C# Online Compiler | .NET Fiddle

using System;
using System.ComponentModel.DataAnnotations;
using System.Web.Mvc;
using System.Collections.Generic;

namespace HelloWorldMvcApp
{
	public class OrderViewModel
	{
		[Display(Name = "Order number")]
		public int OrderNumber { set; get; }
		[Display(Name = "Product")]
		[Required(ErrorMessage = "Please select a product")]
		public int SelectedProductId { get { return 2; }}
		public SelectList ProductList { get; set; }
	}
	
	public class Product
	{
		public int ID { set; get; }
		public string Name { set; get; }
	}
	
	public static class Repository
	{
		public static IEnumerable<Product> FetchProducts()
		{
			return new List<Product>()
			{
				new Product(){ ID = 1, Name = "Mobile" },
				new Product(){ ID = 2, Name = "Laptop" },
				new Product(){ ID = 3, Name = "IPad" }
			};
		}
	}
	
}

 
using System;
using System.ComponentModel.DataAnnotations;
using System.Web.Mvc;
using System.Collections.Generic;
​
namespace HelloWorldMvcApp
{
    public class OrderViewModel
    {
        [Display(Name = "Order number")]
        public int OrderNumber { set; get; }
        [Display(Name = "Product")]
        [Required(ErrorMessage = "Please select a product")]
        public int SelectedProductId { get { return 2; }}
        public SelectList ProductList { get; set; }
    }
    
    public class Product
    {
        public int ID { set; get; }
        public string Name { set; get; }
    }
    
    public static class Repository

@model HelloWorldMvcApp.OrderViewModel
@{
	Layout = null;
}

<!DOCTYPE html>
<!-- template from http://getbootstrap.com/getting-started -->
<html lang="en">
	<head>
		<meta charset="utf-8">
		<meta http-equiv="X-UA-Compatible" content="IE=edge">
		<meta name="viewport" content="width=device-width, initial-scale=1">
		<title>Bootstrap 101 Template</title>
		<!-- CSS Includes -->
		<link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css">
		<style type="text/css">
			.field-validation-error {
				color: #ff0000;
			}
		</style>
	</head>
	<body>
		<div class="container">
			<div class="col-md-6 col-md-offset-3">
				<h1>Order</h1>
				@using (Html.BeginForm())
				{
					<div class="form-group">
						@Html.LabelFor(m => m.OrderNumber)
						@Html.TextBoxFor(m => m.OrderNumber,new {@class="form-control", Value = ""}) 
						@Html.ValidationMessageFor(m => m.OrderNumber)
					</div>
					<div class="form-group">
						@Html.LabelFor(m => m.SelectedProductId)
						@Html.DropDownListFor(m => m.SelectedProductId, Model.ProductList, "- Please select -", new {@class="form-control"}) 
						@Html.ValidationMessageFor(m => m.SelectedProductId)
					</div>
				
					<button type="submit" class="btn btn-success submit">Save</button>
				}
			</div>
		</div>

		<!-- JS includes -->
		<script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
		<script src="//netdna.bootstrapcdn.com/bootstrap/3.1.1/js/bootstrap.min.js"></script>
		<script src="//ajax.aspnetcdn.com/ajax/jquery.validate/1.11.1/jquery.validate.min.js"></script>
		<script src="//ajax.aspnetcdn.com/ajax/mvc/4.0/jquery.validate.unobtrusive.min.js"></script>
	</body>
</html>

 
@model HelloWorldMvcApp.OrderViewModel
@{
    Layout = null;
}
​
<!DOCTYPE html>
<!-- template from http://getbootstrap.com/getting-started -->
<html lang="en">
    <head>
        <meta charset="utf-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <title>Bootstrap 101 Template</title>
        <!-- CSS Includes -->
        <link rel="stylesheet" href="//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css">
        <style type="text/css">
            .field-validation-error {
                color: #ff0000;
            }
        </style>
    </head>
    <body>
        <div class="container">
            <div class="col-md-6 col-md-offset-3">

using System;
using System.Web.Mvc;
using System.Collections.Generic;

namespace HelloWorldMvcApp
{
	public class HomeController : Controller
	{
		[HttpGet]
		public ActionResult Index()
		{
			OrderViewModel model = new OrderViewModel();
			ConfigureViewModel(model);
			return View(model);
		}
		[HttpPost]
		public ActionResult Index(OrderViewModel model)
		{
			if (!ModelState.IsValid)
			{
				ConfigureViewModel(model);
				return View(model);
			}
			// save and redirect
			// but for testing purposes
			ConfigureViewModel(model);
			return View(model);
		}
		
		private void ConfigureViewModel(OrderViewModel model)
		{
			IEnumerable<Product> products = Repository.FetchProducts();
			model.ProductList = new SelectList(products, "ID", "Name");
		}
	}
}

 
using System;
using System.Web.Mvc;
using System.Collections.Generic;
​
namespace HelloWorldMvcApp
{
    public class HomeController : Controller
    {
        [HttpGet]
        public ActionResult Index()
        {
            OrderViewModel model = new OrderViewModel();
            ConfigureViewModel(model);
            return View(model);
        }
        [HttpPost]
        public ActionResult Index(OrderViewModel model)
        {
            if (!ModelState.IsValid)
            {
                ConfigureViewModel(model);
                return View(model);
            }
            // save and redirect
            // but for testing purposes
            ConfigureViewModel(model);
            return View(model);
        }
        
        private void ConfigureViewModel(OrderViewModel model)
        {
            IEnumerable<Product> products = Repository.FetchProducts();
            model.ProductList = new SelectList(products, "ID", "Name");
        }
    }
}