Question

I looked at similar posts but nothing working for my case.
I have a form which loads fine and I see the categories dropdown with all categories in it.
The problem is when I try to post the form.

I get this error:

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

@Html.DropDownList(“Category”, Model.Categories) <– red color

Here is my view:

@using (Html.BeginForm("Save", "Album", FormMethod.Post, new { id = "frmNewAlbum" }))
{                
   @Html.DropDownList("Category", Model.Categories)
}

Here is my model:

public class AlbumModel
{        
        public string Title { get; set; }

        public string Category { get; set; }

        public List<SelectListItem> Categories { get; set; } <-- holds categories
}

This is the controller actions to view the page:

[HttpGet]
public ActionResult Save()
        {
            var model = new AlbumModel();
            var categories = new List<SelectListItem>() { new SelectListItem() { Text = "-- pick --" } };
            categories.AddRange(svc.GetAll().Select(x => new SelectListItem() { Text = x.Name, Value = x.Name }));
            model.Categories = categories;
            return View(model);
        }

Action that receives the post:

[HttpPost]
    public ActionResult Save(AlbumModel model)
    {            
                var album = new AlbumDoc()
                {                        
                    Category = model.Category,
                    Title = model.Title,
                };

                svc.SaveAlbum(album);

        return View(model);
    }

Answer 1

In your POST action you seem to be redisplaying the same view but you are not populating the Categories property on your view model which will contain the dropdown list values. And by the way I would recommend you using strongly typed helper. So:

public class AlbumController: Controller
{
    [HttpGet]
    public ActionResult Save()
    {
        var model = new AlbumModel();
        model.Categories = GetCategories();
        return View(model);
    }

    [HttpPost]
    public ActionResult Save(AlbumModel model)
    {            
        var album = new AlbumDoc()
        {                        
            Category = model.Category,
            Title = model.Title,
        };
        svc.SaveAlbum(album);
        model.Categories = GetCategories();
        return View(model);
    }

    private IList<SelectListItem> GetCategories()
    {
        return svc
            .GetAll()
            .ToList()
            .Select(x => new SelectListItem
            { 
                Text = x.Name, 
                Value = x.Name 
            });
    }
}

and in your view:

@model AlbumModel
...
@using (Html.BeginForm("Save", "Album", FormMethod.Post, new { id = "frmNewAlbum" }))
{                
   @Html.DropDownListFor(
       x => x.Category, 
       Model.Categories,
       -- pick --
   )
}