ConcurrentDictionary.cs

 static ConcurrentDictionary<string, ToDoItem> todoItems = new ConcurrentDictionary<string, ToDoItem>();

 // GET: /Sample/
 public ActionResult Index()
 {
    var data = todoItems.Values;
    return View(data.AsEnumerable<ToDoItem>());
 }

 public ActionResult AddItem()
 {
    return View();
 }

 [HttpPost]
 public ActionResult AddItem(ToDoItem item)
 {
    item.Key = Guid.NewGuid().ToString();
    var data = todoItems.TryAdd(item.Key, item);
    return RedirectToAction("Index");
 }

//Sample Class
 public class ToDoItem
 {
    public string Key { get; set; }
    public string Name { get; set; }
    public bool IsComplete { get; set; }
 }