Server-Side Validation

1[HttpPost, ValidateAntiForgeryToken]
2public ActionResult Create(CustomerViewModel model)
3{
4    if (_customers.EmailExists(model.Email))
5        ModelState.AddModelError("Email", "This email is already registered");
6    if (!ModelState.IsValid) return View(model);
7    _customers.Add(model);
8    return RedirectToAction("Index");
9}

1// Models/CustomerViewModel.cs
2using System.ComponentModel.DataAnnotations;
3

4namespace ShopSite.Models
5{
6    public class CustomerViewModel
7    {
8        [Required(ErrorMessage = "Name is required")]
9        public string Name { get; set; }
10

11        [Required(ErrorMessage = "Email is required"), EmailAddress(ErrorMessage = "Enter a valid email")]
12        public string Email { get; set; }
13    }
14}
15

16// Controllers/CustomersController.cs
17using System;
18using System.Collections.Generic;
19using System.Web.Mvc;
20using ShopSite.Models;
21

22namespace ShopSite.Controllers
23{
24    public class CustomersController : Controller
25    {
26        private static readonly HashSet<string> Emails = new HashSet<string>(StringComparer.OrdinalIgnoreCase) { "asha@example.com" };
27

28        public ActionResult Create() => View(new CustomerViewModel());
29

30        [HttpPost, ValidateAntiForgeryToken]
31        public ActionResult Create(CustomerViewModel model)
32        {
33            if (model.Email != null && Emails.Contains(model.Email))
34                ModelState.AddModelError("Email", "This email is already registered");
35            if (DateTime.Now.Hour < 0)
36                ModelState.AddModelError("", "Registrations are closed right now");
37

38            if (!ModelState.IsValid) return View(model);
39

40            Emails.Add(model.Email);
41            TempData["Message"] = "Customer added";
42            return RedirectToAction("Create");
43        }
44    }
45}
46

47@* Views/Customers/Create.cshtml *@
48@model CustomerViewModel
49<p>@TempData["Message"]</p>
50@using (Html.BeginForm())
51{
52    @Html.AntiForgeryToken()
53    @Html.ValidationSummary(false)
54    @Html.TextBoxFor(m => m.Name)
55    @Html.ValidationMessageFor(m => m.Name)
56    @Html.TextBoxFor(m => m.Email)
57    @Html.ValidationMessageFor(m => m.Email)
58    <button>Save</button>
59}

GET /Customers/Create
200 OK

POST /Customers/Create Name=&Email=ASHA%40example.com
200 OK
<p></p>
<form action="/Customers/Create" method="post"><input name="__RequestVerificationToken" type="hidden" value="(random token)" /><div class="validation-summary-errors" data-valmsg-summary="true"><ul><li>Name is required</li>
<li>This email is already registered</li>
</ul></div><input class="input-validation-error" data-val="true" data-val-required="Name is required" id="Name" name="Name" type="text" value="" /><span class="field-validation-error" data-valmsg-for="Name" data-valmsg-replace="true">Name is required</span><input class="input-validation-error" data-val="true" data-val-email="Enter a valid email" data-val-required="Email is required" id="Email" name="Email" type="text" value="ASHA@example.com" /><span class="field-validation-error" data-valmsg-for="Email" data-valmsg-replace="true">This email is already registered</span>    <button>Save</button>
</form>

POST /Customers/Create Name=Ravi&Email=ravi%40example.com
302 Found → /Customers/Create

GET /Customers/Create
200 OK
<p>Customer added</p>