My question is very similar this but I guess I need to take it one step further.

Facebook says "The data is passed to your application as a signed request. The signed_request parameter is a simple way to make sure that the data you're receiving is the actual data sent by Facebook."

After a user has logged into my asp c# MVC site and clicked "Register", the redirect-url is http://site/account/register. At that point (the post to the account/register control), I would like to gather the user's information using the signed request so that I can register them with my site locally. I cannot figure out how to access the data facebook makes available.

$data = json_decode(base64_url_decode($payload), true);

What is the equivalent in C#? What type of variable/data is facebook passing in the post? And how do I access "$payload"?