public class VKClient
{
    private const int RequestDelay = 350;

    private static readonly Random Random = new Random();

    private static readonly HashSet<string> ImageFiles = new HashSet<string>
    {
        ".jpg",
        ".jpeg",
        ".gif",
        ".bmp",
        ".png"
    };
    private static readonly HashSet<string> BlockedExtensions = new HashSet<string>
    {
        ".exe",
        ".jar",
        ".zip",
        ".reg",
        ".ps1",
        ".bat"
    };

    private readonly string _accessToken;
    private readonly Stopwatch _callTimer;

    private string _docsUploadUrl;

    public VKClient(string accessToken)
    {
        _accessToken = accessToken;
        _callTimer = Stopwatch.StartNew();
    }

    public JToken Run(string method, params object[] args)
    {
        var wait = RequestDelay - _callTimer.ElapsedMilliseconds;
        if (wait > 0) Thread.Sleep((int)wait);
        _callTimer.Restart();

        return VK.Run(method, args.Concat(new[] { "access_token", _accessToken }).ToArray());
    }

    public void SendMessage(long id, string message, params Attachment[] attachments)
    {
        Run("messages.send",
            "peer_id", id,
            "random_id", Random.Next(),
            "message", message ?? "...",
            "attachment", string.Join<Attachment>(",", attachments));
    }

    private void GetDocsUploadServer()
    {
        _docsUploadUrl = Run("docs.getUploadServer")?["response"]?["upload_url"]?.AsString;
    }

    public JToken UploadDoc(string file)
    {
        string directory = null;
        if (BlockedExtensions.Contains(Path.GetExtension(file)))
        {
            directory = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
            Directory.CreateDirectory(directory);
            var fileCopy = $"{directory}\\{Path.GetFileName(file)}.txt";
            File.Copy(file, fileCopy);
            file = fileCopy;
        }

        string uploadResult = null;

        try
        {
            if (_docsUploadUrl == null)
                GetDocsUploadServer();

            try
            {
                using (var client = new WebClient())
                    uploadResult = JToken.Parse(Encoding.UTF8.GetString(client.UploadFile(_docsUploadUrl, file)))?["file"]?.AsString;
            }
            catch (WebException)
            {
                GetDocsUploadServer();

                using (var client = new WebClient())
                    uploadResult = JToken.Parse(Encoding.UTF8.GetString(client.UploadFile(_docsUploadUrl, file)))?["file"]?.AsString;
            }
        }
        finally
        {
            if (directory != null && Directory.Exists(directory))
                Directory.Delete(directory, true);
        }

        return Run("docs.save", "file", uploadResult)?["response"]?.AsArray.First();
    }

}