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.