我们简单的模拟上面功能，

         <cc1:CascadingDropDown ID="CascadingDropDown2" runat="server" TargetControlID="DropDownList2"
         Category="Province"       //唯一性

PromptText="请选择省份..."

         LoadingText="省份加载中..."
         ServicePath="Country.asmx"

          ServiceMethod="GetProvince"  

      ParentControlID="DropDownList1"
         >
         </cc1:CascadingDropDown>
         <cc1:CascadingDropDown ID="CascadingDropDown3" runat="server" TargetControlID="DropDownList3"
         Category="City" PromptText="请选 择城市..." LoadingText="城市加载中..."
         ServicePath="Country.asmx" ServiceMethod="GetCity"   ParentControlID="DropDownList2">
         </cc1:CascadingDropDown>
         <cc1:CascadingDropDown ID="CascadingDropDown1" runat="server" TargetControlID="DropDownList1"
         Category="Country" PromptText="请选择国家..." LoadingText="国家加载中..."
         ServicePath="Country.asmx" ServiceMethod="GetCountry"
         >
         </cc1:CascadingDropDown>

下面是我们的服务方法

/// <summary>
/// Country 的摘要说明
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.Web.Script.Services.ScriptService()]
public class Country : System.Web.Services.WebService
{
     [WebMethod]
     public CascadingDropDownNameValue[] GetCountry(string knownCategoryValues, string category)

//不能修改返回值，形参，唯一可以修改的是函数名，否则，就不能引用
     {

         //如果使用设计的组件，请取消注释以下行
         //InitializeComponent();
         List<CascadingDropDownNameValue> CountryList = new List<CascadingDropDownNameValue>();
         CountryList.Add(new CascadingDropDownNameValue("中国","china"));
         CountryList.Add(new CascadingDropDownNameValue("其他","Other"));
         return CountryList.ToArray();

     }
     [WebMethod]
     public CascadingDropDownNameValue[] GetProvince(string knownCategoryValues, string category)
     {
         StringDictionary kv = CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
         if(kv["Country"]!="china")

       //就是前面的category值
             return null;
         List<CascadingDropDownNameValue> ProvList = new List<CascadingDropDownNameValue>();
         ProvList.Add(new CascadingDropDownNameValue("安徽", "ah"));
         ProvList.Add(new CascadingDropDownNameValue("北京", "bh"));
         return ProvList.ToArray();
     }

     [WebMethod]
     public CascadingDropDownNameValue[] GetCity(string knownCategoryValues, string category)
     {
         StringDictionary kv = CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues);
         if (kv["Province"] != "ah")
             return null;
         List<CascadingDropDownNameValue> CityList = new List<CascadingDropDownNameValue>();
         CityList.Add(new CascadingDropDownNameValue("安庆", "aq"));
         CityList.Add(new CascadingDropDownNameValue("合肥", "bh"));
         return CityList.ToArray();
     }
     [WebMethod]
     public string HelloWorld()
     {
         return "Hello World";
     }

}

可能遇到问题：返回异常：

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