asp.net 动态创建控件和移除控件

<form id="form1" runat="server">
    <div>
        <asp:Panel ID="pnlAdd" runat="server">
            <asp:CheckBox ID="CheckBox1" runat="server" />
            <!-- 制定动态控件的位置 -->
           <placeholder id="p1" runat="server"></placeholder>
            <asp:Calendar ID="Calendar1" runat="server">
            </asp:Calendar>
        </asp:Panel>
    <hr />
        <asp:Button ID="btnCreate" runat="server" Text="CreateControl" 
            onclick="btnCreate_Click" />
        <asp:Button ID="btnRemove" runat="server" Text="RemoveContrel" 
            onclick="btnRemove_Click" />
    </div>
    </form>

Button btn1 = null;
        //PlaceHolder p1 = new PlaceHolder();
        protected void Page_Load(object sender, EventArgs e)
        {
            btn1 = new Button();
        }

        //创建控件
        protected void btnCreate_Click(object sender, EventArgs e)
        {            
            btn1.ID = "btn1";
            btn1.Text = "动态控件";
           // pnlAdd.Controls.Add(btn1);
            p1.Controls.Add(btn1);
            
        }
        //移除控件
        protected void btnRemove_Click(object sender, EventArgs e)
        {
            //btn1 = (Button)pnlAdd.FindControl("btn1");
            pnlAdd.Controls.Remove(btn1);
        }