Posted model is null - asp.net-mvc

Question

I'm not sure what's wrong since I'm very new with MVC. This is a shopping cart. The customer is able to review their cart and edit quantity.
On the HttpPost ViewCart method, the cart is always empty, and number of lines is zero.
Controller:
public ActionResult ViewCart() {
var cart = (CartViewModel)Session["Cart"];
return View(cart);
}
[HttpPost]
public ActionResult ViewCart(CartViewModel cart) {
Session["Cart"] = cart;
return RedirectToAction("Order", "Checkout");
}
View:
#model CartViewModel
using (Html.BeginForm()) {
<h2>Your cart</h2>
<table>
<thead> ... </thead>
<tbody>
#foreach (var item in Model.Lines) {
<tr>
<td>#Html.DisplayFor(modelItem => item.Article.Description)</td>
<td>#Html.EditorFor(modelItem => item.Quantity)</td>
</tr>
}
</tbody>
</table>
<input type="submit" value="Checkout">
}
ViewModel:
public class CartViewModel {
public List<Line> Lines { get; set; }
public CartViewModel() {
Lines = new List<Line>();
}
}

Answer 1

Try changing the view to use indexes:
#model CartViewModel
using (Html.BeginForm()) {
<h2>Your cart</h2>
<table>
<thead> ... </thead>
<tbody>
#for (int i = 0; i < Model.Lines.Count; i++) {
<tr>
<td>#Html.DisplayFor(m => Model.Lines[i].Article.Description) #Html.HiddenFor(m => Model.Lines[i].Article.Id)</td>
<td>#Html.EditorFor(m => Model.Lines[i].Quantity)</td>
</tr>
}
</tbody>
</table>
<input type="submit" value="Checkout">
}