SelectList dropdown list showing multiple = "multiple" for current viewmodel - asp.net-core

Question

This kind of a bizarre issue and I can't figure out a solution how I want.
I'm using .net core 2.1. I have a orders view model like this:
public class OrdersFilterViewModel
{
[Display(Name = "Account Numbers:")]
public IEnumerable<SelectListItem> AccountNumbers { get; set; }
}
My viewmodel and SelectList in my orders controller is called like this:
var vm = new OrdersFilterViewModel
{
AccountNumbers = new SelectList(_context.Account.Where(m => m.UserID == userId), "AccountNumber", "AccountNumber", account)
};
return PartialView("_FilterOrders", vm);
The problem lies when trying to get a dropdown list in the view which looks like this:
<form asp-action="FilterOrders" asp-controller="Order" id="ordersFilterForm" method="post">
<div class="form-group">
<label asp-for="AccountNumbers" class="control-label"></label>
<select asp-for="AccountNumbers" class="form-control" asp-items="#Model.AccountNumbers">
</select>
</div>
<div class="form-group">
<input type="submit" value="Submit" class="btn btn-default" />
</div>
</form>
This somewhat works but gives me a textarea type display where multiple = "multiple" is always tacked on in the browser. I've discovered that if I add something like the following to my viewmodel:
public int? AccountId { get; set; }
Then change my view to:
<select asp-for="AccountId" class="form-control" asp-items="#Model.AccountNumbers">
I can then have my dropdown list. However, I don't need that property for anything as far as I know. I tried a million things so it's possible I made some other slight changes I'm forgetting to get that to work, but that's the gist of it.
Is there any way around adding that extra property? Or do I need it for something I'm not aware of? Or is there any way to set multiple = "false" or something to that effect so I can get my dropdown list with my original viewmodel and such?
I haven't dealt with the post back to the controller yet, so maybe that will reveal the gotchas. I'm basically trying to create a modal type query filter that doesn't really do much other than modify some parameters and send them back to my query to update it. Thanks.

Answer 1

Is there any way around adding that extra property? Or do I need it
for something I'm not aware of?
Yes, you need this extra property, because in your select there are many items, and the user will select one or multiple items, and on the server side you'll need to know what the user selected, this is the purpose of the select tag.
And the multiple = "multiple" depends on what you put in the asp-for in the case of asp-for="AccountId" it is a single int value, so it won't use multiple, is you have an array in the asp-for then it will use the multiple.
Here is a pretty detailed description about the select tag helper:
Select Tag Helper in ASP.NET Core MVC