DropdownList value doesnt add to database - asp.net

Question

I can display the values from my database both in a dropdownlist and where the value is needed.
But I can't get the value from the dropdownlist to my database while creating something. it's getting null.
I've tried some solutions from s.o.f but they didnt't work.
Models 1:
public class Kategori
{
[Key]
public int KategoriID { get; set; }
public string Namn { get; set; }
}
Models 2:
public class Inlägg
{
[Key]
public int InläggsID { get; set; }
public Kategori Kategori { get; set; }
}
Controller:
// POST: Inlägg/Create
[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Create([Bind("Titel,Text,Kategori.Namn")] Inlägg inlägg)
//The Kategori is getting null
{
if (ModelState.IsValid)
{
inlägg.Datum = DateTime.Now;
_context.Add(inlägg);
await _context.SaveChangesAsync();
return RedirectToAction(nameof(Index));
}
return View(inlägg);
}
View:
#Html.DropDownList("Kategori", null, htmlAttributes: new { #class = "form-control" })
I've tried using SelectItemList, select with options values, having a SelectItem inside Models class also a "public Kategori List" inside Inlägg.
Don't really know how to solve this. I've just tried 8 hours today, and 2 hours yesterday.
How can I get the value that the user choosen in the dropdownlist instead of getting null? Tell me if I need to send more codes :-)

Answer 1

You should change it;
#Html.DropDownList("Kategori", null, htmlAttributes: new { #class = "form-control" })
to
#Html.DropDownList("SelectedCategory", ViewData["Kategori"] as SelectList, htmlAttributes: new { #class = "form-control" })
The selected dropdown element is passed to serverside as SelectedCategory.
Also, I strongly suggest you to use Model classes instead of ViewData to carry data between controller and view.

Answer 2

You need to add another property for the foreign key value. Since your other related entity class name is Kategori, you may name this new property KategoriId so that it matches the convention for the foreign key property names.
public class Inlagg
{
[Key]
public int InläggsID { get; set; }
public string Titel { get; set; }
public string Text { get; set; }
public DateTime Datum { get; set; }
public virtual Kategori Kategori { get; set; }
public int? KategoriId { set;get;} // This is the new property
}
Now in your form inside your view, make sure the select element rendered by the DropDownList helper has the same name attribute value as the new property name (check the view source of the page)
#Html.DropDownList("KategoriId", ViewData["Kategori"] as SelectList)
Now finally, make sure you include this new input name/property name inside the Bind attributes Include list so that the model binder will bind that.
public async Task<IActionResult> Create([Bind(Include="Titel,Text,KategoriId")]
Inlagg inlagg)
{
// to do : your code for saving and returning something
}
Another option is to use a view model with only needed properties, instead of using the Bind attribute with your entity class