using
 System;

using
 System.Data;

using
 System.Data.SqlClient;

using
 System.Configuration;

using
 System.Collections;

using
 System.Web;

using
 System.Web.Security;

using
 System.Web.UI;

using
 System.Web.UI.WebControls;

using
 System.Web.UI.WebControls.WebParts;

using
 System.Web.UI.HtmlControls;



/**/
/// <summary>
/// AjaxMethod 的摘要描述
/// </summary>



public
 
class
 AjaxMethod


{
    GetDataSet#region GetDataSet
    public static DataSet GetDataSet(string sql)
    {
        string connectionString = ConfigurationManager.AppSettings["dbConnectionString"];
        SqlDataAdapter sda = new SqlDataAdapter(sql, connectionString);
        DataSet ds = new DataSet();
        sda.Fill(ds);
        return ds;
    }
    #endregion

    [Ajax.AjaxMethod(Ajax.HttpSessionStateRequirement.Read)]
    public static DataSet GetCatList(string id)
    {
        string sql = "SELECT CAT_ID,CAT_NAME FROM CAT WHERE PARENT_ID='" + id.ToString() + "'";
        return GetDataSet(sql);
    }
}

<%

@ Page Language="C#" AutoEventWireup="true" CodeFile="Dropdownlist.aspx.cs" Inherits="Dropdownlist" EnableEventValidation="false" 
%>



<!
DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" 
>


<
HTML
>


<
HEAD
>


<
title
>
DropDownList示例
</
title
>


<
script 
language
="javascript"
>


//Cat联动部分
function catResult() 
{ 
    var bcat=document.getElementById("<%=ddlbCat.ClientID%>");
    if (bcat.value != "")
    {
       AjaxMethod.GetCatList(bcat.value,get_cat_Result_CallBack);
    }
    else
    {
      document.all("<%=ddlCat.ClientID%>").length=0;　　  
      document.all("<%=ddlCat.ClientID%>").options.add(new Option("--Select--",""));
    }
}
            
function get_cat_Result_CallBack(response)
{
    if (response.value != null)
    {                    
        document.all("<%=ddlCat.ClientID%>").length=0;　
        document.all("<%=ddlCat.ClientID%>").options.add(new Option("--Select--",""));　            
        var ds = response.value;
        if(ds != null && typeof(ds) == "object" && ds.Tables != null)
        {                    
            for(var i=0; i<ds.Tables[0].Rows.length; i++)
            {
                var name=ds.Tables[0].Rows[i].CAT_NAME;
                var id=ds.Tables[0].Rows[i].CAT_ID;
                document.all("<%=ddlCat.ClientID%>").options.add(new Option(name,id));
            }
        }
    }           
    return
}

</
script
>


</
HEAD
>


<
body
>


<
form 
id
="Form1"
 method
="post"
 runat
="server"
>


<
asp:DropDownList 
ID
="ddlbCat"
 runat
="server"
 Width
="80px"
></
asp:DropDownList
>
&nbsp;&nbsp;&nbsp;


<
asp:DropDownList 
ID
="ddlCat"
 runat
="server"
 Width
="80px"
><
asp:ListItem 
Value
=""
>
--Select--
</
asp:ListItem
></
asp:DropDownList
>


</
form
>


</
body
>


</
HTML
>

using
 System;

using
 System.Data;

using
 System.Configuration;

using
 System.Web;

using
 System.Web.Security;

using
 System.Web.UI;

using
 System.Web.UI.WebControls;

using
 System.Web.UI.WebControls.WebParts;

using
 System.Web.UI.HtmlControls;

using
 System.Data.SqlClient;



/**/
/// <summary>
/// Dropdownlist 的摘要说明。
/// </summary>



public
 partial 
class
 Dropdownlist : System.Web.UI.Page


{
    private void Page_Load(object sender, System.EventArgs e)
    {
        Ajax.Utility.RegisterTypeForAjax(typeof(AjaxMethod));

        if (!this.Page.IsPostBack)
        {
            //Load Controls
            this.DDLBind(this.ddlbCat, "select [CAT_ID],[CAT_NAME] from CAT where PARENT_ID='0'");
            this.ddlbCat.Attributes.Add("onclick", "catResult();");
        }
    }
    //Load DropDownList
    private void DDLBind(System.Web.UI.WebControls.DropDownList DDL, string SQLString)
    {
        string connectionString = ConfigurationManager.AppSettings["dbConnectionString"];
        SqlConnection connection = new SqlConnection(connectionString);
        connection.Open();
        try
        {
            SqlCommand cmd = new SqlCommand(SQLString, connection);
            SqlDataReader dr = cmd.ExecuteReader();
            DDL.Items.Clear();
            ListItem default_item = new ListItem();
            default_item.Value = "";
            default_item.Text = "--Select--";
            DDL.Items.Add(default_item);

            while (dr.Read())
            {
                ListItem ListItem = new ListItem();
                ListItem.Value = dr.GetValue(0).ToString();
                ListItem.Text = dr.GetString(1);
                DDL.Items.Add(ListItem);
            }
            dr.Close();
        }
        finally
        {
            if (connection.State == ConnectionState.Open)
                connection.Close();
        }
    }
    //Load Selected DropDownList
    public static void DDLOldBind(System.Web.UI.WebControls.DropDownList DDL, string sql, string Id)
    {
        DDL.Items.Clear();
        ListItem default_item = new ListItem();
        default_item.Value = "";
        default_item.Text = "--Select--";
        DDL.Items.Add(default_item);
        try
        {
            DataTable dt = ExecuteQuery(sql);
            if (dt.Rows.Count >= 1)
            {
                foreach (DataRow dr in dt.Rows)
                {
                    ListItem ListItem = new ListItem();
                    ListItem.Value = dr[0].ToString();
                    ListItem.Text = dr[1].ToString();
                    DDL.Items.Add(ListItem);
                }
            }
            DDL.SelectedValue = Id;
        }
        catch (System.Data.SqlClient.SqlException E)
        {
            throw new Exception(DDL.ID.ToString() + E.Message.ToString());
        }
    }

}