UserValidator.cs

public class UserValidator : AbstractValidator<User>
{
    public UserValidator(IUserLogic userLogic)
    {
        RuleFor(u => u.Name)
            .NotEmpty();
 
        RuleFor(u => u.Email)
            .NotEmpty()
            .WithMessage(UserResources.EmailIsRequiredError);
 
        RuleFor(u => u.Email)
            .EmailAddress()
            .WithMessage("Email ma niepoprawny format");
 
        When(u => u.CreateInvoice, () =>
        {
            RuleFor(u => u.Nip)
                .Cascade(CascadeMode.StopOnFirstFailure)
                .NotEmpty()
                .WithMessage("Nip jest wymagany")
                .Must(nip => IsNipValid(nip))
                .WithMessage("Nip ma niepoprawny format") ;
        });
                  
        RuleFor(u => u.Email)
            .Must(email => userLogic.Exist(email))
            .WithMessage(u => string.Format("Użytkownik o adresie {0} już istnieje.", u.Email));
 
        RuleFor(u => u.Address)
            .SetValidator(new AddressValidator());
 
        RuleFor(u => u.Orders)
                .SetCollectionValidator(new OrderValidator());
    }
 
    private bool IsNipValid(string nip)
    {
        if (nip == null)
        {
            return false;
        }
 
        if (nip.Length != 10)
        {
            return false;
        }
 
        return true;
    }
}