Are Guids guaranteed to be unique on creation?

Question

I've been developing a WCF Service where clients logon with their guid that they obtain when logging in to the client and the guid gets registered at the WCF Service. But every once in a while... about almost everytime I logon with the same user login. The guid from user 'x' gets re-used..

This is the code:

public void Logon(ReceiveClient rc, string name)
    {
        try
        {
            inst = new InstanceContext(rc);
            DMSNotificationClient = new DMSCSSendServiceClient(inst);

            var ci = Guid.NewGuid();
            ClientId = ci;

            LoggedOnSince = DateTime.Now;

            DMSNotificationClient.Logon(name);

            ReceiveMsg += OnReceiveMsg;
            FolderChange += OnFolderChange;

        }
        catch (Exception ex)
        {
            var dmsEx = new DmsException(ex);
            DmsException.WriteErrorsToLog(dmsEx);
        }
    }

After some analyses of my C# Code I'm asking you this question:

Can I say that a GUID is NOT unique?

There is more than one "type" of GUID, and it's conceivable that you might come across duplicates in certain cases (see [Wikipedia](http://en.wikipedia.org/wiki/Universally_unique_identifier)). But it's hard to answer meaningfully without more details. — Jon, Feb 11 '14 at 09:44
It aint a duplicate. I'm asking if its NOT unique since my software is creating guids but when I login with the same account the guid is the same... — ValarmorghulisHQ, Feb 11 '14 at 09:45
why is the logon method wiring up event handlers (each time)? — Mitch Wheat, Feb 11 '14 at 09:49
@MitchWheat does that actually matter? We're talking about Guids not events........ — ValarmorghulisHQ, Feb 11 '14 at 09:50
@RaZor It's not relevant to the question but he raises a good point... — DGibbs, Feb 11 '14 at 09:50
would I have mentioned it if it didn't matter? You decide... — Mitch Wheat, Feb 11 '14 at 09:50
Then that's the problem (see answer). `static` is not what you're looking for here. — Matt Tester, Feb 11 '14 at 18:52
but shouldn't it be static to maintain it's value through out the whole application?? — ValarmorghulisHQ, Feb 12 '14 at 10:05

score 2 · Answer 1 · answered Feb 11 '14 at 09:48

2

From MSDN:

A GUID is a 128-bit integer (16 bytes) that can be used across all computers and networks wherever a unique identifier is required. Such an identifier has a very low probability of being duplicated.

answered Feb 11 '14 at 09:48

boris6

121
5

but still i get the same guid – ValarmorghulisHQ Feb 11 '14 at 09:49

score 1 · Answer 2 · answered Feb 11 '14 at 09:52

In certain cases yes. In this specific case, no. The method you are calling clearly states that

The chance that the value of the new Guid will be all zeros or equal to any other Guid is very low.

So the chance is decidedly non-zero. Admittedly I would expect "very low" to be in the range of "astronomically low" so I don't know why you get duplicates so easily, but the fact is that you cannot blindly trust the generated guid to be unique.

Matt Tester · Accepted Answer · 2014-02-11T18:55:24.880

Guids can be considered unique, as per the other answers.

Given that, the problem will lie in the ClientId property which must be either static or something similar (even ThreadStatic), which will cause the logon sessions to appear to "blend":

Session 1 Login ClientId == GuidA
Session 2 Login ClientId == GuidB
Session 1 Service Call ClientId == GuidB (as it was overridden by Session 2)

Edit

So just to clarify, static variables are available to all threads (therefore all requests) in an AppDomain. Therefore there is only one such variable, and each request is updating that same variable. How to avoid this in your scenario? ... that would be a different question.

score 0 · Answer 4 · answered Feb 11 '14 at 09:45

0

Can I say that a GUID is NOT unique?

No. See:

about almost everytime I logon with the same user login. The guid from user 'x' gets re- used..

So hey do not regenerate it but look up an existing Guid in a database. Simple like that. They do not use the Guid as "unique per session" but it defines a specific user.

This has nothing to do with GUID's guaranteed to be unique in creation because obviously they are not created for every login.

answered Feb 11 '14 at 09:45

TomTom

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They are generated at logon..... – ValarmorghulisHQ Feb 11 '14 at 09:46
1

Obviously not. Guid.NewGuid is randm enough to not return the same UI on every user logon. Heck, every random is that random. Either the geneation code is buggy or it is not generated on logon. – TomTom Feb 11 '14 at 09:49
Look at the code, it is ;) – ValarmorghulisHQ Feb 11 '14 at 09:51
Only if the method is called. Lots of things in between, including possibl iis caching. – TomTom Feb 11 '14 at 09:52

score 0 · Answer 5 · answered Feb 11 '14 at 09:46

0

Technically, they're not (Pidgeon Hole Principle). But then again, it is very unlikely that you experience collisions on a regular basis. My best guess is that the reuse stems from some kind of caching, but I can't say more based on the information.

answered Feb 11 '14 at 09:46

Janis F

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Okay, If you look at the code that I added. Can you give more info on that? Cuz it's generating the guid at the Logon Method – ValarmorghulisHQ Feb 11 '14 at 09:48
No, sorry. From what I see I wouldn't expect collisions either. – Janis F Feb 11 '14 at 09:53

Are Guids guaranteed to be unique on creation?

5 Answers5