多级联动Dropdownlist(刷新版)

添加三个dropdownlist

<asp:DropDownList ID="a" runat="server" OnSelectedIndexChanged="a_SelectedIndexChanged" AutoPostBack="true"></asp:DropDownList>
            <asp:DropDownList ID="d" runat="server" OnSelectedIndexChanged="d_SelectedIndexChanged" AutoPostBack="true"></asp:DropDownList>
            <asp:DropDownList ID="m" runat="server"></asp:DropDownList>

后台代码

 protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                this.bindAcaData();
            }
        }

        private void bindAcaData()//绑定学院
        {
            AcadamyBLL ab = new AcadamyBLL();
            this.a.DataSource = ab.SelectAll();
            this.a.DataTextField = "academyName";
            this.a.DataValueField = "id";
            this.a.DataBind();

        }
        private void bindDepData()//绑定学系
        {
            DepartmentBLL db = new DepartmentBLL();
            this.d.DataSource = db.SelectDepartmentByAcademy(this.a.SelectedValue);
            this.d.DataTextField = "departmentName";
            this.d.DataValueField = "id";
            this.d.DataBind();
        }

        private void bindMajData()//绑定专业
        {
            MajorBLL mb = new MajorBLL();
            this.m.DataSource = mb.GetMajorByDepartment(this.d.SelectedValue);
            this.m.DataTextField = "majorName";
            this.m.DataValueField = "id";
            this.m.DataBind();
        }

        protected void a_SelectedIndexChanged(object sender, EventArgs e)
        {
            this.bindDepData();
        }

        protected void d_SelectedIndexChanged(object sender, EventArgs e)
        {
            this.bindMajData();
        }