ModalPopupExtender PostBack

Question

我的Project里用到了一个ModalPopupExtender，我用了一个TextBox，然后写了个TextChanged事件，用来异步查询是否有这个用户，这个TextBox是在ModalPopupExtender里面的，当我TextChanged的时候它就会出发PostBack，从而导致弹出窗口关闭，我不希望此时关闭弹出窗口，该如何做？前台代码：（缩略）        <asp:Panel ID="Panel1" runat="server" Height="50px" Width="125px"   style="display:none">
                       <table align="center" cellpadding="4" cellspacing="1" class="tableborder"  style="width: 580px; height: 433px">
                <tr>
                    <td style="width: 119px; height: 32px" class="altbg1">
                        用户登陆名：</td>
                    <td style="width: 690px; height: 32px" class="altbg2">
                        <asp:TextBox ID="TxtUser" runat="server" AutoPostBack="True" OnTextChanged="TxtUser_TextChanged1"></asp:TextBox></asp:Panel>
</table>
<cc1:ModalPopupExtender ID="ModalPopupExtender1" runat="server"
         TargetControlID="LinkButton2" PopupControlID="Panel1" BackgroundCssClass="bgcss" CancelControlID="Button1" Drag="true" PopupDragHandleControlID="table1" X="365" Y="75">
        </cc1:ModalPopupExtender>
后台代码：
    protected void TxtUser_TextChanged1(object sender, EventArgs e)
    {
        
        int rs = B2CBll.UserInfoManager.SelectUserInfoByLoginName(this.TxtUser.Text);
        if (rs > 0)
        {
            this.Label2.Text = "该用户名已被注册！";
        }
        else
        {
            this.Label2.Text = "";
        }
    }拜托各位大虾了！