Form submitting DropDownListFor with Required field - asp.net-mvc

Question

I have a dropdownlist from a table called StampsCSEMonths_test. Im trying to make the dropdown required before submitting the form.
The problem it always submits the form no matter if I select or not.
What could be the issue?
View:
#model WINHRAdmin.Models.CTS_Stamps
#using (Html.BeginForm())
{
#Html.AntiForgeryToken()
#Html.ValidationSummary(true, "", new { #class = "text-danger" })
#Html.DropDownListFor(model => model.stampscsemonths_id, ViewBag.StampsCSEMonths_test as IEnumerable<SelectListItem>, "", new { #class = "form-control " })
#Html.ValidationMessageFor(model => model.stampscsemonths_id, "", new { #class = "text-danger" })
}
Model:
namespace WINHRAdmin.Models
{
public class CommitmentToServiceModel
{
}
public class CTS_Stamps
{
[Required]
[Range(1,Int32.MaxValue,ErrorMessage ="MonthYerar is required")]
public int? stampscsemonths_id { get; set; }
}
}
Controller:
public ActionResult CTS_Stamps()
{
ViewData["StampsCSEMonths_test"] =
new SelectList((from s in _IntranetEntities.StampsCSEMonths_test.OrderByDescending(x => x.stampscsemonths_id).ToList()
select new
{
stampscsemonths_id = s.stampscsemonths_id,
FullName = s.month + "/" + s.year
}),
"stampscsemonths_id",
"FullName",
null);
LoadEmployeer();
return View();
}
[HttpPost]
public ActionResult CTS_Stamps(FormCollection collection)
{
var testvar = collection["stampscsemonths_id"];//after submission it does get the value of ID
return View();
}

Answer 1

Typically you would want to use a client-side library like jquery.validate to prevent form submission if it's invalid. Here is a good article on the subject: https://www.codeproject.com/Articles/718004/ASP-NET-MVC-Client-Side-Validation.
The usual pattern for server-side MVC validation is almost what you're doing. You put the [Required] attribute above your data model attribute. That's good. You have a #ValidationMessageFor element in your view. Also good.
You forgot, however, to check if the model is valid in your controller. Add a check for ModelState.IsValid before you process the results. Something like this:
if (!ModelState.IsValid)
{
return View(collection); // assumes collection is the model
}
If the user submits an invalid form, after the round-trip they will see a validation error message next to the missing field.
Hope that helps.