ASP.NET：UpdateProgress不适用于ClientIDMode =“Static”的控件

Look at this markup:

看看这个标记:

<asp:UpdatePanel ID="Panel1" runat="server" UpdateMode="Conditional">
    <ContentTemplate>
        <asp:DropDownList ID="cboBox1" ClientIDMode="Static" AutoPostBack="true" runat="server" />
    </ContentTemplate>
</asp:UpdatePanel>

<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
    <ContentTemplate>
        <asp:DropDownList ID="cboBox2" runat="server" />
        <asp:UpdateProgress ID="UpdateProgress1" style="display: inline" AssociatedUpdatePanelID="Panel1" DynamicLayout="false" DisplayAfter="0" runat="server">
            <ProgressTemplate>
                <img src='<%= ResolveClientUrl("~/Images/indicator.gif")%>' border="0" alt="" />
            </ProgressTemplate>
        </asp:UpdateProgress>
    </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="cboBox1" EventName="SelectedIndexChanged" />
    </Triggers>
</asp:UpdatePanel>
<asp:UpdatePan