Руслан Ахметов

string AuthUrl = "https://m.vk.com/login";
string logoutId = "act=logout";
string MainUrl = "https://vk.com";

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 httpHandler = new HttpClientHandler
                              {
                                  CookieContainer = new CookieContainer(),
                                  AllowAutoRedirect = true,
                                  AutomaticDecompression = DecompressionMethods.Deflate | DecompressionMethods.GZip
                              };

 httpClient = new HttpClient(httpHandler) {Timeout = TimeSpan.FromSeconds(150)};
            httpClient.DefaultRequestHeaders.Add("Accept-Encoding", "gzip, deflate");
            httpClient.DefaultRequestHeaders.Add("Accept-Language", "ru-RU,ru;q=0.8,en-US;q=0.6,en;q=0.4");
            httpClient.DefaultRequestHeaders.Add("Cache-Control", "no-cache");
            httpClient.DefaultRequestHeaders.Add("Accept", "*/*");
            httpClient.DefaultRequestHeaders.Add("User-Agent", "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36");

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Uri authUri = new Uri(AuthUrl);
            string data = await httpClient.GetStringAsync(authUri);
            IsAuth = data.Contains(logoutId);
            if (!IsAuth)
            {
                string result = data.Substring("<form method=\"post\" action=\"", "\"");//достаем ссылку для авторизации
                authUri = new Uri(result);
                var body = new Dictionary<string, string> {{"act", "login"}, {"email", Email}, {"pass", Pass}};//тут заполняем логин и пароль
                var response = await httpClient.PostAsync(authUri, new FormUrlEncodedContent(body));
                var page = await response.Content.ReadAsStringAsync();
                IsAuth = page.Contains(logoutId);//если вошли на странице появится ссыль для выхода :)
                Uri mainUri = new Uri(MainUrl);//переходим на главную страницу
                data = await httpClient.GetStringAsync(mainUri);
            }
            return IsAuth;