Custom Validators

public class PasswordValidator : IValidatorRuleSync<RegisterRequest>
{
    public string Name => "Password";
    public string ErrorMessage => "Password must be at least 8 characters";

    public ValidationResult<RegisterRequest> Validate(RegisterRequest request) =>
        request.Password.Length >= 8
            ? ValidationResult<RegisterRequest>.Success(request)
            : ValidationResult<RegisterRequest>.Failure(
                new ValidationError("Password", ErrorMessage));
}

var rules = new ValidatorRuleBuilder<RegisterRequest>()
    .AddRule(new PasswordValidator())
    .Build();