UUID version

[pappasa]

Does NewGuid() produce a UUID version 4 according to RFC4122?

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• ID: 63c02116-1b9d-69f5-7446-76e2f756207f
• Version Independent ID: 2c3d5439-e4d0-fcf1-dbb1-b5cf14abfdd7
• Content: Guid.NewGuid Method (System)
• Content Source: xml/System/Guid.xml
• Product: dotnet-api
• GitHub Login: @dotnet-bot
• Microsoft Alias: dotnetcontent

[BillWagner]

@karelz Do you know who can answer this question?

[BillWagner]

Thanks for raising this issue @pappasa We've investigating the correct answer.

[karelz]

@pappasa version 4 seems to defined as "randomly or pseudo-randomly generated version". That's what NewGuid does. Did you check the bits in the output?

What is motivation for your question? Confirmation that the number is randomly/pseudo-randmly generated? Or do you need to know how exactly?

[pappasa]

Yes, I checked the bits in the output, and it seems to produce version 4. But will it allways produce version 4 UUIDs, even if I call it a trillion times?

[karelz]

That sounds more like implementation question - check the soruce code: https://github.com/dotnet/corefx#useful-links

• .NET Framework calls Windows API: https://referencesource.microsoft.com/#mscorlib/system/guid.cs,b7fae6994c22c22e
• .NET Core:
  • Also calls Windows API: https://source.dot.net/#System.Private.CoreLib/shared/System/Guid.Windows.cs,b7fae6994c22c22e
  • Calls some Unix API: https://github.com/dotnet/coreclr/blob/master/src/System.Private.CoreLib/shared/System/Guid.Unix.cs
    • https://github.com/dotnet/coreclr/blob/030a3ea9b8dbeae89c90d34441d4d9a1cf4a7de6/src/System.Private.CoreLib/shared/Interop/Unix/System.Native/Interop.GetRandomBytes.cs#L17-L20
    • etc.

For context: UUID versions: https://tools.ietf.org/html/rfc4122#section-4.1.3

[pappasa]

* .NET Framework calls Windows API: https://referencesource.microsoft.com/#mscorlib/system/guid.cs,b7fae6994c22c22e -> Guid.NewGuid calls Win32Native<https://referencesource.microsoft.com/mscorlib/microsoft/win32/win32native.cs.html#619db4d994b020d5>.CoCreateGuid<https://referencesource.microsoft.com/mscorlib/microsoft/win32/win32native.cs.html#78204c597ae067fb> not documented if Variant 2, Version 4. * .NET Core: * Also calls Windows API: https://source.dot.net/#System.Private.CoreLib/shared/System/Guid.Windows.cs,b7fae6994c22c22e -> Guid.NewGuid calls Interop<https://source.dot.net/System.Private.CoreLib/P/0c5afec88b013285.html#0c5afec88b013285>.Ole32<https://source.dot.net/System.Private.CoreLib/shared/Interop/Windows/Ole32/Interop.CoCreateGuid.cs.html#8107506c279e441c>.CoCreateGuid<https://source.dot.net/System.Private.CoreLib/shared/Interop/Windows/Ole32/Interop.CoCreateGuid.cs.html#ee111d9e318539d6> not documented if Variant 2, Version 4. * Calls some Unix API: https://github.com/dotnet/coreclr/blob/master/src/System.Private.CoreLib/shared/System/Guid.Unix.cs -> Guid.NewGuid calls Interop.GetRandomBytes, and sets the bits for Variant 2, Version 4. Therefore from the above, we know that .NET CORE Guid.New Guid on Unix produces Variant 2, Version 4. On Windows we do not know if Guid.NewGuid produces Variant 2, Version 4.

[xneoron]

Guid may be duplicate or never ?

[karelz]

Good question for the underlying OS APIs. AFAIK they use MAC address + current time, which makes it fairly unique for normal purposes when you are not generating GUIDs in a tight loop.

The original question seems to be as answered as it can be, closing.

[xneoron]

Good question for the underlying OS APIs. AFAIK they use MAC address + current time, which makes it fairly unique for normal purposes when you are not generating GUIDs in a tight loop.

The original question seems to be as answered as it can be, closing.

current time use milliseconds or nanosecond it very difficult to duplicate and i can use lock thread prevent duplicates, maybe that right ?

[karelz]

@xneoron you're asking about OS implementation details. The best option IMO is to try it out and see if it is a problem at all.