title: Why my model in post lose dropdownlist items?
body: <p>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?</p>
  
  <p>Here is the code that I will return an error:
  Value cannot be null.</p>
  
  <p><strong>controller action:</strong></p>
  
  <pre><code>public ActionResult Index()
  {
      var model = new Models.UploadVideoPage.UploadVideoPageModel();
      model.categoria = new List&lt;Models.UploadVideoPage.ListCategorie&gt;();
      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);
  }
  </code></pre>
  
  <p><strong>index.cshtml</strong></p>
  
  <pre><code> @Html.LabelFor(x =&gt; x.categoria)
              @Html.DropDownListFor(x =&gt; x.categoria.First().idCategoria, new SelectList(Model.categoria, "idCategoria", "TitoloCategoria"))
  </code></pre>