How Random is System.Guid.NewGuid()?

Here's an excellent breakdown by Raymond Chen

No fixed-length value can ever guarantee to be 100% unique (just call it enough times, give or take the universe ending ;-p) - but it can be very, very, very unlikely to duplicate.

I can't speak to the predictability of sequential numbers but it will be unique. I think you'd be better off using a random number generator from System.Security.Cryptography, though. Tie a random number with a monotonically increasing value (time) to generate your unique key and you can be sure that it is unique and not predictable.

The documentation for System.Guid.NewGuid() makes no guarantees for randomness, so while the current implementation is based on a random number generator (it's version 4 of the algorithm, which was devised after privacy concerns arose from version 1 which used the MAC address; other system's like Apple's OS X still use version 1 of the algorithm).

So while you have a very high probabilty of System.Guid.NewGuid() generating a unique value, you can't make any assumptions about its predictability because that's not specified by the documentation.

Assuming that System.Guid.NewGuid uses CoCreateGuid, it is not random at all. Historically, the algorithm used for creating guids was to incorporate the MAC address from a network adapter, in addition to some other things like the time. I'm not sure if the algorithm has changed. While it certainly is not random, it is guaranteed to be unique.