c# - How to generate 8 bytes unique id from GUID?

Question

I try to use long as unique id within our C# application (not global, and only for one session) for our events. Do you know if the following will generate an unique long id?

public long GenerateId()
{
 byte[] buffer = Guid.NewGuid().ToByteArray();
 return BitConverter.ToInt64(buffer, 0);
}

Why we not use GUID directly? We think 8 bytes long is good enough.

Answer 1

No, it won't. As highlighted many times on Raymond Chen's blog, the GUID is designed to be unique as a whole, if you cut out just a piece of it (e.g. taking only 64 bytes out of its 128) it will lose its (pseudo-)uniqueness guarantees.

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Here it is:

A customer needed to generate an 8-byte unique value, and their initial idea was to generate a GUID and throw away the second half, keeping the first eight bytes. They wanted to know if this was a good idea.

No, it's not a good idea. (...) Once you see how it all works, it's clear that you can't just throw away part of the GUID since all the parts (well, except for the fixed parts) work together to establish the uniqueness. If you take any of the three parts away, the algorithm falls apart. In particular, keeping just the first eight bytes (64 bits) gives you the timestamp and four constant bits; in other words, all you have is a timestamp, not a GUID.

Since it's just a timestamp, you can have collisions. If two computers generate one of these "truncated GUIDs" at the same time, they will generate the same result. Or if the system clock goes backward in time due to a clock reset, you'll start regenerating GUIDs that you had generated the first time it was that time.

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I try to use long as unique id within our C# application (not global, and only for one session.) for our events. do you know the following will generate an unique long id?

Why don't you just use a counter?