如何在ViewState中保存数组并能够在页面卸载后检索它？

Question

请使用以下代码:

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());
        }
    }
}

它不起作用.每次我单击Add按钮并重新加载页面时,我会得到一个Null的NullReferenceException,CustomerList = new List<Customer>((Customer[])ViewState["Customers"]);因为它在ViewState中不存在.这是为什么？

Answer 1

页面卸载设置ViewState变量为时已晚

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

在页面完全呈现,发送到客户端并准备好被丢弃之后引发Unload事件.此时,将卸载响应和请求等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,如果您已将页面发送到客户端,则以后无法添加到ViewState.

尝试使用LoadComplete或PreRender事件？