Every .aspx page starts with a Page directive. This Page directive specifies the language for the page,
and it also tells ASP.NET where to find the associated code

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="FileName.aspx.cs" Inherits="FileName"%>

Notice that Visual Studio uses a slightly unusual naming syntax for the source code file. It has
the full name of the corresponding web page, complete with the .aspx extension, followed by the .cs
extension at the end.