Question

I’ve been working on a till/cash-register application. We need to prevent the users from quitting the application by using Alt + Esc, Ctrl + Alt + Del or Alt + Tab etc combinations and we’ll be providing a separate custom hotkey for quiting. The application will be full-screen and without any close button. I’ve already done the full screen no cross button part but stuck with disabling the low system level hotkeys. How do I do this in C#? Any help or alternative methods (ie. setting up user group permissions to prevent star menu/task manager or any other alternative to achieve the goal) are appreciated.

Edit: We’re using WinForms. The application will run in Windows XP/Vista/7.

Answer 1

There is no direct way to escape from the standard behavior of Windows. A user of Windows must have the possibility to kill applications. If the application doesn’t obey, the system will force-kill the application.

However, there are a few tricks, often employed by virus-writers, here are some (not extensive):

• Disallowing all system hotkeys is relatively trivial, but Ctrl-Alt-Del is not amongst them. Look up this excellent code-project article with a sample application.
• Build a DLL and inject yourself in other processes that are vital to the system. Easy to do, but make sure you port your application from .NET to standalone C++ or other language, you don’t want system processes to be dependent on the version of .NET that you load.
• In lieu of the dll-suggestion, you can create something like a shell extension with a library like this one which allows you to continue using .NET. It’s not meant for this kind of thing, but it forces the user to kill explorer.exe to get rid of your application.
• Create a service that can interact with the desktop. Restrict the rights of the service by normal means, which essentially has the same effect of disallowing users to kill your app through the ways you mention.
• Obviously, you should cancel any WM_CLOSE, WM_EXIT or WM_KILL message and simply continue “living”.
• Use a sentinel application, this is what most virus scanners use: if you kill one process, the sentinel will restart it. If you kill the sentinel, it should be restarted by the main app. Bad behavior, but works.

But, general advice: don’t do this. Allow your users to potentially kill the program if needed, otherwise it will be very hard to do any maintenance. The only “supported” way is through services, where you can allow special users to stop your service and it cannot be stopped through task manager.