C# - How to detect a Windows shutdown/logoff and cancel that action (after asking the user)

Some explanation: for a project I'm working on I have to write a program that is running in the background, detects changes to files, and uploads the modified files to a web service to mak开发者_运维百科e it available to others. Quite simple synchronization if it were not for the case when a user modifies a big file and decides to shutdown its computer right after the edit.

I could cancel the upload and wait for the next reboot to do the upload, but I can imagine the user downloading the file from the web to another computer the next morning and don't understanding why his changes from last night aren't there.

So my idea was to detect when the users logs off or reboots Windows, and if I'm in the middle of an upload, just asking the user "We're still synchronizing file Foo.txt that you just changed. Are you sure you want to reboot ? You're changes won't be available to others until you restart your computer !". If the users says no, I'd need to cancel the reboot/loging off

Is this possible?

There is a static class called SystemEvents that exposes this behaviour:

http://msdn.microsoft.com/en-us/library/microsoft.win32.systemevents.aspx

However, it cannot differentiate between certain actions and doesn't pause the OS process time-out guard. I used it once, but the default time-out as configured in the registry is a little short so will likely need increasing.

To cut a long story short, it all felt a little hackish.

To add to @Adam's answer, if you need to tell the difference between logoff and shutdown/reboot, you can handle the WM_QUERYENDSESSION message.

"Shutdown Changes for Windows Vista" is a useful article for understanding the shutdown timeout.

Trying to block a shutdown is a lossy proposition these days, it's no longer possible to do so in Vista and up. A prompt isn't readable nor reachable. Using a service is highly indicated here, lets you survive a user log-off. And a reboot, your service will start running again automatically, letting you complete the job.