public class HomeController : Controller
            {
                public ActionResult Index()
                {
                    var list = new SelectList(new[]
                    {
                        new { ID = "1", Name = "Select Id" },
                        new { ID = "2", Name = "1001" },
                        new { ID = "3", Name = "1002" },
                        new { ID = "4", Name = "1003" },
                    },
                    "ID", "Name", 1);

                    ViewData["list"] = list;

                    return View();
                }

                public List<Customer> GetCustomerById(String CustomerId)
                {
                    var objCustomer = new List<Customer>();

                    objCustomer.Add(new Customer {CustomerId="1001", FirstName="John", LastName="Travolta", EmailAddress="John.Travolta@yahoo.com"});
                    objCustomer.Add(new Customer {CustomerId = "1002", FirstName = "Bayo", LastName = "Adenuga", EmailAddress = "Bayo.Adenuga@yahoo.com" });
                    objCustomer.Add(new Customer { CustomerId = "1003", FirstName = "Feyi", LastName = "Olawoye", EmailAddress = "feyi.olawoye@yahoo.com" });


                    var objResult = from c in objCustomer
                                    where c.CustomerId == CustomerId
                                    select c;

                    return objResult.ToList();
                }

                [HttpPost]
                public ActionResult Action(string customerId)
                {
                    var results = GetCustomerById(customerId);
                    return Json(results);
                }

            }

@model DropdownSelectedFillTextBox.Models.Customer

        @{
            ViewBag.Title = "Home Page";
        }

        <script src="//ajax.aspnetcdn.com/ajax/jQuery/jquery-1.9.1.min.js">
        </script>

        @using (@Html.BeginForm("Action","Home",FormMethod.Post))
        {

             <h2>Dotuns Test</h2>

            <div>
                @Html.DropDownList("drpList",ViewData["list"] as SelectList)
            </div>

            <div>First Name</div>
            <div>
                  @Html.TextBox("FirstName", null, new { @class = "form-control" })
            </div>

            <div>Last Name</div>
            <div>
                  @Html.TextBox("LastName", null, new { @class = "form-control" })
            </div>

            <div>Email Address</div>
            <div>
                 @Html.TextBox("EmailAddress", null, new { @class = "form-control" })
            </div>

        }

           <script type="text/javascript">

               function Action(customerId) {
            $.ajax({
                url: '@Url.Action("Action", "Home")',
                type: "POST",
                data: { "customerId": customerId },
                "success": function (data) {
                    if (data != null) {
                        var vdata = data;
                        $("#FirstName").val(vdata[0].FirstName);
                        $("#LastName").val(vdata[0].LastName);
                        $("#EmailAddress").val(vdata[0].EmailAddress);
                    }
                }
            });
        }
        </script>