Are C# GUIDs v4 UUIDs?

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.