为什么我们还需要在HttpPost Action中传递SelectList的ViewBag呢？

Question

I have select-list items that are stored in a ViewBag. I don't understand why we should declare this ViewBag in both, HttpGet and HttpPost actions?

If we don't declare it again in HttpPost action, the validation will not work and we will get exception. But how does this happen technically?

---

Actions:

@Html.DropDownListFor(m => m.CategoryId, (IEnumerable<SelectListItem>)ViewBag.Categories, "Select Category", new { @class = "form-control" })

List:

 @Html.DropDownListFor(m => m.CategoryId, (IEnumerable<SelectListItem>)ViewBag.Categories, "Select Category", new { @class = "form-control" }) 

The raised exception when I delete the ViewBag from HttpPost Action. I wonder why that
key = CategoryId? how does this key work?

Screenshot :

Answer 1

You may be overthinking it. Consider it simply...

Both actions return the same view:

public ActionResult New()
{
    //...
    return View();
}

And that view uses a value from the ViewBag :

(IEnumerable<SelectListItem>)ViewBag.Categories

In order for the view to use that value, that value has to be in the ViewBag . So any action which returns that view will need to populate that value. Which is why there's an error when you don't populate that value. The error is telling you that you're trying to use a ViewBag value that doesn't exist.