Why can't I find the CS file in the ES0en. net page

 
<%@ Page language="c#" Codebehind="Main.aspx.cs" AutoEventWireup="false" Inherits="Hr.WebForm1" %> 

Do not use CodeBehind on the top line. This is used to put Dll into the Bin directory after generating CS files.
If the cs file is not compiled, change to

 
<%@ Page language="c#" Src="Main.aspx.cs" AutoEventWireup="false" Inherits="Hr.WebForm1" %>