c# - 更新面板和回发问题

Question

我有 2 个 ddl、ddlCountry 和 ddlState，在选择 ddlCountry 时，ddlState 会根据所选国家/地区进行更新，但是...当我选择 ddlState 时，ddlState 中的所选项目未标记为选中，因此我无法获得选定的值。

这是代码：

<asp:DropDownList 
    ID="ddlReceiverCountry" 
    runat="server" 
    CssClass="textBox" 
    OnSelectedIndexChanged="ddlReceiverCountry_SelectedIndexChanged" 
    AutoPostBack="true" >
</asp:DropDownList>
    <br />
<asp:UpdatePanel ID="updatePanel" runat="server" UpdateMode="Conditional"  >
    <ContentTemplate>
        <asp:Label ID="lblReceiverState" runat="server" Text="State" CssClass="newReceiverlabel">
        </asp:Label>
        <asp:DropDownList 
            ID="ddlReceiverState" 
            runat="server" 
            CssClass="textBox" OnSelectedIndexChanged="ddlReceiverState_SelectedIndexChanged">     
        </asp:DropDownList>
    </ContentTemplate>
    <Triggers>
        <asp:PostBackTrigger ControlID="ddlReceiverState" />
        <asp:AsyncPostBackTrigger ControlID="ddlReceiverCountry" EventName="SelectedIndexChanged"   />
    </Triggers>
</asp:UpdatePanel>