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Questions about GUID's: Are they always fixed in length, and is the middle number always 4?

Answer 1

+5 A:

Yes, the length is fixed and yes, the middle number is always 4 when you use the standard tostring format. Some of the bits in GUID (known as a UUID almost anywhere that isn't windows) are fixed to indicate things like version etc..

http://en.wikipedia.org/wiki/Uuid

EDIT I should add that the "4" only applies to Guids that have been generated according to the Guid.NewGuid algorithm as implemented in .NET. There is nothing to stop you from taking any arbitrary byte[16] and converting it to Guid. So, you can only bank on it being 4 for the current implementation of the algorithm in .Net. If you are getting Guids from another source, you can't bank on the 4. An update to .Net or possibly windows(depending if .Net uses its own or Windows' generator) may change the fixed numbers of the GUID

e.g. the following is completely working code and will not have the 4 in position:

        var rand = new Random();
        var byteArray = new byte[16];
        rand.NextBytes(byteArray);
        var g = new Guid(byteArray);

Jim Leonardo 2010-09-18 02:50:12

The 4 is the version number. It depends on how it was generated, *not* how you convert it to a string.

Gabe 2010-09-18 02:53:43

If you displayed the GUID as a base-10 number, there wouldn't be a 4 in that position always. Mind you, doing that would be insane, it's just that you COULD.

mjfgates 2010-09-18 02:58:56

@Gabe - to shorten the string length, a semi- common trick is to do a base 64 conversion, in which case the string would not have the 4 in position, it would mask the fixed values. eg: Convert.ToBase64String(guid.ToByteArray());

Jim Leonardo 2010-09-18 03:06:20

@Jim Leonardo - Will the Base64 GUID always have the same length?

MakerOfThings7 2010-09-18 03:21:38

MakerOfThings7: a GUID is by definition 128 bits. Any reasonable algorithm for converting one to a string will always generate strings of the same length.

Gabe 2010-09-18 03:33:32

@Maker - A GUID is always 16 bytes, the byte array is therefore byte[16], so I don't see how the length ToBase64String would ever change for the Guid.ToByteArray.

Jim Leonardo 2010-09-18 03:36:44

Answer 2

+3 A:

From the documentation on Guid.ToString (with no parameters):

The value of this Guid, formatted as follows: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx where the value of the GUID is represented as a series of lowercase hexadecimal digits in groups of 8, 4, 4, 4, and 12 digits and separated by hyphens. An example of a return value is "382c74c3-721d-4f34-80e5-57657b6cbc27".

So the answer is "yes", it will always be the same length.

As for the 4, it is a version number (according to http://en.wikipedia.org/wiki/Uuid). Every GUID that you generate with that algorithm will have a 4 in that position, but older GUIDs will have a 1, 2, or 3. Future ones might have a 5 or something higher.

Gabe 2010-09-18 03:27:55

Answer 3

+1 A:

No - a GUID does not have to be a type 4 UUID in fact many GUIDS under windows are UUID's of TYPE 1.

Type 1 takes the primary MAC, a clock and a sequence. This in fact "Leaks" data since all the UUID1s that are created on the same system will have the same MAC. That is why most GUID functions will take this data and hash it and turn it into a hash-based UUID

Dominik Weber 2010-09-28 20:06:05

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Questions about GUID's: Are they always fixed in length, and is the middle number always 4?

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