>>>Yet another method involves the use of a "dongle" (i.e. a hardware device that plugs into one of the PC's ports).
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>I had to work with a company that took the dongle approach for some desktop software. Customers hated it.
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>These days most of my customers expect to use Citrix/TS especially if they decide to go to Azure or other cloud, so per PC licensing probably is getting closer to how the old dongle was perceived... we do still have one product that absolutely needs a per PC license and we carved out own combination in VFP C++ Compiler to phone home and allocate an encrypted license key that's moderately difficult to spoof.

Yep, dongles sort of fell out of favor a decade or two ago -- mainly due to the hassles they typically entailed -- mostly from what I recall were the older type that hooked up to the now "legacy" serial or parallel ports. The ones that plugged into USB were generally a bit less problematic -- mostly since you weren't going to be using them in a "passthru" mode to connect another device. On the other hand there are issues with "plug-n-play" to deal with. Sometimes there was a network component that you could add -- on the computer where the dongle was plugged in you can install a "server" application and on other workstations you could install "client" software -- this would allow you to share the dongle across the network (though generally you were restricted to only one instance of the program at any single moment -- unless there was an option for multi-user licensing). The problem of course is this software could be finicky -- especially across different versions and editions of Windows.
I suppose an alternative would be to use a variation on the old "key disk" idea -- back in the day when floppies were common, the key disk was a specially-authored disk (often formatted in a non-standard fashion so that normal copying would not duplicate the behavior), or disks that had been physically altered (e.g. laser-burned spots). -- such that original disk would have very specific behavior (e.g. laser-burned sections could store data -- a behavior that could not be duplicated if you'd copied the disk). It might be possible to create uniquely authored CD-ROM and DVD-ROM -- e.g. using a non-standard format that normal copying methods for recordable optical disc would not be able to duplicate. Of course the problem with "key disk" would be that floppies are NOT something you're going to find on most computers, and it's increasingly rare to find PCs with optical drives. I suppose some form of low-level hacking might be possible with flash media ( https://www.bunniestudios.com/blog/?p=3554 ) to create media that could not be (easily) copied. On the other hand, this goes *way* beyond what would be practical (i.e. probably requires *a lot* more effort than would be worthwhile).