I'm writing an app that has to compare files to previous versions of that file, to see if it's changed. So far, I've read all the files, recording their 'last modified' attribute - to compare the files (as there may be a lot), I first compare this attribute. If it has changed, I need a way to see if the file has changed - I cannot hold a copy of the file to compare them directly (this would be slow anyway), I need to do some kind of checksum. Anyone know anything about this? Thanks.