You get signed request when your web page is loaded within facebook canvas app; you should be able to parse signed request something similar to following:
if (Request.Params["signed_request"] != null)
{
string payload = Request.Params["signed_request"].Split('.')[1];
var encoding = new UTF8Encoding();
var decodedJson = payload.Replace("=", string.Empty).Replace('-', '+').Replace('_', "https://stackoverflow.com/");
var base64JsonArray = Convert.FromBase64String(decodedJson.PadRight(decodedJson.Length + (4 - decodedJson.Length % 4) % 4, '='));
var json = encoding.GetString(base64JsonArray);
var o = JObject.Parse(json);
var lPid = Convert.ToString(o.SelectToken("page.id")).Replace("\"", "");
var lLiked = Convert.ToString(o.SelectToken("page.liked")).Replace("\"", "");
var lUserId= Convert.ToString(o.SelectToken("user_id")).Replace("\"", "");
}
You need to add reference to json libraries in order to parse signed requestin C#, download from http://json.codeplex.com/
Also refere to How to decode OAuth 2.0 for Canvas signed_request in C#? if you are worndering about signed request.