File system operations (e.g. copy, delete, move) performed through ActiveSync - are they atomic?

Question

I am developing an application in .NET Compact Framework 3.5, for Windows CE 6. I need to automate some data synchronization between the mobile application and a PC. So what I do is that, in the .NETCF application, I have a while loop that checks every few seconds, using System.IO.File.Exists(fullFilePath), whether the file I am expecting has been copied from the PC to the mobile device, through an ActiveSync connection.

Once the mobile app finds the file, it will open and read it using a StreamReader.

My problem is that, some times, File.Exists(fullFilePath) would return true, yet the subsequent use of StreamReader may fail with a FileNotFoundException.

Ruling out the possibility of the file getting deleted between the call to File.Exists() and the instantiation of the StreamReader, would anybody know how the exception may occur? Is it because file copy through ActiveSync is not atomic, such that the file is not actually ready for reading even though File.Exists() returns true?

Answer 1

A file copy is most certainly not going to be atomic. When you start the copy, the file is created, but has no data in it. As the file data gets written, the file "exists", but is not yet fully complete. You'd want to know not only that it exists, but that it's not currently in use (meaning the writer is done). Trying to open it exclusively would verify that.