How to keep DropDownList values between a server call in ASP.NET MVC3 - asp.net-mvc-3

Question

I have an ASP.NET MVC3 application. I have the following ViewModel:
public class MyViewModel
{
public string Year { get; set; }
public string Month {get; set; }
public IEnumerable<SelectListItem> Years
{
get
{
return Enumerable.Range(2000, DateTime.Now.Year - 2000).Select(x => new SelectListItem
{
Value = x.ToString(),
Text = x.ToString()
});
}
}
}
And the following View:
#model MyNamespace.MyViewModel
#Html.DropDownListFor(
x => x.Year,
new SelectList(Model.Years, "Value", "Text"),
"-- select year --"
)
#Html.DropDownListFor(
x => x.Month,
Enumerable.Empty<SelectListItem>(),
"-- select month --"
)
I fill the DropDownList for Month with a jQuery function that is triggered onchange from the DropDownList for Year, which works perfectly. When the form is posted to the server and then the view is rendered back I want to keep the values in the 'Month' DropDownList as it happens correctly for Year. Therefore I tried (besides the jQuery script):
public class MyViewModel
{
public string Year { get; set; }
public string Month {get; set; }
public IEnumerable<SelectListItem> Months
{
get
{
if(Year != null)
{
return Enumerable.Range(1, 12).Select(x => new SelectListItem
{
Value = x.ToString(),
Text = x.ToString()
});
}
}
}
And in the View:
#Html.DropDownListFor(
x => x.Month,
new SelectList(Model.Months, "Value", "Text"),
"-- select month --"
)
I left Years out of the code to make things shorter. This code throws a NullReferenceException at the first run because the IEnumerable is empty. how can I achieve my goal?

Answer 1

You have to repopulate the month list in your action.
The POST is stateless is not like asp.net webforms.
Just do something like this in your code:
public ViewResult MyAction(MyViewModel model)
{
if (ModelState.IsValid)
{
//.. do your stuff ...
//return redirect to index if everything went ok
}
//something went wrong return the model
model.Years = new SelectList(/* code to populate years */, "Value", "Text", Model.Year);
model.Months = new SelectList(/* code to populate months */, "Value", "Text", Model.Month);
}
As a side note I prefer to use SelectList instead of IEnumerable<SelectListItem> beacuse I can set the default value in the controller (a better fit for the controller responsibility in my opinion) and not in the View.