Why my model in post lose dropdownlist items?

Question

I have one simple page mvc with one action. Get method in the index action I create an instance of the property model.categoria and I value with 3 items. The problem is that if I run the post index of action, as you can see below, I have an error because the dropdownlist associated model.categoria is null. My question is: I always instantiated at every post the model.categoria order to avoid this problem, one that today I wonder, is rightly so?

Here is the code that I will return an error: Value cannot be null.

controller action:

public ActionResult Index()
{
    var model = new Models.UploadVideoPage.UploadVideoPageModel();
    model.categoria = new List<Models.UploadVideoPage.ListCategorie>();
    model.categoria.Add(new ListCategorie { TitoloCategoria = "Volontariato", idCategoria = 1 });
    model.categoria.Add(new ListCategorie { TitoloCategoria = "Interno", idCategoria = 2 });
    model.categoria.Add(new ListCategorie { TitoloCategoria = "Sindacato", idCategoria = 3 });
    return View(model);
}
[HttpPost]
public ActionResult Index(Models.UploadVideoPage.UploadVideoPageModel model) 
{
    return View(model);
}

index.cshtml

 @Html.LabelFor(x => x.categoria)
            @Html.DropDownListFor(x => x.categoria.First().idCategoria, new SelectList(Model.categoria, "idCategoria", "TitoloCategoria"))

Answer 1

You do have to populate the list every time you do a post back unless you were to put hidden field for all of the values in your form. It only takes a few lines of code though to make this work a different way

public ActionResult Index()
    {
        var model = new Models.UploadVideoPage.UploadVideoPageModel();
        PopulateDependencies(model);
        return View(model);
    }
    [HttpPost]
    public ActionResult Index(Models.UploadVideoPage.UploadVideoPageModel model)
    {
        PopulateDependencies(model);
        return View(model);
    }

    private void PopulateDependencies(Models.UploadVideoPage.UploadVideoPageModel model)
    {
        model.categoria = new List<Models.UploadVideoPage.ListCategorie>();
        model.categoria.Add(new ListCategorie { TitoloCategoria = "Volontariato", idCategoria = 1 });
        model.categoria.Add(new ListCategorie { TitoloCategoria = "Interno", idCategoria = 2 });
        model.categoria.Add(new ListCategorie { TitoloCategoria = "Sindacato", idCategoria = 3 });
    }