To support FTP/SFTP or any other protocol in C# you can do it in 3 ways:
1. NEW APP PROCESS - Start an app that does the FTP communication in separate process, and be able to control what file to download, where to save it and to tell the app to terminate when download is finished. This way, you can use FileZilla only if it lets you pass certain parameters in command line, like the URI of the resource you want to transfer through FTP/SFTP, and the path where the file should be saved to. And as I can see HERE this could work.
To start the process and pass it command line arguments in C# you would do something like this:
static void StartNewProcess(string app, string args)
{
ProcessStartInfo startInfo = new ProcessStartInfo();
startInfo.FileName = app; //full app path
startInfo.Arguments = args; //command line arguments
startInfo.CreateNoWindow = true; //dont create app window
startInfo.WindowStyle = ProcessWindowStyle.Hidden; //hide app from taskbar
Process.Start(startInfo);
}
Now you can execute FileZila app, pass it args containing file URL and let it do its job... But you cant know how long will it take to download the file, when the download is ended, do you need to log in to get it...
2. EXISTING CLASS LIBRARY - Include a Class Library that is written by someone else, that does the job. This way you are in TOTAL control of the process. And as many other suggested, this would be a perfect way for you. Many answers here contain good class libraries that you can use and be happy with the results.
3. HOME-MADE CLASS LIBRARY - Open RFC 959, read it all and write your code... (Now 2. sounds better, doesn't it? :D)