using FluentValidation;

public class CustomerValidator: AbstractValidator<Customer> {

  public CustomerValidator() {

    RuleFor(customer => customer.Surname).NotEmpty();

    RuleFor(customer => customer.Forename).NotEmpty().WithMessage("Please specify a first name");

    RuleFor(customer => customer.Discount).NotEqual(0).When(customer => customer.HasDiscount);

    RuleFor(customer => customer.Address).Length(20, 250);

    RuleFor(customer => customer.Postcode).Must(BeAValidPostcode).WithMessage("Please specify a valid postcode");

  }

  private bool BeAValidPostcode(string postcode) {

    // custom postcode validating logic goes here

  }

}

Customer customer = new Customer();

CustomerValidator validator = new CustomerValidator();

ValidationResult results = validator.Validate(customer);

bool validationSucceeded = results.IsValid;

IList<ValidationFailure> failures = results.Errors;