如何使用下拉列表填充/输入数据？

Here's my existing code:

这是我现有的代码:

 <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" 
            DataSourceID="dsEmployees" DataTextField="Last_First" 
            DataValueField="EmpNum" 
            onselectedindexchanged="DropDownList1_SelectedIndexChanged">
 </asp:DropDownList>
 <asp:Dr