页面文件

                 
       
        城市1
                   
       
        城市2
        
       
       
      

程序文件

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)        {
    if (DropDownList1.SelectedItem.Text == "城市1")                {                       DropDownList2.Items.Clear(); DropDownList2.Items.Add("区1"); DropDownList2.Items.Add("区2");                } else if (DropDownList1.SelectedItem.Text == "城市2")              {
                          DropDownList2.Items.Clear(); DropDownList2.Items.Add("区3"); DropDownList2.Items.Add("区4");              }       }

 注意第一个dropdownlist的AutoPostBack="True"一定要设置 其实如果这种省市区联动的一般不用数据库，直接在js里面写，或者找现成的有很多，

放数据库里浪费服务器资源，而且刷新效果不好。

提供一个比较好的省市三级联动的DropDownList+Ajax的三种框架(aspnet/Jquery/ExtJs)

示例：

 

附加一个使用数据库数据的例子：

前台控件代码：

后台代码段：

protected string SqlModuleList = ""; protected string SqlTypeList = "";     SqlHelper sqlhelper = new SqlHelper(); protected void Page_Load(object sender, EventArgs e)     {
   if (!IsPostBack)         {
               ModuleList(); //首次加载模块的下拉数据         }     } protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)     {
   if (DropDownList1.SelectedItem.Text != "")         {
               SqlTypeList = "select TypeID,TypeName from KY_SendMessage_Type as T,KY_SendMessage_Module as M                             where T.ModuleID=M.ModuleID and T.ModuleID=" + DropDownList1.SelectedValue + "";             DataSet ds = sqlhelper.readDataSet(SqlTypeList);             DataTable dt = ds.Tables[0]; if (dt.Rows.Count != 0)//判断模块下面是否有子分类             {
                   Label2.Visible = true;                 DropDownList2.Visible = true;                 DropDownList2.Items.Clear();             } else             {
                   Label2.Visible = false;                 DropDownList2.Visible = false;             }             DropDownList2.DataSource = dt;             DropDownList2.DataValueField = "TypeID";             DropDownList2.DataTextField = "TypeName";             DropDownList2.DataBind();         }     } protected void ModuleList()     {
           SqlModuleList = "select * from KY_SendMessage_Module as M";         DataSet ds = sqlhelper.readDataSet(SqlModuleList);         DataTable dt = ds.Tables[0];         DropDownList1.DataSource = dt;         DropDownList1.DataValueField = "ModuleID";         DropDownList1.DataTextField = "ModuleName";         DropDownList1.DataBind();     }

部分效果图：

                         初始加载图

　　　　　　　点击选项加载子下拉图