怪怪怪问题,不能触发DropDownList1_SelectedIndexChanged(object sender, EventArgs e)事件

今天学习母版的时候遇到一件很怪的事情,就是不能触发DropDownList1_SelectedIndexChanged(object sender, EventArgs e)事件,检查了很久发现代码没有错误,于是在一个新的页面里拖放了一个Label控件和一个DropDownList控件,当触发DropDownList1_SelectedIndexChanged(object sender, EventArgs e)事件的时候把值传给Label的text属性来检查是否能触发DropDownList1_SelectedIndexChanged(object sender, EventArgs e)事件,但运行后无论如何选择DropDownList控件的值,Label的text都没有变,怪怪怪啊!!!!

代码如下:
Default.aspx

 1 <% @ Page Language = " C# "  AutoEventWireup = " true "   CodeFile = " Default.aspx.cs "  Inherits = " _Default "   %>
 2
 3 <! DOCTYPE html PUBLIC  " -//W3C//DTD XHTML 1.0 Transitional//EN "   " http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd " >
 4
 5 < html xmlns = " http://www.w3.org/1999/xhtml "   >
 6 < head runat = " server " >
 7      < title > Untitled Page </ title >
 8 </ head >
 9 < body >
10      < form id = " form1 "  runat = " server " >
11      < div >
12          < asp:Label ID = " Label1 "  runat = " server "  Text = " Label " ></ asp:Label >
13                           < asp:DropDownList ID = " DropDownList1 "  runat = " server "  OnSelectedIndexChanged = " DropDownList1_SelectedIndexChanged " >
14                              < asp:ListItem > 123 </ asp:ListItem >
15                              < asp:ListItem > 456 </ asp:ListItem >
16                              < asp:ListItem > 789 </ asp:ListItem >
17                              < asp:ListItem > abc </ asp:ListItem >
18                          </ asp:DropDownList >
19      </ div >
20      </ form >
21 </ body >
22 </ html >
23

Default.aspx.cs

 1 using  System;
 2 using  System.Data;
 3 using  System.Configuration;
 4 using  System.Web;
 5 using  System.Web.Security;
 6 using  System.Web.UI;
 7 using  System.Web.UI.WebControls;
 8 using  System.Web.UI.WebControls.WebParts;
 9 using  System.Web.UI.HtmlControls;
10
11 public  partial  class  _Default : System.Web.UI.Page 
12 {
13    protected void Page_Load(object sender, EventArgs e)
14    {
15
16    }
17    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
18    {
19        Label1.Text = DropDownList1.SelectedValue;
20    }
21}
22