创建要部署到 SharePoint 上的可视 Web 部件时出现问题

嗨，我是 C# 的新手，我正在尝试实现一个与此网站相关的示例：http://www.devmanuals.com/tutorials/ms/aspdotnet/dropdownlist.html

我要做的就是创建 Web 部件并将其部署到 SharePoint 上：法典：

<%@ Page Title="" Language="C#" MasterPageFile="~/RoseindiaMaster.master"
AutoEventWireup="true" CodeFile="DropDownList.aspx.cs" Inherits="DropDownList" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
<style type="text/css">
.style3
{
color: #800000;
}
</style>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<div>
<h2 style="color:Green">DropDownList in ASP.NET 4 , C#</h2>
<strong><span class="style3">Enter first number:</span>
 </strong>
 <br />
 <asp:TextBox ID="txt1" runat="server" Text="12"/>
   <br />
  <br />
 <span class="style3">
 <strong>Enter second number:
  </strong>
 </span>

逻辑代码：

protected void drp1_SelectedIndexChanged(object sender, EventArgs e)
{
double firstno = Convert.ToDouble(txt1.Text);
double secondno = Convert.ToDouble(txt2.Text);
if(drp1.SelectedIndex == 1)
{
double add = firstno + secondno;
label1.Text = "Addition is :" + add;
}

我不断收到错误：无法识别 TXT1，请指教，我是 C# 的新手

protected TextBox txt1;