How to catch the file before it is written to the disk, during upload.

I'm wanting to implement a checkout/checkin feature.

I've got the check out working, it sees who the current user is, and makes an entry in a new table that records UserID, RootFolderID, RelativePath, and Filename.  This will uniquely id a file in the system.  That's no problem.  The question is where is the best place to check the uploads?  I've found the place that checks if the file exists before it deletes it so the user can overwrite, and thought this would be a great place, but I'm not sure how to get a message back to the user if he is trying to check something in that the current user doesn't have checked out.  I need to tell him that they can't check it in.  Any help or direction would be appreciated.

For controlling upload behaviour you need to examine FileVistaUploadStorageProvider class. You can place your checkin code into StartFile method of this class.