gistfile1.cs

public interface IFileSaverService {
    void SaveFile(byte[] bytes, string filename);
}
 
public class LocalFileSystemSaver : IFileSaverService {
    public void SaveFile(byte[] bytes, string filename) {
        ...
    }
}
 
public class FtpFileSaver : IFileSaverService {
    IConnection _connection = null;
    public void SaveFile(byte[] bytes, string filename) {
        if (_connection == null)
            throw new NotConnectedException();
 
        ...
    }
 
    // FTP is special...you need to login first
    public void LoginToFtpSiteFirst(string name, string password) {
        _connection = GetConnectionSomehow(name, password);
    }
}
 
public class FileManager {
    IFileSaverService _fileSaver;
 
    public void Save(Document doc) {
        var bytes = doc.GetBytes();
        var fileName = doc.GetFileName();
        _fileSaver.SaveFile(bytes, fileName);
    }
}