省市县三级联动

先在页面上加了这个：

  <asp:ScriptManager ID="ScriptManager1" runat="server" AllowCustomErrorsRedirect="false"
        EnablePartialRendering="true" AsyncPostBackErrorMessage="cuowu">
    </asp:ScriptManager>

实现功能的控件：

  <asp:UpdatePanel ID="UpdatePanel1" runat="server">
                        <ContentTemplate>
                            <asp:DropDownList ID="ddlProvince" runat="server">
                            </asp:DropDownList>
                            <asp:DropDownList ID="ddlCity" runat="server">
                            </asp:DropDownList>
                            <asp:DropDownList ID="ddlVilliage" runat="server">
                            </asp:DropDownList>
                            <ajaxToolkit:CascadingDropDown ID="CascadingDropDown2" runat="server" TargetControlID="ddlProvince"
                                ServicePath="PCAService.asmx" ServiceMethod="GetProvinceContents" Category="Province"
                                PromptText="请选择省份" LoadingText="省份加载中">
                            </ajaxToolkit:CascadingDropDown>
                            <ajaxToolkit:CascadingDropDown ID="CascadingDropDown1" runat="server" ParentControlID="ddlProvince"
                                ServicePath="PCAService.asmx" ServiceMethod="GetCityContents" Category="City"
                                TargetControlID="ddlCity" PromptText="请选择城市" LoadingText="城市加载中">
                            </ajaxToolkit:CascadingDropDown>
                            <ajaxToolkit:CascadingDropDown ID="CascadingDropDown3" runat="server" Category="Villiage"
                                LoadingText="区县加载中" ParentControlID="ddlCity" PromptText="请选择区县" ServiceMethod="GetViliageContents"
                                ServicePath="PCAService.asmx" TargetControlID="ddlVilliage">
                            </ajaxToolkit:CascadingDropDown>
                        </ContentTemplate>
                    </asp:UpdatePanel>

App_Code文件夹下PCAService.cs文件：

using System;
using System.Web;
using System.Collections;
using System.Web.Services;
using System.Web.Services.Protocols;
using System.Web.Script.Services;
using System.Collections.Generic;
using System.Collections.Specialized;
using AjaxControlToolkit;
using System.Data.SqlClient;
using System.Data;
using System.Configuration;

/**//// <summary>
///PCAService 的摘要说明
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.Web.Script.Services.ScriptService]
public class PCAService : System.Web.Services.WebService
{
    public SqlConnection sqlConn;

    public PCAService()
    {

        //如果使用设计的组件，请取消注释以下行 
        //InitializeComponent(); 
        this.sqlConn = new SqlConnection(ConfigurationManager.ConnectionStrings["LocalSqlServer"].ConnectionString);
    }

    /**//// <summary>
    /// 获取省份数据
    /// </summary>
    /// <param name="knownCategoryValues"></param>
    /// <param name="category"></param>
    /// <returns></returns>
    [WebMethod]
    public CascadingDropDownNameValue[] GetProvinceContents(string knownCategoryValues, string category)
    {
        List<CascadingDropDownNameValue> provinceList = new List<CascadingDropDownNameValue>();

        sqlConn.Open();

        string strSql = "Select * From Province";

        SqlCommand sqlCmd = new SqlCommand(strSql, sqlConn);

        SqlDataReader dtrProvince = sqlCmd.ExecuteReader();

        while (dtrProvince.Read())
        {
            provinceList.Add(new CascadingDropDownNameValue(dtrProvince["Name"].ToString(), dtrProvince["Code"].ToString()));
        }

        dtrProvince.Close();

        sqlConn.Close();

        return provinceList.ToArray();
    }
    /**//// <summary>
    /// 获取市数据
    /// </summary>
    /// <param name="knownCategoryValues"></param>
    /// <param name="category"></param>
    /// <returns></returns>
    [WebMethod]
    public CascadingDropDownNameValue[] GetCityContents(string knownCategoryValues, string category)
    {
        StringDictionary provinceList = CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);

        sqlConn.Open();

        string strSql = "Select * From city Where ProvinceId='" + provinceList["Province"] + "'";

        SqlCommand sqlCmd = new SqlCommand(strSql, sqlConn);

        SqlDataReader dtrCity = sqlCmd.ExecuteReader();

        List<CascadingDropDownNameValue> cityList = new List<CascadingDropDownNameValue>();

        while (dtrCity.Read())
        {
            cityList.Add(new CascadingDropDownNameValue(dtrCity["Name"].ToString(), dtrCity["code"].ToString()));
        }

        dtrCity.Close();

        return cityList.ToArray();
    }
    /**//// <summary>
    /// 获取乡镇数据
    /// </summary>
    /// <param name="knownCategoryValues"></param>
    /// <param name="category"></param>
    /// <returns></returns>
    [WebMethod]
    public CascadingDropDownNameValue[] GetViliageContents(string knownCategoryValues, string category)
    {
        StringDictionary cityList = CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);

        sqlConn.Open();

        string strSql = "Select * From area Where CityId='" + cityList["City"] + "'";

        SqlCommand sqlCmd = new SqlCommand(strSql, sqlConn);

        SqlDataReader dtrViliage = sqlCmd.ExecuteReader();

        List<CascadingDropDownNameValue> viliageList = new List<CascadingDropDownNameValue>();

        while (dtrViliage.Read())
        {
            viliageList.Add(new CascadingDropDownNameValue(dtrViliage["Name"].ToString(), dtrViliage["id"].ToString()));
        }

        dtrViliage.Close();

        return viliageList.ToArray();
    }

}