How to return a message warning that there is already a record?

Question

I have a method in Cepcontroller Register that before making the registration makes some validations and between them, checks if there is already registration for that Zip Code. I would like to implement a notice to inform the user about the existence of registration without giving a Post on the page.

Controller method:

public IActionResult CadastrarCep(CepViewModel cep)
        {
            using (var sqlConnection = new SqlConnection(_config.GetConnectionString("DBConect")))
            {
                if (!ModelState.IsValid)
                {
                    return View("Cadastro", cep);
                }

                if (cep.TakeCepById(cep.CepCodigo, _config) != null)
                {
                    ModelState.AddModelError("Cep", "Cep já existente.");
                  //  return View("Cadastro", cep);

                }
                cep.CepSetor = 0;
                cep.CepRegiao = "";
                cep.CepRota = "";

                StringBuilder strSql = new StringBuilder ("INSERT INTO Cep (CepCodigo, CepLogr, CepEnd, CepCompl, CepBairro, CepCidade, CepUF, CepSetor, CepRegiao, CepRota) ");
                strSql.Append(" VALUES( @CepCodigo, @CepLogr, @CepEnd, @CepCompl, @CepBairro, @CepCidade, @CepUF, @CepSetor, @CepRegiao, @CepRota)");
                sqlConnection.Execute(strSql.ToString(), cep);


                return View("Cadastro", new CepViewModel());

            }
        }

View:

@model WebApplication1.Models.CepViewModel
<h2>Manutenção de Cep</h2>
<div class="container" style="margin-top: 50px;">
    <form asp-controller="Cep" asp-action="Cadastro" method="post">
        <div class="form-group">
            <label class="control-label">CEP</label>
            <input asp-for="CepCodigo" maxlength="8" min="8" class="form-control" type="text" />
            <span asp-validation-for="CepCodigo"  class="text-danger"></span>
        </div>

        <div class="form-group">
            <label class="control-label">LOGRADOURO</label>
            <input asp-for="CepLogr"  class="form-control" type="text"  value="@Model.CepLogr" />
        </div>
        <div class="form-group">
            <label class="control-label">ENDEREÇO</label>
            <input asp-for="CepEnd"  class="form-control" type="text"  value="@Model.CepEnd" />
        </div>
        <div class="form-group">
            <label class="control-label">COMPLEMENTO</label>
            <input asp-for="CepCompl"  class="form-control" type="text"  value="@Model.CepCompl" />
        </div>
        <div class="form-group">
            <label class="control-label">BAIRRO</label>
            <input asp-for="CepBairro"  class="form-control" type="text"  value="@Model.CepBairro" />
        </div>
        <div class="form-group">
            <label class="control-label">CIDADE</label>
            <input asp-for="CepCidade"  class="form-control" type="text"  value="@Model.CepCidade" />
        </div>
        <div class="form-group">
            <label class="control-label">UF</label>
            <input asp-for="CepUF"  class="form-control" type="text"  value="@Model.CepUF" />
        </div>
        <div class="form-group">
            <input type="submit" class="btn btn-primary" value="Consultar" asp-action="ConsultarCep" />
            <input type="submit" class="btn btn-default" value="Cadastrar" asp-action="CadastrarCep" />
            <input type="submit" class="btn btn-danger" value="Excluir" asp-action="ExcluirCep" />
            <input type="submit" class="btn btn-success" value="Alterar" asp-action="AlterarCep" />

        </div>
</div>
</div>
</form>

Answer 1

You should add the error state before checking if the model is valid. And the error should be pointed to some attribute of your model, after seeing your view, it seems to me that CepCodigo would be the most appropriate.

public IActionResult CadastrarCep(CepViewModel cep)
{
    using (var sqlConnection = new SqlConnection(_config.GetConnectionString("DBConect")))
    {
        if (cep.TakeCepById(cep.CepCodigo, _config) != null)
        {
            ModelState.AddModelError("CepCodigo", "Cep já existente.");                      

        }

        if (!ModelState.IsValid)
        {
            return View("Cadastro", cep);
        }

        cep.CepSetor = 0;
        cep.CepRegiao = "";
        cep.CepRota = "";

        StringBuilder strSql = new StringBuilder ("INSERT INTO Cep (CepCodigo, CepLogr, CepEnd, CepCompl, CepBairro, CepCidade, CepUF, CepSetor, CepRegiao, CepRota) ");
        strSql.Append(" VALUES( @CepCodigo, @CepLogr, @CepEnd, @CepCompl, @CepBairro, @CepCidade, @CepUF, @CepSetor, @CepRegiao, @CepRota)");
        sqlConnection.Execute(strSql.ToString(), cep);


        return View("Cadastro", new CepViewModel());

    }
}