accessing signed_request

My ultimate goal is to access app_data, but signed_request is always null for me. I made a very simple facebook iframe app with facebook c#.

dynamic signedRequest = FacebookWebContext.Current.SignedRequest.Data; //null error
dynamic dat开发者_如何学Ca = signedRequest.Data;

I read that this may be related to tab url. My app has urls such as localhost:52212/ , localhost:52212/Home/About, and localhost:52212/Home/MoreStuff. I set my tab url to localhost:52212/Home/

I tried a few JSON methods I saw posted here using FacebookSignedRequest.Parse but those were null as well.

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I think the Tab URL should always be under the Canvas URL like this:

http://site:port/canvasdir => for canvas URL

http://site:port/canvasdir/tabdir => for tab URL