Coinbase IPN test page not posting data?
Instant Payment Notification callbacks are POSTed in the form of JSON data.  The issue may be that you're attempting to iterate over regular http params that aren't there, instead of parsing the response body as JSON.  I've never used ASP.net before but this topic looks like it may help:

https://stackoverflow.com/questions/3398926/how-to-retrieve-json-via-asp-net-context-request/3411775