MVC List<SelectListItem> with groups creating optgroup for each item

Question

I'm having an issue where I'm creating a List<SelectListItem> with optgroups but instead of creating an optgroup per group of SelectListItem it's creating a new SelectListGroup per SelectListItem. It's got me a bit confused because there aren't any duplicated SelectListGroup's in my code.

Here is an example:

Expected Result:

<select datatag="data-States=''" class="form-control filter-select" data-multi-select="" id="States" multiple="multiple" name="States">
    <optgroup label="MA">
    <option value="01602">01602</option>
    <option value="02743">02743</option>
    <option value="01107">01107</option>
    </optgroup>
    </select>

Actual Result:

<select datatag="data-States=''" class="form-control filter-select" data-multi-select="" id="States" multiple="multiple" name="States">
<optgroup label="MA">
<option value="01602">01602</option>
</optgroup>
<optgroup label="MA">
<option value="02743">02743</option>
</optgroup>
<optgroup label="MA">
<option value="01107">01107</option>
</optgroup>
</select>

Method:

 public ManifestFilterDropDownItem ReturnManifestFilterDataBasedOnTotalDataSet(IEnumerable<ManifestTableItem> data, bool isUserASR) {
            IEnumerable<SelectListGroup> stateGroups = data.Select(x => x.AddrState.ToUpper()).Distinct().Select(x => new SelectListGroup() {
                Name = x
            });

            IList<SelectListItem> stateZipSelectListItems = data.GroupBy(x => x.AddrZip).Select(x => new SelectListItem() {
                Text = string.IsNullOrWhiteSpace(x.Key) ? "Empty" : x.Key,
                Value = string.IsNullOrWhiteSpace(x.Key) ? "" : x.Key,
                Group = stateGroups.Where(y => y.Name == data.Where(p => p.AddrZip == x.Key).First().AddrState.ToUpper()).Single()
            }).OrderBy(x => x.Group.Name).ToList();


            var manifestItem = new ManifestFilterDropDownItem {
                States = stateZipSelectListItems
            return manifestItem;
        }

ViewModel:

using System.Collections.Generic;
using System.Web.Mvc;

namespace FSVendor.Models.Manifest {
    public class ManifestFilterViewModel {
        public ManifestFilterViewModel() {

        }

        public string Name { get; set; }
        public string DataTag => $"data-{Name}=''";
        public IEnumerable<SelectListItem> SelectListItems { get; set; }
    }
}

View:

@model FSVendor.Models.Manifest.ManifestFilterViewModel

<label>States:</label>
@Html.DropDownList(Model.Name, Model.SelectListItems, new { @class = "form-control filter-select", data_multi_select = "", multiple = "multiple", @Model.DataTag })

Answer 1

You query is creating a new SelectListGroup for each SelectListItem, even though each SelectListGroup has the same value.

Modify the query to group the data, and then create a new SelectListGroup for each group

// Initialize model
var model = new ManifestFilterDropDownItem
{
    States = new List<SelectListItem>
}
var states = data.GroupBy(x => x.AddrState); // group by state
foreach (var group in states)
{
    // Create a SelectListGroup
    var optionGroup = new SelectListGroup() { Name = group.Key };
    // Add SelectListItem's
    foreach (var item in group)
    {   
        model.States.Add(new SelectListItem()
        {
            Value = item.AddrZip,
            Text = item.AddrZip,
            Group = optionGroup
        })
    }
}
return model;

Alternatively, you can use one of the overloads of the SelectList constructor

var model = new ManifestFilterDropDownItem
{
    States = new SelectList(data, "AddrZip", "AddrZip", "AddrState", null, null)
};
return model;

As a side note, do not use DropDownList() to create a <select multiple>. You need to use ListBoxFor() in order for 2-way model binding to work. Refer Why does the DropDownListFor lose the multiple selection after Submit but the ListBoxFor doesn't?