DropDownList 控件的SelectedIndexChanged事件触发不了

protected void Page_Init(object sender, EventArgs e)
{
DropDownList ddl = new DropDownList();
ddl.AutoPostBack = true;
ddl.Items.Add(new ListItem("国家", "c"));
ddl.Items.Add(new ListItem("中國", "c1"));
ddl.Items.Add(new ListItem("美國", "c2"));
ddl.SelectedIndexChanged += new EventHandler(ddl_SelectedIndexChanged);
this.Form.Controls.Add(ddl);
}

protected void Page_Load(object sender, EventArgs e)
{
//if (!IsPostBack)
//{
//    ShowPage();
//}
}

//public void ShowPage()
//{
//    DropDownList ddl = new DropDownList();
//    ddl.AutoPostBack = true;
//    ddl.Items.Add(new ListItem("国家", "c"));
//    ddl.Items.Add(new ListItem("中國", "c1"));
//    ddl.Items.Add(new ListItem("美國", "c2"));
//    ddl.SelectedIndexChanged += new EventHandler(ddl_SelectedIndexChanged);
//    this.Form.Controls.Add(ddl);
//}
protected void ddl_SelectedIndexChanged(object sender, EventArgs e)
{
DropDownList dl = (DropDownList)sender;
Response.Write("<script>alert('" + dl.SelectedValue + "')</script>");
}