Asp.net DropDownList 四级联动

                        DataTextField="c1_name" DataValueField="id">
                   
                                            DataValueField="id" onselectedindexchanged="c2_id_SelectedIndexChanged" AutoPostBack="True">
                   
                                            DataValueField="id" onselectedindexchanged="c3_id_SelectedIndexChanged">
                   
                   
                   

后台：
private ProductClassService service = new ProductClassService();
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            UserDataBind();
            c1_id_SelectedIndexChanged(sender, e);
        }
    }
    //绑定一级分类
    private void UserDataBind()
    {
        c1_id.DataSource = service.getPClassList1();
        c1_id.DataBind();

    }
   //绑定二级分类
    protected void c1_id_SelectedIndexChanged(object sender, EventArgs e)
    {
        string id = c1_id.SelectedValue;
        c2_id.DataSource = service.getPClassList2(id);
        c2_id.DataBind();
        c2_id_SelectedIndexChanged(sender,e);
    }
    //绑定三级分类
    protected void c2_id_SelectedIndexChanged(object sender, EventArgs e)
    {
        string id = c2_id.SelectedValue;
        IList list = service.getPClassList3(id);
        c3_id.DataSource = list;
        c3_id.DataBind();
        if (service.getPClassList3(id).Count <= 0)
        {
            ListItem item = new ListItem("没有选择", "0");
            c3_id.Items.Insert(0, item);
        }
        c3_id_SelectedIndexChanged(sender, e);
    }
   //绑定四级分类
    protected void c3_id_SelectedIndexChanged(object sender, EventArgs e)
    {
        string id = c3_id.SelectedValue;
        IList list = service.getPClassList4(id);
        c4_id.DataSource = list;
        c4_id.DataBind();
        if (list.Count <= 0)
        {
            ListItem item = new ListItem("没有选择", "0");
            c4_id.Items.Insert(0, item);
        }
    }

http://blog.csdn.net/wengman/article/details/4095913

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注意：
asp:DropDownList控件触发OnSelectedIndexChanged，AutoPostBack="true" 必须设置
设置默认值
ListItem item = new ListItem("请选择", "0");
this.DropDownList1.Items.Insert(0, item);