string host = "";
string username = "";
string password = "";
string localFileName = System.IO.Path.GetFileName(localFile);
string remoteFileName = "";

using (var sftp = new SftpClient(host, username, password))
{
    sftp.Connect();

    using (var file = File.OpenWrite(localFileName))
    {
        sftp.DownloadFile(remoteFileName, file);
    }
    sftp.Disconnect();
}