Aspx.cs doesn't recognize controls declared in aspx?
<p>I created a branch new web project:
File -> New -> Project  -> ASP.NET Web Application</p>

<p>Then I tried to add a simple control asp:Label but the aspx.cs doesn't recognize this declaration for some reasons. I didn't make any changes to the default project. It contains 6 pages:</p>

<p><img src="http://i.stack.imgur.com/NAEex.png" alt="enter image description here"></p>

<p>Does anyone know what could cause this issue? Thanks.</p>

<p><strong>EDIT</strong><br>
Here what I added to the file <code>About.aspx</code> is a label with id = <code>id1</code></p>

<pre><code>&lt;%@ Page Title="About Us" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"
    CodeBehind="About.aspx.cs" Inherits="WebApplication2.About" %&gt;

&lt;asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent"&gt;
&lt;/asp:Content&gt;
&lt;asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent"&gt;
    &lt;asp:label ID="id1" runat="server" &gt;A Label&lt;/asp:label&gt;
    &lt;h2&gt;
        About
    &lt;/h2&gt;
    &lt;p&gt;
        Put content here.
    &lt;/p&gt;
&lt;/asp:Content&gt;
</code></pre>

<p>I ran the project without calling that variable <code>id1</code>, everything works fine. </p>

<p><img src="http://i.stack.imgur.com/WfpU8.png" alt="enter image description here"></p>

<p>But if I called <code>id1</code> within <code>About.aspx.cs</code>, it gave me error said the variable was not found.</p>

<pre><code>using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace WebApplication2 {
    public partial class About : System.Web.UI.Page {
        protected void Page_Load(object sender, EventArgs e) {
            id1.Text = "What the heck?";
        }
    }
}
</code></pre>
