Question

I created a branch new web project:
File -> New -> Project -> ASP.NET Web Application

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:

Does anyone know what could cause this issue? Thanks.

EDIT
Here what I added to the file About.aspx is a label with id = id1

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

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

I ran the project without calling that variable id1, everything works fine.

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

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?";
        }
    }
}

Answer 1

Based on you comment it seems your your About.aspx.designer.cs file is missing the Label definition. Delete your About.aspx.designer.cs file and to regenerate it, right click on the Project (WebApplication2) and Choose ‘Convert To Web Application‘ this will regenerate your About.aspx.designer.cs file.