Download.cs

public void Download(string fileToDownload, string host, string username, string password)
{
    try
    {
        using (var sftpClient = new SftpClient(host, username, password))
        using (var fs = new FileStream(Path.GetFileName(fileToDownload), FileMode.OpenOrCreate))
        {
            sftpClient.Connect();

            sftpClient.DownloadFile(
                "/ftproot/" + fileToDownload,
                fs,
                downloaded =>
                    {
                        Console.WriteLine($"Downloaded {(double)downloaded / fs.Length * 100}% of the file.");
                    });

            sftpClient.Disconnect();
        }
    }
    catch (Exception e)
    {
        Console.WriteLine(e.Message);
    }
}