Have Selected Dropdown Appear on Edit Razor Page

Question

NOTE: REVISED FROM ORIGINAL POST. SOME OF THE MODEL/PROPERTY names have changed with more editing.

I have a create page which will utilizes a couple of dropdown lists. For the edit page, I would like the selected value from the create page to appear as the selected dropdown choice, but would still like it to be a dropdown list of all values so that users could select a new value if the value selected on the create page was incorrect.

For example, if a user selected 'peaches' from a dropdown list of 'apples', 'peaches' and 'pears' on the create page and posted it, when they go to the edit page, I would like the dropdown list to show 'peaches', as the selected value, but allow the user to pick 'apples' or 'pears' if they wished to change it.

Here is what I have on my edit.cshtml page for my dropdown lists (both are set up the same way):

<td style="width: 25%">
   <div class="mb-3">
      <label asp-for="InfoSite.Specialty"></label>
      <select asp-for="InfoSite.Specialty" id="Select1" class="form-select"> 
         <option value="">---Select Specialty---</option>
         @foreach(var item in Model.DisplayMDSpecialtyData)
             {
               <option value="@item.ID" selected="@(item.ID.ToString()==Model.InfoSite.Specialty?true:false)" disabled="@(item.ID.ToString()==Model.InfoSite.Specialty?true:false)">@item.Description</option>
             }
       </select>
   </div>
</td>
<td style="width: 25%">
   <div class="mb-3">
       <label asp-for="InfoSite.ResType"></label>                      
       <select asp-for="InfoSite.ResType" id="Select2" class="form-select">
           <option value="">---Select Residence Type---</option>
           @foreach(var item in Model.DisplayResTypeData)
              {
                 <option value="@item.ID" selected="@(item.ID.ToString()==Model.InfoSite.ResType?true:false)" disabled="@(item.ID.ToString()==Model.InfoSite.ResType?true:false)">@item.Description</option>
              }
        </select>
    </div>
</td>

And here is what I have on my edit.cshtml.cs (includes info for the 2 dropdowns):

namespace PHECWeb.Pages.InfoSites;

[BindProperties]

public class EditModel : PageModel
{
    private readonly ApplicationDbContext _db;
    public InfoSite InfoSite { get; set; }
    public EditModel(ApplicationDbContext db)
    {
        _db = db;
    }

    public IEnumerable<ResType> DisplayResTypeData { get; set; }
    public IEnumerable<MDSpecialty> DisplayMDSpecialtyData { get; set;  }

    public async void OnGet(int? ID)
    {
        InfoSite = _db.InfoSite.Find(ID);
        DisplayResTypeData = await _db.ResType.ToListAsync();
        DisplayMDSpecialtyData = await _db.MDSpecialty.ToListAsync();
    }

    public async Task<IActionResult> OnPost()
    {
        if (ModelState.IsValid)
        {
            _db.InfoSite.Update(InfoSite);
            await _db.SaveChangesAsync();
            TempData["success"] = "Site Information updated successfully.";
            return RedirectToPage("Index");
        }
        return Page();
    }
}

I am getting the dreaded System.NullReferenceException: 'Object reference not set to an instance of an object.' on the @foreach(var item in Model.DisplayMDSpecialtyData).

I desperately need help to get past this. :( Would someone please be able to add some insight (in layman's terms as I am a newbie to all of this) as to what edits I would need to this code to rid myself of this error? I have Googled and YouTubed, but because my knowledge level is so low at this point, I don't understand much of what I am seeing. I understand the concept, I just don't understand the placement of a fix (view vs .cs) or the exact syntax of a fix. Trying my best to learn and as stated above, asking the question was only after MUCH searching on my own. Please be kind.

Answer 1

This was another stumper with a very simple change to make the code work.

<option value="@item.ID" selected="@(item.ID.ToString()==Model.InfoSite.Specialty?true:false)" disabled="@(item.ID.ToString()==Model.InfoSite.Specialty?true:false)">@item.Description</option>

Just needed the "disabled=..." removed to look like this:

<option value="@item.ID" selected="@(item.ID.ToString()==Model.InfoSite.Specialty?true:false)">@item.Description</option>

And then the selected value displays on the Edit page just fine.