Is a GUID unique 100% of the time?

Is a GUID unique 100% of the time?

回答1

While each generated GUID is not guaranteed to be unique, the total number of unique keys (2¹²⁸ or 3.4×10³⁸) is so large that the probability of the same number being generated twice is very small. For example, consider the observable universe, which contains about 5×10²² stars; every star could then have 6.8×10¹⁵ universally unique GUIDs.

From Wikipedia.

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These are some good articles on how a GUID is made (for .NET) and how you could get the same guid in the right situation.

https://ericlippert.com/2012/04/24/guid-guide-part-one/

https://ericlippert.com/2012/04/30/guid-guide-part-two/

https://ericlippert.com/2012/05/07/guid-guide-part-three/

 

Summing up:

• GUIDs are guaranteed to be unique but not guaranteed to be random. Do not use them as random numbers.
• GUIDs that are random numbers are not cryptographic strength random numbers.
• GUIDs are only unique when everyone cooperates; if someone wants to re-use a previously-generated GUID and thereby artificially create a collision, you cannot stop them. GUIDs are not a security mechanism.
• GUIDs have an internal structure; at least six of the bits are reserved and have special meanings.
• GUIDs are allowed to be generated sequentially, and in practice often are.
• GUIDs are only unique when taken as a whole.
• GUIDs can be generated using a variety of algorithms.
• GUIDs that are generated randomly are statistically highly unlikely to collide in the foreseeable future.
• GUIDs could reveal information about the time and place they were created, either directly in the case of version one GUIDs, or via cryptanalysis in the case of version four GUIDs.
• GUIDs might be generated by some entirely different algorithm in the future.