Codebehind

I´m writing an application with 2 files(aspx and cs). This is my first application on that style, I´m following a book, but I´m getting this error once I try to run the application:
Parser Error
Description: An error occurred during the parsing of a resource required to service this request. Please review the following specific parse error details and modify your source file appropriately.
Parser Error Message: Could not load type 'HelloCodeBehind'.
Source Error:

Line 1:  <%@ Page Language="C#" CodeFile="HelloCodeBehind.cs"
Line 2:      Inherits="HelloCodeBehind"%>
Line 3:  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
 
 I try with the original samples provided by this book, but it does not work. What is my problem?
This is my code:
HelloCodeBehind.aspx
& ...