MVC4:视图似乎独立于控制器维护状态

我有一个下拉列表（客户）和复选框列表（销售订单），具体取决于当前选定的客户。如果我选择新客户，我希望复选框会清除，但尽管在回发中清除了模型，但它们会从一个保留到另一个。

我不是一个经验丰富的MVC开发人员，但我不确定为什么会这样。调试我要发送回视图的 ViewModel 时，它为所有复选框显示 IsSelected = false，但在视图中，它们被选中。我做错了什么？（提前致谢）

查看模型：

namespace MvcTest1.Models
{
    public class Customer
    {
        public int CustomerID { get; set; }
        public string Name { get; set; }
    }
    public class SalesOrder
    {
        public int SalesOrderID { get; set; }
        public string Reference { get; set; }
        public bool IsSelected { get; set; }
    }
    public class SalesOrderPageViewModel
    {
        public List<Customer> Customers { get; set; }
        public int SelectedCustomerID { get; set; }
        public List<SalesOrder> SalesOrders { get; set; }
    }
}

控制器：

namespace MvcTest1.Controllers
{
    public class SalesOrderPageController : Controller
    {
        [HttpGet]
        public ActionResult Index()
        {
            SalesOrderPageViewModel viewModel = BuildViewModel(1);
            return View(viewModel);
        }
        [HttpPost]
        public ActionResult Index(SalesOrderPageViewModel viewModelInput)
        {
            SalesOrderPageViewModel viewModel = BuildViewModel(viewModelInput.SelectedCustomerID);
            return View(viewModel);
        }
        public SalesOrderPageViewModel BuildViewModel(int customerID)
        {
            SalesOrderPageViewModel viewModel = new SalesOrderPageViewModel
            {
                Customers = new List<Customer> 
                { 
                    new Customer { CustomerID = 1, Name = "Alan" },
                    new Customer { CustomerID = 2, Name = "Bob" },
                    new Customer { CustomerID = 3, Name = "Charlie" }
                }
            };
            viewModel.SelectedCustomerID = customerID;
            if (customerID == 1)
            {
                viewModel.SalesOrders = new List<SalesOrder>
                {
                    new SalesOrder { SalesOrderID = 11, Reference = "AA11" },
                    new SalesOrder { SalesOrderID = 12, Reference = "AA22" },
                    new SalesOrder { SalesOrderID = 13, Reference = "AA33" }
                };
            }
            if (customerID == 2)
            {
                viewModel.SalesOrders = new List<SalesOrder>
                {
                    new SalesOrder { SalesOrderID = 21, Reference = "BB11" },
                    new SalesOrder { SalesOrderID = 22, Reference = "BB22" },
                    new SalesOrder { SalesOrderID = 23, Reference = "BB33" }
                };
            }
            if (customerID == 3)
            {
                viewModel.SalesOrders = new List<SalesOrder>
                {
                    new SalesOrder { SalesOrderID = 31, Reference = "CC11" },
                    new SalesOrder { SalesOrderID = 32, Reference = "CC22" },
                    new SalesOrder { SalesOrderID = 33, Reference = "CC33" }
                };
            }
            return viewModel;
        }
    }
}

视图：

@model MvcTest1.Models.SalesOrderPageViewModel
@{
    ViewBag.Title = "SalesOrderPage";
    Layout = "~/Views/Shared/_Layout.cshtml";
}
<h2>SalesOrderPage</h2>
<br /><br />
@using (Html.BeginForm())
{
    @Html.DropDownListFor(model =>
        model.SelectedCustomerID,
        new SelectList(
            Model.Customers, 
            "CustomerID", 
            "Name", 
            Model.SelectedCustomerID
        ),
        new { id = "customerSelect" }
        );
    <script type="text/javascript">
        $(function () {
            $('#customerSelect').change(function () {
                this.form.submit();
            });
        })
    </script>
    <br /><br />
    for (int i = 0; i < Model.SalesOrders.Count(); i++)
    {
        @Html.DisplayFor(m => Model.SalesOrders[i].Reference) 
        @Html.CheckBoxFor(m =>
            m.SalesOrders[i].IsSelected
        )
        <br />
    }
}