Are C# GUIDs v4 UUIDs?

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No. No they’re not. There’s too much misinformation floating around about this issue and it’s easy to prove they’re not. Although when using Guid.NewGuid() you seem to always get a v4 UUID, this doesn’t mean that all instances of the GUID struct in C# are that way. The GUID struct is just a 128 bit integer - nothing more. This means Guid.Parse(42.ToString("00000000000000000000000000000000")) will get the C# GUID ‘00000000-0000-0000-0000-000000000042’ even though it’s not a valid v4 UUID.