使用隐藏字段.在设置下拉值的地方设置其值.
例如

Use hidden field. Set its value where dropdown value is set.
e.g

<asp:dropdownlist id="ddl_GetLists" >
                    SelectedValue='<%#Bind("listID") %>'
                    runat="server"
                    >
</asp:dropdownlist>

如果要更改隐藏字段值，可以根据需要在DropDownList selectedindexChanged事件中对其进行更改.

If you want to change hidden field value you can change it in DropDownList selectedindexChanged event as per your requirement.