How to Check if a user is following you on Twitter using twitter API 1.1

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C#

twitter

I want to check whether if a user is following you on Twitter using twitter API 1.1 but I am getting an error while getting the response

The remote server returned an error: (401) Unauthorized.

Am I doing something wrong?

var postData = "source_screen_name=" + source_screen_name + "&target_screen_name=" + target_screen_name;
        var updateStatusURL = "https://api.twitter.com/1.1/friendships/show.json" ;
        // oauth implementation details
        var oauth_version = "1.0";
        var oauth_signature_method = "HMAC-SHA1";

        // unique request details
        var oauth_nonce = Convert.ToBase64String(new ASCIIEncoding().GetBytes(DateTime.Now.Ticks.ToString()));

        var timeSpan = DateTime.UtcNow- new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);
        var oauth_timestamp = Convert.ToInt64(timeSpan.TotalSeconds).ToString();
        // create oauth signature
        var baseFormat = "oauth_consumer_key={0}&oauth_nonce={1}&oauth_signature_method={2}" +
                        "&oauth_timestamp={3}&oauth_token={4}&oauth_version={5}&oauth_callback={6}";

        var baseString = string.Format(baseFormat,
                                    oauth_consumer_key,
                                    oauth_nonce,
                                    oauth_signature_method,
                                    oauth_timestamp,
                                    oauth_token,
                                    oauth_version,
                                    Uri.EscapeDataString(strcallbackurl)
                                    );


        baseString = string.Concat("GET&", Uri.EscapeDataString(updateStatusURL), "&", Uri.EscapeDataString(baseString));

        var compositeKey = string.Concat(Uri.EscapeDataString(oauth_consumer_secret),
                                "&", Uri.EscapeDataString(oauth_verifier));

        string oauth_signature;
        using (HMACSHA1 hasher = new HMACSHA1(ASCIIEncoding.ASCII.GetBytes(compositeKey)))
        {
            oauth_signature = Convert.ToBase64String(
                hasher.ComputeHash(ASCIIEncoding.ASCII.GetBytes(baseString)));
        }

        // create the request header
        var headerFormat = "OAuth oauth_nonce=\"{0}\", oauth_signature_method=\"{1}\", " +
                           "oauth_timestamp=\"{2}\", oauth_consumer_key=\"{3}\", " +
                           "oauth_token=\"{4}\", oauth_signature=\"{5}\", " +
                           "oauth_version=\"{6}\"";

        var authHeader = string.Format(headerFormat,
                                Uri.EscapeDataString(oauth_nonce),
                                Uri.EscapeDataString(oauth_signature_method),
                                Uri.EscapeDataString(oauth_timestamp),
                                Uri.EscapeDataString(oauth_consumer_key),
                                Uri.EscapeDataString(oauth_token),
                                Uri.EscapeDataString(oauth_signature),
                                Uri.EscapeDataString(oauth_version)
                        );
        ServicePointManager.Expect100Continue = false;

        //var postBody = "q=" + Uri.EscapeDataString(q);//
        updateStatusURL += "?" + postData;
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(updateStatusURL);
        request.Headers.Add("Authorization", authHeader);
        request.Method = "GET";
        request.ContentType = "application/x-www-form-urlencoded";
        var response = (HttpWebResponse)request.GetResponse();
        var reader = new StreamReader(response.GetResponseStream());
        var objText = reader.ReadToEnd();

        return request;