Label doesnt exist in the current context

why m i getting this error "The name 'lblHelloWorld' does not exist in the current context"? How do i fix it?

<%@ Page Language="C#" AutoEventWireup="True" Inherits="_Default"      Codebehind="Default.aspx.cs" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0     Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title>Hello, world!</title>
</head>
<body>
<form id="form1" runat="server">
    <asp:ScriptManager ID="MainScriptManager" runat="server" />
    <asp:UpdatePanel ID="pnlHelloWorld" runat="server">
        <ContentTemplate>
            <asp:Label runat="server" ID="lblHelloWorld" Text="Click the button!" />
            <br /><br />
            <asp:Button runat="server" ID="btnHelloWorld" OnClick="btnHelloWorld_Click" Text="Update label!" />
        </ContentTemplate>
    </asp:UpdatePanel>
</form>

using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace ajaxTesting
{
public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void btnHelloWorld_Click(object sender, EventArgs e)
    {
        lblHelloWorld.Text = "Hello, world - this is a fresh message from ASP.NET AJAX! The time right now is: " + DateTime.Now.ToLongTimeString();
    }
}
}

I tried cleaning and rebuliding; deleting the designer.cs file and recreating it but was of no use.

Solution

I think the problem in your designer.cs file.Try to add a reference manually for this label in the designer.cs file.

EDIT:

The problem after revision to your code is in the name space.

To fix your problem::

Replace your line by this:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="ajaxTesting._Default" %>