How to write File Synchronization Function with C#

There is a filesystemwatcher component that will notify you when a file, directory or other filesysem object has been modified. If you want to copy the file, this will help. If you only want to write CHANGES, you'll have to devise the scheme you're describing, but it would be far easier (and probably faster) to copy the file instead of just applying changes, even on a large file. To update just the changed bytes in a file, you will have to have a backup of the previous version of the file to compare against, and you'll have to build a "patch" file that contains the differences. Send this patch file to the other location and update the file with those changes.