如何将数组保存在ViewState中并在页面卸载后能够检索它？

Question

采取以下代码：

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Collections.Generic;

namespace Test
{
    public partial class _Default : System.Web.UI.Page
    {
        private List<Customer> CustomerList;
        protected void Page_Load(object sender, EventArgs e)
        {
            // Quickly Illustrate  Creation of a List of that contains information;
            //CustomerList = (!this.IsPostBack) ? new List<Customer>() : new List<Customer>((Customer[])ViewState["Customers"]);
            if (IsPostBack)
                CustomerList = new List<Customer>((Customer[])ViewState["Customers"]);
            else
                CustomerList = new List<Customer>();
            // Convert the List to Array of Customers and Save To View State
            // Rather than Handling Serialization Manually
            //ViewState.Add("Customers", CustomerList.ToArray());

            // While Reading the View State Information - Of course
            // use correct checks to see the item is Not Null and All that... and then do:
            //Customer[] newArray = (Customer[])ViewState["Customers"];
            //List<Customer> newList = new List<Customer>(newArray);
            //for (int i = 0; i < CustomerList.Count; i++)
            //    Response.Write(CustomerList[i].CustomerName + "\r\n");

        }

       protected void Page_Unload(object sender, EventArgs e)
        {
            for (int i = 0; i < CustomerList.Count; i++)
                Response.Write(CustomerList[i].CustomerName + "\r\n");
            ViewState.Add("Customers", CustomerList.ToArray());
        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            Customer SingleCustomer = new Customer();
            SingleCustomer.CustomerName = TextBox1.Text;
            CustomerList.Add(SingleCustomer);
            ViewState.Add("Customers", CustomerList.ToArray());
        }
    }
}

没用 每次单击“添加”按钮并重新加载页面时，都会收到NullReferenceException，因为“ CustomerList = new List<Customer>((Customer[])ViewState["Customers"]); ”，因为它不在ViewState中。 这是为什么？

Answer 1

Page Unload太迟了，无法设置ViewState变量

从http://msdn.microsoft.com/en-us/library/ms178472.aspx ：

页面完全呈现，发送给客户端并准备丢弃后，将引发Unload事件。 此时，将卸载诸如响应和请求之类的页面属性，并执行清理。

ViewState本身作为页面中的隐藏字段发送-因此，如果您已经将页面发送给客户端，则以后就无法添加到ViewState。

尝试使用LoadComplete或PreRender事件？