[SOLVED] SelectList always null on POST causes Model State to be Invalid

Issue

I understand that the SelectListItems are meant to not return back the items on post and perhaps be null, but because of this my ModelState.IsValid condition is always false and the Studies field which contains the list is the one with the invalid attribute. I tried different methods and am stuck on resolving this

View

 <label asp-for="Studies" class="form-label"></label>
 <select asp-for="SelectedStudyGuid" asp-items="@Model.Studies" class="form-control" autocomplete="off">
   <option value="">Select Study</option>
 </select>
 <span asp-validation-for="SelectedStudyGuid" class="text-danger"></span>

ViewModel

public class AddStudyViewModel
{
    public string ParticipantGuid { get; set; }

    public IEnumerable<SelectListItem> Studies { get; set; }
    public string SelectedStudyGuid { get; set; }
    
    [DataType(DataType.Date)]
    public DateTime? StartDate { get; set; }
    
    [DataType(DataType.Date)]
    public DateTime? EndDate { get; set; }
}

Controller

  private List<SelectListItem> GetStudyCatalog()
    {
        return _repo.GetStudyCatalog().Select(s => new SelectListItem
        {
            Text = s.Name,
            Value = s.Guid
        }).ToList();
    }

    [HttpGet]
    public IActionResult AddStudy(string participantGuid)
    {
        var model = new AddStudyViewModel
        {
            Studies = GetStudyCatalog(),
            ParticipantGuid = participantGuid
        };
        
        return PartialView("_AddStudyModal", model);
    }

    [HttpPost]
    public IActionResult AddStudy(AddStudyViewModel model)
    {
        if (ModelState.IsValid)
        {
           var response= _repo.AddStudy(model.SelectedStudyGuid, model.ParticipantGuid, model.StartDate, model.EndDate);
           
            if (response.Success)
            {
                TempData["Message"] = response.Message;
                return RedirectToAction("EditParticipant", new {id = response.NewGuid});
            }
        }

        model.Studies = GetStudyCatalog();
        return PartialView("_AddStudyModal", model);
    }

Solution

This can happen only if you use net 6. To fix it simply make Studies nullable

public List<SelectListItem>? Studies { get; set; }

or what is the smart developers do , they just remove nullables from the projests

 <TargetFramework>net6.0</TargetFramework>
    <!--<Nullable>enable</Nullable>-->

to use a ViewBag instead of a ViewModel is very unprofessional

Answered By – Serge

Answer Checked By – Jay B. (BugsFixing Admin)