显示出选中的复选框的值 （要求必须使用HTML服务器控件，如下） 为什么不行？？

前台代码<form id="form1" runat="server">
    <div id="DIV1" runat="server">
        <input id="Checkbox1" runat="server" type="checkbox" />
        <input id="Checkbox2" runat="server" type="checkbox" />
        <input id="Checkbox3" runat="server" type="checkbox" />
        <input id="Checkbox4" runat="server" type="checkbox" />
        <input id="Checkbox5" runat="server" type="checkbox" />
        
        <asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" /></div>
    </form>
后台代码
 protected void Page_Load(object sender, EventArgs e)
        {            this.Checkbox1.Attributes["name"] = "my";
            this.Checkbox2.Attributes["name"] = "my";
            this.Checkbox3.Attributes["name"] = "my";
            this.Checkbox4.Attributes["name"] = "my";
            this.Checkbox5.Attributes["name"] = "my";
            this.Checkbox1.Attributes["value"] = "val1";
            this.Checkbox2.Attributes["value"] = "val2";
            this.Checkbox3.Attributes["value"] = "val3";
            this.Checkbox4.Attributes["value"] = "val4";
            this.Checkbox5.Attributes["value"] = "val5";
        }        protected void Button1_Click(object sender, EventArgs e)
        {
            Response.Write(Request.Form["my"]);
        }