I have some problem When multiple users will encounter this problem.

My Controller : When the user enters for the first time

public class ShoppingCart
{
public string ProductID { get; set; }
public string Amount { get; set; }
}

public ActionResult Index()
{
var uuid = Guid.NewGuid().ToString();
List ShoppingCartList = new List();
var storage = new LocalStorage();-----------(1)
storage.Store(uuid , ShoppingCartList );
storage.Persist();

ShoppingCartList = storage.Get<List>(uuid); -----------(2)
int Amount = ShoppingCartList.Count;
return View();
}

line --(1) error : System.IO.IOException: The process cannot access the file 'C:\xxx\yyy\zzz\ .localstorage' because it is being used by another process.

line --(2) error : System.NullReferenceException: Object reference not set to an instance of an object.

Both errors are not always present.
Where am I going wrong?