Force an application to be single instance?

Question

Is there a way I can force an application on Windows to be a single-instance application, i.e., not launch if another instance of the same application is already running? Ideally, I want to be able to do this without needing to replace the actual application with a stub, but I would not mind using one if really necessary.

Answer 1

normally you have to implement that "singleton" feature right into the application. if you want to guard it "from the outside", you have to use a wrapper which checks, if you have another instance running already. there might be more solutions to this but i once did the trick with nsis. the idea is this:

• create a "fake installer" which itself is only a little wrapper around the app you want to start
• make that installer a singleton, see here http://nsis.sourceforge.net/Allow_only_one_installer_instance or http://nsis.sourceforge.net/Check_whether_your_application_is_running
• launch your app from that "wrapper installer", see here http://nsis.sourceforge.net/ExecCmd_plug-in
• "hide" the normal application icon from the user and present that fake-installer. since you can add a nice looking icon to it, you should be able to make it look like the original program.

Answer 2

Unless there is a workaround you can apply with the Application Compatibility Kit that enables this – which I doubt (it isn't the kind of thing that itself breaks compatibility¹) – then there is no setting in Windows.

Other than a launcher application the other approach would be to monitor for new processes (eg. WMI instance creation events for Win32_Process objects – filtered by session of course) and perform some action to block the second instance.

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¹ Of course a fix might be needed where an application uses an unsupported mechanism to detect multiple instances of itself so the detection mechanism needs a shim.