Question

I am passing a List<Listing> object to another action method and making that method call a View with the parameter.

For some reason, the parameter which I am passing is null.

The following works fine:

        public ActionResult SortListing(string categoryGuid)
    {
        var listingCategory = new ListingCategory();
        listingCategory = _tourismAdminService.GetByGuid<ListingCategory>(Guid.Parse(categoryGuid));
        var listings = new List<Listing>();

        foreach (var listing in _tourismAdminService.ListAllEntities<Listing>())
        {
            if (listing.CategoryId == listingCategory.Id)
            {
                listings.Add(listing);
            }
        }

        return RedirectToAction("Index", "Listing", listings);
    }

The following shows a Parameter which is coming up as null.

        public ActionResult Index(List<Listing> listing)
    {
        var model = new ListListingsViewModel();
        IEnumerable<ListingCategory> categories = _service.ListAllEntities<ListingCategory>();

        if (categories != null)
        {
            model.Categories =
                categories.Select(
                    cat =>
                    new SelectListItem
                        {
                            Text =
                                cat.GetTranslation(stringA,
                                                   stringB).Name,
                            Value = cat.Guid.ToString()
                        }).ToList();
        }
        model.Listings = listing ?? _service.ListAllEntities<Listing>();

        return View(model);
    }

EDIT

Error Message:

The ViewData item that has the key ‘SelectedCategoryGuid’ is of type ‘System.Guid’ but must be of type ‘IEnumerable’.

On:

@Html.DropDownListFor(
m => m.SelectedCategoryGuid, 
Model.Categories, 
"Select a Category", 
new {
    id = "hhh",
    data_url = Url.Action("SortListing", "Listing") 
}
  )

Answer 1

RedirectToAction method returns a Http 302 response to the browser,which cause the browser to make a GET request to the specified action.

Remember HTTP is stateless. You can not pass such complex object like that.

You should either pass a querystring (an Id) and get the value in the second action again or keep the data in a persitant medium between the calls. You may consider using Session or TempData (session is the backed up storage for that) for that.

http://rachelappel.com/when-to-use-viewbag-viewdata-or-tempdata-in-asp.net-mvc-3-applications

EDIT : As per the comment. Yes you can call the view from the first method itself. the below code will pass the string collection the Index view (index.cshtml).

public ActionResult SortedList(string categoryGuid)
{
   var listings = new List<Listing>();
   //fill the collection from the data from your db
   return View("Index",listings)
}

If you want to pass the data to a view in a different controller, you can specify the full path when calling the View method.

return View("~/Views/User/Details.cshtml",listings)

Assuming your view is strongly typed to a list of strings like this

@model List<string>
foreach(var item in Model)
{
 <p>@item</p>
}