How to decode OAuth 2.0 for Canvas signed_request in C#?

I'm able to successfully validate the signed request for a Facebook canvas app using the example here, but I'm unable to decode the payload. The Facebook documentation states that the 2nd parameter in signed_request is a base64url encoded JSON object. In PHP the payload is decoded using json_decode:

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

What is the equivalent in C#?