Validate Required on Non Model MVC Dropdownlist
<p>I have a Dropdownlist from a Viewbag and I need to validate it like "Required"</p>

<p>My Controller</p>

<pre><code>public ActionResult EsperaPorHora()
        {
            var cliente = new UsuarioData().Id_LicenciadoPorId(User.Identity.GetUserId());
            var Cli = !string.IsNullOrEmpty(cliente.ToString()) ? Convert.ToInt32(cliente) : 0;
            var cliData = new LicenciadoData();
            var agora = DateTime.Now;
            ViewBag.Data1 = agora.ToShortDateString();
            ViewBag.Data2 = agora.ToShortDateString();

            if (Cli != 0)
            {
                ViewBag.IdCliente = new SelectList(cliData.ListaClienteId(Cli), "Id", "Fantasia");
            }
            else
            {
                ViewBag.IdCliente = new SelectList(cliData.ListarClientes(), "Id", "Fantasia");
            }           
            return View();
        }

        [HttpGet]
        public JsonResult EsperaHora(string data1, string data2, int? cliente)
        {
            var voiceData = new KiperVoiceData(cliente);
            var media = voiceData.GetEsperaData(data1, data2);
            var atend = voiceData.GetEsperaHora(data1, data2);

            var result = new { atend, media };
            return Json(result, JsonRequestBehavior.AllowGet);
        }
</code></pre>

<p>I tried:</p>

<pre><code>@Html.DropDownList("IdCliente", null, "SELECIONE A EMPRESA", htmlAttributes: new { @class = "form-control combo2", @required = "required" })                              

@Html.DropDownList("IdCliente", null, "SELECIONE A EMPRESA", htmlAttributes: new { @class = "form-control combo2", @required = true })                              

@Html.DropDownList("IdCliente", null, "SELECIONE A EMPRESA", htmlAttributes: new { @class = "form-control combo2", required = true })  
</code></pre>

<p>But no one worked to me, if I click button without select it run to an exception. What im doing wrong?</p>