class Program
    {
        private static void Main(string[] args)
        {
            FileSystemWatcher fileWatcher = new FileSystemWatcher();
            fileWatcher.NotifyFilter = NotifyFilters.LastAccess | NotifyFilters.LastWrite | NotifyFilters.FileName | NotifyFilters.DirectoryName;
            fileWatcher.Created += fileWatcher_Created;
            fileWatcher.EnableRaisingEvents = true;
            Console.ReadLine();
        }

        private static void fileWatcher_Created(object sender, FileSystemEventArgs e)
        {
            WorkOnFile(e.FullPath);
        }


        //must be done completely. How do I ensure it?
        private static void WorkOnFile(string fileName)
        {
            using (FileStream f = File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.None))
            {
                Thread.Sleep(40000); // some long operations
            }
        }
  }

var scanDirectoryIn = new DirectoryInfo(folderIn);
foreach (var fileInfo in scanDirectoryIn.GetFiles())
{
  if (fileInfo.Extension != ".csv") continue;
  if (DateTime.UtcNow.Subtract(fileInfo.LastWriteTimeUtc).TotalMinutes < 5) continue;
  try
  {
    fileInfo.MoveTo(folderOut + "\" + fileInfo.Name);
  }
  catch (Exception) {}
}

//must be done completely. How do I ensure it?
    private static void WorkOnFile(string fileName)
    {
        while(true){
            try{
                using (FileStream f = File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.None))
                {
                    Thread.Sleep(40000); // some long operations

                    break; //exit while() infinite loop
                }
            }
            catch(Exception e){
                //file is locked because being written. wait a few seconds then retry
                Thread.Sleep(10000);
            }
        }
    }