How to implement Paypal IPN, Instant Payment Notification, in ASP.NET?

Step one:
Understand IPN working flow:

https://cms.paypal.com/us/cgi-bin/?&cmd=_render-content&content_ID=developer/e_howto_admin_IPNIntro


Step two:
The code for Step 3
https://cms.paypal.com/cms_content/CA/en_US/files/developer/IPN_ASP_NET_C.txt