FB signed_request issue ASP.net c#

As you probably already know, when your page tab app is firstly loaded, Facebook makes a POST request to the URL you specified in application's Page Tab URL (or Secure Page Tab URL) along with the signed_request string. This is why its value is not null on your landing page. But when you click on a link to navigate to another page inside your tab app, it suddenly becomes null. This is how it should be because you are making a GET request with no signed_request. To make your other pages able to access signed_request, you need to store it. I came up with the following code which works for me but if someone comes with a better solution, I would be glad to hear it:

public static string StoredSignedRequest
{
    get
    {
        string signedRequest = HttpContext.Current.Request["signed_request"];

        // If signed_request is provided, store it
        if (!String.IsNullOrEmpty(signedRequest))
        {
            WriteCookie("fb-app-signed-request", signedRequest);
            return signedRequest;
        }
        else
        {
            return ReadCookie("fb-app-signed-request");
        }
    }
}

public static void WriteCookie(string strCookieName, string strCookieValue)
{
    var hcCookie = new HttpCookie(strCookieName, strCookieValue);
    HttpContext.Current.Response.Cookies.Set(hcCookie);
}

public static string ReadCookie(string strCookieName)
{
    foreach (string strCookie in HttpContext.Current.Response.Cookies.AllKeys)
    {
        if (strCookie == strCookieName)
        {
            return HttpContext.Current.Response.Cookies[strCookie].Value;
        }
    }

    foreach (string strCookie in HttpContext.Current.Request.Cookies.AllKeys)
    {
        if (strCookie == strCookieName)
        {
            return HttpContext.Current.Request.Cookies[strCookie].Value;
        }
    }

    return null;
}

Then you can use a JSON parser to parse the value of StoredSignedRequest. In my case, I use Newtonsoft:

public static JObject GetSignedRequestJsonObject()
{
    string signed_request = StoredSignedRequest;

    if (String.IsNullOrEmpty(signed_request))
    {
        // If signed_request is null on all pages except the landing page, add the following code to all pages so that it is stored:
        // <input type="hidden" name="signed_request" value="<%= FacebookAppHelper.StoredSignedRequest %>" />
        return null;
    }

    string payload = signed_request.Split('.')[1];
    UTF8Encoding encoding = new UTF8Encoding();
    string decodedJson = payload.Replace("=", string.Empty).Replace('-', '+').Replace('_', '/');
    byte[] base64JsonArray = Convert.FromBase64String(decodedJson.PadRight(decodedJson.Length + (4 - decodedJson.Length % 4) % 4, '='));
    string json = encoding.GetString(base64JsonArray);

    return JObject.Parse(json);
}

The critical part is to not forget to add the hidden field (see commented line above) to all pages that you plan to access using a GET request inside your page tab application.