关于asp:DropDownList SelectedValue方面的问题

我是ASP.NET自学新手，有ASP.NET方面的问题请高手帮助解答一下：

在辑编表Info记录时：
如信息表Info中的字段有Id, Title,Area1,Area2,Area3,Area4　..等；
地区分类有四级：表Category1（国家,对应该字段class1），表Category2（省市,对应该字段class1,class2），表Category3（城市,,对应该字段class1,class2,class3）,表Category4（区/县,,对应该字段class1,class2,class3,class4）

当我想编辑表Info中的记录时,选择国家-省-城市-地区　在没有SelectedValue='<%# Bind("Area2") %>'这样的Bind时是可以正确请的，但加了SelectedValue='<%# Bind("Area2") %>'后选择失效，
想请高手帮忙处理一下，我是新手，谢谢了!

没有加SelectedValue='<%# Bind(表Info字段) %>'时是可以选择国家-省-城市-地区　的
<asp:UpdatePanel ID="up1" runat="server">
                    <ContentTemplate>
                    <%-- Area1  ------%>
                    <asp:DropDownList ID="ddlArea1" AutoPostBack="true" DataSourceID="obsArea1" DataTextField="Class1" DataValueField="Class1" runat="server">
                    </asp:DropDownList>
                    <asp:ObjectDataSource ID="obsArea1" TypeName="Category1" SelectMethod="SelectByClasses" runat="server">
                        <SelectParameters>
                            <asp:Parameter Name="claType" DefaultValue="国家" Type="String" />
                        </SelectParameters>
                    </asp:ObjectDataSource>

                    <%-- Area2  -------%>
                    <asp:DropDownList ID="ddlArea2" AutoPostBack="true" DataSourceID="obsArea2" DataTextField="Class2" DataValueField="Class2" runat="server">
                    </asp:DropDownList>
                    <asp:ObjectDataSource ID="obsArea2" TypeName="Category2" SelectMethod="SelectByClasses" runat="server">
                        <SelectParameters>
                            <asp:ControlParameter Name="class1" ControlID="ddlArea1"  />
                        </SelectParameters>
                    </asp:ObjectDataSource>

                    <%-- Area3  --------%>
                    <asp:DropDownList ID="ddlArea3" AutoPostBack="true" DataSourceID="obsArea3" DataTextField="Class3" DataValueField="Class3" runat="server">
                    </asp:DropDownList>
                    <asp:ObjectDataSource ID="obsArea3" TypeName="Category3" SelectMethod="SelectByClasses" runat="server">
                        <SelectParameters>
                            <asp:ControlParameter Name="class1" ControlID="ddlArea1" />
                            <asp:ControlParameter Name="class2" ControlID="ddlArea2" />
                    </SelectParameters>
                    </asp:ObjectDataSource>

                    <%-- Area4  --------%>
                    <asp:DropDownList ID="ddlArea4" DataSourceID="obsArea4" DataTextField="class4" DataValueField="class4" runat="server">
                    </asp:DropDownList>
                    <asp:ObjectDataSource ID="obsArea4" TypeName="Category4" SelectMethod="SelectByClasses" runat="server">
                        <SelectParameters>
                            <asp:ControlParameter Name="class1" ControlID="ddlArea1" />
                            <asp:ControlParameter Name="class2" ControlID="ddlArea2" />
                            <asp:ControlParameter Name="class3" ControlID="ddlArea3" />
                        </SelectParameters>
                    </asp:ObjectDataSource>
                    </ContentTemplate>
                    </asp:UpdatePanel>

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但修改SelectedValue后就不能选择了（如下），选择国家-省-城市-地区失效，请高手帮助一下，谢谢：
<asp:UpdatePanel ID="up1" runat="server">
                    <ContentTemplate>
                    <%-- Area1  -----%>
                    <asp:DropDownList ID="ddlArea1" AutoPostBack="true" DataSourceID="obsArea1" DataTextField="Class1" DataValueField="Class1" SelectedValue='<%# Bind("Area1") %>' runat="server">
                    </asp:DropDownList>
                    <asp:ObjectDataSource ID="obsArea1" TypeName="Category1" SelectMethod="SelectByClasses" runat="server">
                        <SelectParameters>
                            <asp:Parameter Name="claType" DefaultValue="国家" Type="String" />
                        </SelectParameters>
                    </asp:ObjectDataSource>

                    <%-- Area2  ------%>
                    <asp:DropDownList ID="ddlArea2" AutoPostBack="true" DataSourceID="obsArea2" DataTextField="Class2" DataValueField="Class2" SelectedValue='<%# Bind("Area2") %>' runat="server">
                    </asp:DropDownList>
                    <asp:ObjectDataSource ID="obsArea2" TypeName="Category2" SelectMethod="SelectByClasses" runat="server">
                        <SelectParameters>
                            <asp:ControlParameter Name="class1" ControlID="ddlArea1"  />
                        </SelectParameters>
                    </asp:ObjectDataSource>

                    <%-- Area3  ---------------------------------------------------------------------------------------%>
                    <asp:DropDownList ID="ddlArea3" AutoPostBack="true" DataSourceID="obsArea3" DataTextField="Class3" DataValueField="Class3" SelectedValue='<%# Bind("Area3") %>' runat="server">
                    </asp:DropDownList>
                    <asp:ObjectDataSource ID="obsArea3" TypeName="Category3" SelectMethod="SelectByClasses" runat="server">
                        <SelectParameters>
                            <asp:ControlParameter Name="class1" ControlID="ddlArea1" />
                            <asp:ControlParameter Name="class2" ControlID="ddlArea2" />
                    </SelectParameters>
                    </asp:ObjectDataSource>

                    <%-- Area4  ---------------------------------------------------------------------------------------%>
                    <asp:DropDownList ID="ddlArea4" DataSourceID="obsArea4" DataTextField="class4" DataValueField="class4" SelectedValue='<%# Bind("Area4") %>' runat="server">
                    </asp:DropDownList>
                    <asp:ObjectDataSource ID="obsArea4" TypeName="Category4" SelectMethod="SelectByClasses" runat="server">
                        <SelectParameters>
                            <asp:ControlParameter Name="class1" ControlID="ddlArea1" />
                            <asp:ControlParameter Name="class2" ControlID="ddlArea2" />
                            <asp:ControlParameter Name="class3" ControlID="ddlArea3" />
                        </SelectParameters>
                    </asp:ObjectDataSource>
                    </ContentTemplate>
                    </asp:UpdatePanel> --------------------编程问答-------------------- 绑定好后
在后台设置一下SelectedIndex就可以了绑它做什么

补充：.NET技术 ,  ASP.NET