CODE EXAMPLE FOR CSHARP

C# winform get access token facebook

protected override string QueryAccessToken(Uri returnUrl, string authorizationCode)
        {
            // Note: Facebook doesn't like us to url-encode the redirect_uri value
            var builder = new UriBuilder("https://graph.facebook.com/oauth/access_token");
            builder.AppendQueryArgument("client_id", this.appId);
            builder.AppendQueryArgument("redirect_uri", NormalizeHexEncoding(returnUrl.GetLeftPart(UriPartial.Path)));
            builder.AppendQueryArgument("client_secret", this.appSecret);
            builder.AppendQueryArgument("code", authorizationCode);

            using (WebClient client = new WebClient())
            {
                //Get Accsess  Token
                string data = client.DownloadString(builder.Uri);
                if (string.IsNullOrEmpty(data))
                {
                    return null;
                }

                var parsedQueryString = HttpUtility.ParseQueryString(data);
                return parsedQueryString["access_token"];
            }
        }
 private static string NormalizeHexEncoding(string url)
        {
            var chars = url.ToCharArray();
            for (int i = 0; i < chars.Length - 2; i++)
            {
                if (chars[i] == '%')
                {
                    chars[i + 1] = char.ToUpperInvariant(chars[i + 1]);
                    chars[i + 2] = char.ToUpperInvariant(chars[i + 2]);
                    i += 2;
                }
            }
            return new string(chars);
        }