If it has no value to compare, save without comparing

Question

I’m trying to make my application save if it has no value to compare, however you are always going through the comparison, I tried to put an if but still problem, this is the class that makes the comparison between my view and my database.

Controller:

[HttpPost]
    public ActionResult Adiciona(RotaModel viewModel)
    { 

            var rotas = ckm.Consulta(viewModel.NumCarroId);
            //  Aqui busca todas as rotas deste veículo

            var maiorRota = rotas.OrderByDescending(r => r.Km).FirstOrDefault();
            //  Aqui você tem a última rota cadastrada, considerando a regra geral  


            if (viewModel.Km < maiorRota.Km)
            {
                ModelState.AddModelError("Km_Atual.Invalido",
                "A quilometragem precisa ser maior que a anterior");
            }
        if (ModelState.IsValid)
            {
            Rota rota = viewModel.CriaRota();
            dao.Adiciona(rota);
            //return View();
            return RedirectToAction("Index");
            }
        else
            {
            ViewBag.Usuarios = usuarioDAO.Lista();
            ViewBag.Veiculo = veiculoDAO.Lista();
            return View("Form", viewModel);
            }
    }

When I try to save without any pre-registered value, it appears the following error.

Description: An unhandled Exception occurred During the Execution of the Current web request. Please review the stack trace for more information about the error and Where it originated in the code.

Exception Details: System.Nullreferenceexception: Object Reference not set to an instance of an Object.

Answer 1

The Enumerable.FirstOrDefault() no argument for default brings the null value when not found a value within your collection. What happens is that if it does not find anything you will be accessing a property (.km) of a null reference, this will cause your NullReferenceException

You need to validate when there is no major route to make the comparison. An example of how to validate:

var maiorRota = rotas.OrderByDescending(r => r.Km).FirstOrDefault();

if (maiorRota != null && viewModel.Km < maiorRota.Km)
{
    ModelState.AddModelError("Km_Atual.Invalido",
            "A quilometragem precisa ser maior que a anterior");
}