public ActionResult SelectionClientSide(string id)
        {
            ViewData["Customers"] = GetCustomers();
            ViewData["Orders"] = GetOrdersForCustomer(id ??"ALFKI");
            ViewData["id"] ="ALFKI";
            return View();
        }
        [GridAction]
        public ActionResult _SelectionClientSide_Orders(string customerID)
        {
            customerID = customerID ??"ALFKI";
            return View(new GridModel<Order>
            {
                Data = GetOrdersForCustomer(customerID)
            });
        }

var x = y ?? z;

// is equivalent to:
var x = (y == null) ? z : y;

// also equivalent to:
if (y == null)
{
    x = z;
}
else
{
    x = y;
}