c# – 如何在不重新加载页面的情况下刷新下拉列表？

<asp:DropDownList AutoPostBack="True" OnSelectedIndexChanged="ddlMain_SelectedIndexChanged" ClientIDMode="Static" ID="ddlMain" name="searchPhys" style="width: 365px;" class="default" runat="server" AppendDataBoundItems="true">
    <asp:ListItem Text="BY PHYSICIAN" Value="0" Selected="True" />
    <asp:ListItem Text="BY LOCATION" Value="1" />
    <asp:ListItem Text="BY SPECIALTY" Value="2" />
</asp:DropDownList>

<br /><br />

<asp:DropDownList ClientIDMode="Static" ID="ddlDrillDown" name="searchPhys" style="width: 365px;" class="default" runat="server" AppendDataBoundItems="true">
</asp:DropDownList>

我处理下拉列表更改的代码隐藏是：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Xml.Linq;
using System.Configuration;
using System.Windows.Forms;
using System.Data;

public partial class physicians : System.Web.UI.Page
{

    protected void Page_Load(object sender,EventArgs e) {

    if (!Page.IsPostBack) {
        PopulatePhysician();
    }
    //PopulateSpecialty();
    //PopulateLocation();

    }

    public void PopulatePhysician() {
        SqlCommand cmd = new SqlCommand("getPhysicians",new SqlConnection(ConfigurationManager.AppSettings["ConnString"]));
        //cmd.CommandType = Data.CommandType.StoredProcedure
        cmd.Connection.Open();

        SqlDataReader ddlValues = default(SqlDataReader);
        ddlValues = cmd.ExecuteReader();

        //if (!IsPostBack) {
            ddlDrillDown.Items.Clear();
            ddlDrillDown.DataSource = ddlValues;
            ddlDrillDown.DataValueField = "content_id";
            ddlDrillDown.DataTextField = "content_title";
            ddlDrillDown.DataBind();
            //set the default value for the drop down
            ListItem Item = new ListItem();
            Item.Text = "Select a Physician's Name";
            Item.Value = "0";
            //Item.Selected = True
            ddlDrillDown.Items.Insert(0,Item);
        //}
    cmd.Connection.Close();
    cmd.Connection.Dispose();
    }

    public void PopulateSpecialty() {
        SqlCommand cmd = new SqlCommand("getSpecialties",new SqlConnection(ConfigurationManager.AppSettings["ConnString"]));
        cmd.Connection.Open();

        SqlDataReader ddlValues = default(SqlDataReader);
        ddlValues = cmd.ExecuteReader();

        //if (!IsPostBack) {
            ddlDrillDown.Items.Clear();
            ddlDrillDown.DataSource = ddlValues;
            ddlDrillDown.DataValueField = "content_id";
            ddlDrillDown.DataTextField = "content_title";
            ddlDrillDown.DataBind();
            //set the default value for the drop down
            ListItem Item = new ListItem();
            Item.Text = "Select a Specialty";
            Item.Value = "0";
            ddlDrillDown.Items.Insert(0,Item);
        //}
        cmd.Connection.Close();
        cmd.Connection.Dispose();
    }

    public void PopulateLocation() {
        SqlCommand cmd = new SqlCommand("getLocations",new SqlConnection(ConfigurationManager.AppSettings["ConnString"]));
        cmd.Connection.Open();

        SqlDataReader ddlValues = default(SqlDataReader);
        ddlValues = cmd.ExecuteReader();

        //if (!IsPostBack) {
            ddlDrillDown.Items.Clear();
            ddlDrillDown.DataSource = ddlValues;
            ddlDrillDown.DataValueField = "content_id";
            ddlDrillDown.DataTextField = "content_title";
            ddlDrillDown.DataBind();

            //set the default value for the drop down
            ListItem Item = new ListItem();
            Item.Text = "Select a Location";
            Item.Value = "0";
            ddlDrillDown.Items.Insert(0,Item);
        //}
        cmd.Connection.Close();
        cmd.Connection.Dispose();
    }

    public void ddlMain_SelectedIndexChanged(object sender,System.EventArgs e) {
        switch(ddlMain.SelectedIndex) {
            case 0:
                PopulatePhysician();
                break;
            case 1:
                PopulateLocation();
                break;
            case 2:
                PopulateSpecialty();
                break;
        }
    }
}

我试图添加到上面的功能是,当用户从ddlMain下拉列表中选择一个选项,根据选项刷新ddlDrillDown下拉列表而不重新加载页面.

我怎样才能实现它？

更新：

<asp:ScriptManager ID="ScriptManager" 
                               runat="server" />
            <asp:UpdatePanel ID="UpdatePanel1" 
                             UpdateMode="Conditional"
                             runat="server">
                <ContentTemplate>
                    <asp:DropDownList AutoPostBack="True" OnSelectedIndexChanged="ddlMain_SelectedIndexChanged" ClientIDMode="Static" ID="ddlMain" style="width: 365px;" class="default" runat="server" AppendDataBoundItems="true">
                        <asp:ListItem Text="BY PHYSICIAN" Value="0" Selected="True" />
                        <asp:ListItem Text="BY LOCATION" Value="1" />
                        <asp:ListItem Text="BY SPECIALTY" Value="2" />
                    </asp:DropDownList>
                    <br /><br />
                    <asp:DropDownList ClientIDMode="Static" ID="ddlDrillDown" style="width: 365px;" class="default" runat="server" AppendDataBoundItems="true">
                    </asp:DropDownList>
                    </ContentTemplate>
            </asp:UpdatePanel>