FileUpload 在updatepanel 中 出现" 未将对像引用到实例" 错误 或

在asp:UpdatePanel控件中直接使用FileUpload，服务端是无法找到上传文件的。

只要在<Trigger>中添加button就可以解决这个问题

<asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<table border="1" cellpadding="0" cellspacing="0" class="tableframe" width="100%">
<tr>
<td class="titlebar" style="background-color: #cce4f7; width: 100%; height: 30px"
colspan="6" valign="middle" align="center">
Order of display</td>
</tr>
<tr>
<td style="width: 15%; height: 27px; background-color: #cce4f7;" valign="middle"
align="right">
Manually re-arrange:</td>
<td align="left" colspan="1" style="width: 35%">
<asp:TextBox ID="tbNum" runat="server" Width="30px" Height="17px" MaxLength="2"
TabIndex="13">0</asp:TextBox>
<ajaxToolkit:FilteredTextBoxExtender ID="ft1" runat="server" FilterType="Custom, Numbers"
TargetControlID="tbNum">
</ajaxToolkit:FilteredTextBoxExtender>
</td>
<td align="right" colspan="1" style="width: 15%; height: 27px; background-color: #cce4f7">
Data:</td>
<td align="left" colspan="3" style="width: 35%">
<asp:FileUpload ID="FileUpload1" runat="server" Height="20px" Width="95%" TabIndex="14" /></td>
</tr>
<tr>
<td align="center" colspan="6" style="height: 30px">
<asp:Button ID="Button1" runat="server" CssClass="submit" Height="20px" _disibledevent="Button1" />
</Triggers>
</asp:UpdatePanel>