ASP.NET MVC variable number of dropdown lists - asp.net-mvc-4

Question

Consider the following question and potential answer: ASP.Net MVC Multiple Drop Downs, Single List, Only Allow Unique Selections
Ignoring most of the details, we can see that we can implement many dropdowns for a 1-m relationship like this:
<%: Html.DropDownListFor(model => model.DropwdownId1,Model.DropdownEntities) %>
<%: Html.DropDownListFor(model => model.DropwdownId2,Model.DropdownEntities) %>
<%: Html.DropDownListFor(model => model.DropwdownId3,Model.DropdownEntities) %>
DropdownId1, DropdownId2 and DropdownId3 are properties easily added to the model if and only if we know exactly how many dropdowns we are going to display and postback.
However, I would like to implement a variable number of dropdowns. New dropdowns could, for example, be added dynamically by Javascript. Or the number of dropdowns displayed into the view could be dependent on some variable Property in model. say, model.NumberOfDropDowns.
How can I implement this? How do I write a viewmodel, and controller action that can handle a variable number of dropdowns?
I have done a lot of reading on complex things like editor templates and blog posts where form elements are added dynamically, but I'm really having difficulty trying to figure out how this could be done. Any assistance would be greatly appreciated

Answer 1

A drop-down input still has only 1 submitted form value, so it's the same as any other variable-length view-model property: use a List<T>, like so:
ViewModel
class FooViewModel {
public List<String> DropDownFields { get; set; }
}
Controller Actions
[HttpGet]
ActionResult Index() {
ViewData["dropDownSource"] = new List<SelectListItem>
{
new SelectListItem
{
Text = "Test",
Value= "1"
},
new SelectListItem
{
Text = "Text",
Value= "2"
}
};
return View( new FooViewModel() );
}
[HttpPost]
ActionResult Index(FooViewModel vm) {
for(int i = 0; i < vm.DropDownFields.Count; i++) {
// for each dropdown
}
}
View (aspx syntax)
<% for(int i = 0; i < vm.DropDownFields.Count; i++) { %>
<%= Html.DropDownListFor( m => m.DropDownFields[i], (IEnumerable<SelectListItem>)ViewData["dropDownSource"] ) %>
<% } %>