Have you tried using a view model instead of ViewData? Strongly typed helpers that end with For and take a lambda expression cannot work with weakly typed structures such as ViewData.
Personally I don't use ViewData/ViewBag. I define view models and have my controller actions pass those view models to my views.
For example in your case I would define a view model:
public class MyViewModel
{
[HiddenInput(DisplayValue = false)]
public string CRN { get; set; }
}
have my controller action populate this view model:
public ActionResult Index()
{
var model = new MyViewModel
{
CRN = "foo bar"
};
return View(model);
}
and then have my strongly typed view simply use an EditorFor helper:
@model MyViewModel
@Html.EditorFor(x => x.CRN)
which would generate me:
<input id="CRN" name="CRN" type="hidden" value="foo bar" />
in the resulting HTML.