ViewBag multiple SelectListItems - asp.net-mvc-3

Question

Getting some very annoying behaviour from my MVC3 app. So I've got a model with 3 values:
[DisplayName("Airlines ")]
[StringLength(2, ErrorMessage = "Airline codes can only be 2 characters in length")]
public string AirlineCode1 { get; set; }
[DisplayName("Airlines")]
[StringLength(2, ErrorMessage = "Airline codes can only be 2 characters in length")]
public string AirlineCode2 { get; set; }
[DisplayName("Airlines")]
[StringLength(2, ErrorMessage = "Airline codes can only be 2 characters in length")]
public string AirlineCode3 { get; set; }
Now these are populated from a DropDowList so I'm popping the DropDownListItems in the ViewBag and rendering them in the view such:
#Html.LabelFor(l => l.AirlineCode1) <span>(select/enter code in box on right)</span>
<div id="airportCode1">
#Html.DropDownListFor(d => d.AirlineCode, ViewBag.AirLines as List<SelectListItem>) <input type="text" maxlength="2" value="#Model.AirlineCode1" />
</div>
<div id="airportCode2" style="#Model.AirlineCode2Style">
#Html.DropDownListFor(d => d.AirlineCode, ViewBag.AirLines2 as List<SelectListItem>) <input type="text" maxlength="2" value="#Model.AirlineCode2" />
</div>
<div id="airportCode3">
#Html.DropDownListFor(d => d.AirlineCode3, ViewBag.AirLines3 as List<SelectListItem>) <input type="text" maxlength="2" value="#Model.AirlineCode3" />
</div>
So my controller looks like:
IEnumerable<SelectListItem> airLines = PopulateAirlines(user);
ViewBag.AirLines = airLines;
ViewBag.AirLines2 = airLines;
ViewBag.AirLines3 = airLines;
Now in some circumstances I want to prepoulate AirLineCode in the model. so I set the model value in the controller. This resulted in some odd behaviour. Suddenly all my DropDownLists contained the prepopulated value!
Checked the model, value only set in AirLineCode1. Checked the ViewBag, no selected SelectListItems. So I figured the ViewBag must be maintaining a reference. So I changed my code to:
ViewBag.AirLines = PopulateAirlines(user);
ViewBag.AirLines2 = PopulateAirlines(user);
ViewBag.AirLines3 = PopulateAirlines(user);
Boom, fixed. Problem is PopulateAirlines is an expensive process!
Problem is the ViewBag appears to be maintaing a reference between the 3 SelectListItem Lists. How do I stop it doing this and still only make one call to PopulateAirlines(user);?
I tried the below code and this completely blew up:
IEnumerable<SelectListItem> airLines = PopulateAirlines(user);
ViewBag.AirLines = airLines;
ViewBag.AirLines2 = airLines.Select(s => new SelectListItem() { Text = s.Text, Value = s.Value, Selected = s.Selected });
ViewBag.AirLines3 = airLines.Select(s => new SelectListItem() { Text = s.Text, Value = s.Value, Selected = s.Selected });
with the error:
There is no ViewData item of type 'IEnumerable' that
has the key 'AirlineCode2'.
What??!

Answer 1

It may be because of in this code:
IEnumerable<SelectListItem> airLines = PopulateAirlines(user);
ViewBag.AirLines = airLines;
ViewBag.AirLines2 = airLines;
ViewBag.AirLines3 = airLines;
all the ViewBag properties are referring to the same list. A change to the list will therefore reflect when referencing any of the properties. When you call the PopulateAirlines() method you're creating 3 different lists.
You'll need make three separate lists but only create the first using the PopulateAirlines method and then clone it twice. That will not be as expensive.