Pick up a selected item on Dropdownlistfor using an ASP.NET MVC Viewmodel

Question

I am trying to register a Subcategory that needs a Category. I have Subcategoriaviewmodel, where I created the fields public IEnumerable<CategoriaViewModel> Categorias { get; set; } and public Guid CategoriaId { get; set; }. Tbm have public virtual Categoria Categoria { get; set; } pro EF.

My question is: how I would implement for me to receive in my Controller the Category selected in View?

In my Viewmodel have it:

public class SubCategoriaViewModel
    {
        public SubCategoriaViewModel()
        {
        }

        [Key]
        public Guid SubCategoriaId { get; set; }

        [Required(ErrorMessage = ("Preencha o nome da SubCategoria."))]
        [MaxLength(60, ErrorMessage = ("Máximo {0} caracteres."))]
        [MinLength(1, ErrorMessage = ("Mínimo {0} caracteres."))]
        [DisplayName("Nome")]
        public string SubCategoriaNome { get; set; }

        public IEnumerable<CategoriaViewModel> Categorias { get; set; }
        public Guid CategoriaId { get; set; }

        //[ScaffoldColumn(false)]
        //public DomainValidation.Validation.ValidationResult ValidationResult { get; set; }

        public virtual Categoria Categoria { get; set; }
        //public ICollection<Produto> Produtos { get; set; }

    }

In my View have it:

<div class="form-group">
            @Html.LabelFor(model => model.Categoria, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @* Este aqui tá dando certo aparecer na tela, mas não imagino um implementação para pegar um item selecionado *@
                @Html.DropDownListFor(model => model.CategoriaId,
                 new SelectList(Model.Categorias, "CategoriaId", "CategoriaNome"), new { @class = "form-control" })

                @* Esse aqui é so pra teste *@
                @Html.DropDownListFor(model => model.CategoriaId,
                 new SelectList(Model.Categorias, "CategoriaId", "CategoriaNome"), ((IEnumerable < Categorias)), new { @class = "form-control" })
            </div>
        </div>

In my Controller of the Subcategory I have this:

 // GET: SubCategorias/Create
        public ActionResult Create()
        {
            subCategoriaViewModel.Categorias = _categoriaAppService.ObterTodas();
            return View(subCategoriaViewModel);
        }

        // POST: SubCategorias/Create
        [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult Create(
            SubCategoriaViewModel subCategoriaViewModel)
        {
            subCategoriaViewModel =
                _subCategoriaAppService
                .Adicionar(subCategoriaViewModel);


            return View(subCategoriaViewModel);
        }

Answer 1

How to receive the selected category in the dropdown?

When you use:

    @Html.DropDownListFor(model => model.CategoriaId,
                new SelectList(Model.Categorias, "CategoriaId", "CategoriaNome"), 
                new { @class = "form-control" })

Will be generated:

<select id="CategoriaId" name="CategoriaId">
    <option value="1">Categoria 1</option>
    <option value="2">Categoria 2</option>
                   .
                   .
                   .
    <option value="N">Categoria N</option>
</select>

This means that when you select an item in the dropdown, the field CategoriaId will receive the "value" that select generated.

Select by returning null:

When the post is performed, by your code, you are sending only the CategoriaId, but not the list, so it’s not filled in.

To solve this, just in the post, when you do the Return view, set again these lines (as shown in Pablo Vargas' reply):

subCategoriaViewModel.Categorias = _categoriaAppService.ObterTodas();
return View(subCategoriaViewModel); 

An alternative would be to run this get from the list directly on your template (if this doesn’t hurt your solution architecture), assign the list once and every time you do get that property checks if the list this set, if not, get it again.

Answer 2

Iago, I believe you only need to repopulate the categories in the post again.

// POST: SubCategorias/Create
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Create(SubCategoriaViewModel subCategoriaViewModel)
{
    subCategoriaViewModel =
                _subCategoriaAppService
                .Adicionar(subCategoriaViewModel);

    subCategoriaViewModel.Categorias = _categoriaAppService.ObterTodas();
    return View(subCategoriaViewModel);
}