how to invoke class method asp.net C#
<p>this code is a excerpt from the book asp.net all in one reference for dummies on pages 18 and 19. The problem with this is that it doesn't compile. The compiler says txtFirst, txtSecond and lblAnswer are not found in the current context. I am new in this field, could someone please help me out ?</p>

<pre><code>&lt;%@ Page Language="C#" AutoEventWireup="true"
    CodeFile="Default.aspx.cs" Inherits="_Default" %&gt;

&lt;!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0
Transitional//EN" “http://www.w3.org/TR/xhtml1/DTD/xhtml1-
transitional.dtd"&gt;
&lt;html xmlns="http://www.w3.org/1999/xhtml"&gt;
&lt;head id="Head1" runat="server"&gt;
    &lt;title&gt;Simple Calculator&lt;/title&gt;
&lt;/head&gt;
&lt;body&gt;
    &lt;form id="form2" runat="server"&gt;
        &lt;div&gt;
            &lt;h1&gt;The Simple Calculator&lt;/h1&gt;
            First number:
            &lt;asp:TextBox ID="txtFirst" runat="server" /&gt;
            &lt;br /&gt;
            &lt;br /&gt;
            Second number:
            &lt;asp:TextBox ID="txtSecond" runat="server" /&gt;
            &lt;br /&gt;
            &lt;br /&gt;
            &lt;asp:Button ID="btnAdd" runat="server"
                OnClick="btnAdd_Click" Text="Add" /&gt;
            &lt;br /&gt;
            &lt;br /&gt;
            The answer is:
            &lt;asp:Label ID="lblAnswer" runat="server" /&gt;
        &lt;/div&gt;
    &lt;/form&gt;
&lt;/body&gt;
&lt;/html&gt;



using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
namespace WebApplication2
{
    public partial class _Default : System.Web.UI.Page
    {

        protected void btnAdd_Click(object sender,EventArgs e)
        {
            decimal a = decimal.Parse(txtFirst.Text);
            decimal b = decimal.Parse(txtSecond.Text);
            decimal c = a + b;
            lblAnswer.Text = c.ToString();
        }
    }
}
</code></pre>
