How to do server-side validation using MVC 5

Question

I am working on a simple auction website for a charity. I have an Item model for the sale items, and a Bid view where the user can enter a bid and submit it. This bid is received inside the Item controller:

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Bid(int itemID, int bidAmount)
    {
        if (ModelState.IsValid)
        {
            Item item = db.Items.Find(itemID);
            if (bidAmount >= item.NextBid)
            {
                item.Bids++;
                item.CurrentBid = bidAmount;
                item.HighBidder = HttpContext.User.Identity.Name;
                db.Entry(item).State = EntityState.Modified;
                db.SaveChanges();
            }
            else
            {
                // Already outbid
            }
            return View(item);
        }
        return RedirectToAction("Auction");
    }

I would like to know how to display server-side validation to the user. For example, in the above code, it may be that the submitted bid amount is no longer sufficient. In that case, I would like to display a message to the user that they have been outbid etc.

How can I pass this information back to the view to display an appropriate message? I want the user to see the same item page view as before, updating the value in the edit box and displaying the message - similar to eBay. Thanks.

Answer 1

You should have a look at the AddModelError method of the ModelState property.

   [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Bid(int itemID, int bidAmount)
    {
        if (ModelState.IsValid)
        {
            Item item = db.Items.Find(itemID);
            if (bidAmount >= item.NextBid)
            {
                item.Bids++;
                item.CurrentBid = bidAmount;
                item.HighBidder = HttpContext.User.Identity.Name;
                db.Entry(item).State = EntityState.Modified;
                db.SaveChanges();
            }
            else
            {
                // Already outbid
                ModelState.AddModelError("", "Already outbid");
            }
            return View(item);
        }
        return RedirectToAction("Auction");
    }

To display the message in your view, you need a ValidationSummary:

@Html.ValidationSummary(true)