ASP.NET MVC multiple select dropdown

In your view:

@Html.ListBoxFor(m => m.location_code, Model.location_type)

That's all you need. You're using a ListBox control so it's already a multiple select list.

Then back in your controller you can get the selected items like this:

[HttpPost]
public string SaveResults(List<int> location_code)
{

    if (location_code!= null)
    {
        return string.Join(",", location_code);
    }
    else
    {
        return "No values are selected";
    }
}