How to make sure a generated guid is unique globally?

Question

Let's say I want to set a guid to be my application's assembly guid. As searched from internet, we can use (new Guid()).Next() to get a new unique value.

I cannot figure out how my guid is warranted to be unique against others? Please explain if you know how to.

Answer 1

The only guarantee you have is that probability is on your side. 2^128 possible GUIDs and some cleverness in the creation process makes it very unlikely you will ever see a duplicate.

It seems V4 is the standard GUID on Windows now. If that one is purely based on a pseudo-random number generator, as Wikipedia seems to indicate, it's affected by the Birthday problem.

I've seen several examples using 128-bits to show that a duplicate is almost impossible. Those often miss two things. The Birthday problem and that a V4 GUID actually is 124 bits.

You need 1/2+sqrt(1/4-2*2^124*ln(0,5)) ≈ 5.4*10^18 GUIDs to reach a 50% chance of a duplicate. That is still a lot, but 50% may not be the deal you are looking for. Say you want it to be one in a million to get a duplicate, then you can have sqrt(2*2^124*ln(1/(1-0,000001))) ≈ 6,5*10^15 GUIDs. If you create a thousand GUIDs per second you could keep on doing that for almost 206667 years before reaching a one to a million risk of getting a duplicate. 6,52191054316287e15/(3600*24*365,25*1000) ≈ 206666,874006986

The chance of me getting all of those calculations correct →0.

Answer 2

It isnt, but the way it is generated and the way it is represented makes probability of generating two same GUIDs in this milenium almost zero.

See: Simple proof that GUID is not unique