UpdatePanel异步回发不调用TextBox的TextChanged事件

Question

为什么UpdatePanel异步回发不调用TextBox的TextChanged事件，在下面显示的代码中？

<%@ Page Title="" Language="VB" MasterPageFile="~/Master.master" CodeFile="Default2.aspx.vb" Inherits="Default2" AutoEventWireup="false" %>
<asp:Content ID="Content1" ContentPlaceHolderID="cphHead" runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ConPhameNet" runat="Server">
    <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
        <ContentTemplate>
            <asp:TextBox ID="TextBox1" runat="server" AutoPostBack="True"></asp:TextBox>
            <asp:Label ID="Labelabc1" runat="server" Text="Label"></asp:Label>
        </ContentTemplate>
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="TextBox1" EventName="TextChanged" />
        </Triggers>
    </asp:UpdatePanel>
</asp:Content>