gistfile1.cs

static IObservable<FileInfo> GetFiles(DirectoryInfo root, 
                                      Func<FileInfo, bool> fileFilter, 
                                      Func<DirectoryInfo, bool> directoryFilter, 
                                      IScheduler scheduler = null)
{
    scheduler = scheduler ?? Scheduler.Default;

    return Observable.Create<FileInfo>(obs => {
        Action<DirectoryInfo, Action<DirectoryInfo>> iterator = (dir, self) => {
            foreach (var file in dir.GetFiles().Where(fileFilter))                        
                obs.OnNext(file);

            foreach (var childDir in dir.GetDirectories().Where(directoryFilter))                                                
                self(childDir);
        };

        return scheduler.Schedule(root, iterator);
    });
}