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;