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;