Guid.CreateVersion7

Program.cs

using System.Runtime.InteropServices;

var now = DateTimeOffset.UtcNow;

TryGenerate(() => Guid.NewGuid(), "  v4");
TryGenerate(() => Guid.CreateVersion7(now), "  v7");
TryGenerate(() => CreateVersion7(now), "MBv7");
TryGenerate(() => Ulid.NewUlid(now).ToGuid(), "Ulid");

static void TryGenerate(Func<Guid> generator, string name)
{
    for (var i = 0; i < 5; i++)
    {
        var guid = generator();
        Console.WriteLine($"{name}: {guid}");
    }
}

/*Output
  v4: 9c695f61-6c00-41d7-a191-efc8b2d6b5c4
  v4: 5b8a53e7-01f7-4f89-aa1c-325b1b03d8d1
  v4: 4d7c6daf-91b1-4d44-a277-ba5626b17f8c
  v4: e3039fc9-0dbe-406b-83a9-7aaebe97aa4f
  v4: f5bf0aa8-e865-4916-97a4-9ae965e0b82f

  v7: 01933a32-b2b8-7dc2-a605-1f49057b06be
  v7: 01933a32-b2b8-77d3-9b69-a7ce07550dc9
  v7: 01933a32-b2b8-78b9-8d25-325ec1eeb7f1
  v7: 01933a32-b2b8-7fb3-95cd-4470245d95f1
  v7: 01933a32-b2b8-79e0-90f3-912566945738

Ulid: 01933a32-b2b8-835b-4e7a-be129fb30dda
Ulid: 01933a32-b2b8-e09c-16d9-e1118adaf181
Ulid: 01933a32-b2b8-bfd4-fac0-18b026635ce9
Ulid: 01933a32-b2b8-a481-8d60-eef345331de0
Ulid: 01933a32-b2b8-1e49-7dba-22788da45197 
 */


static Guid CreateVersion7(DateTimeOffset? timestamp)
{
    const byte Variant10xxMask = 0xC0, Variant10xxValue = 0x80;
    const ushort VersionMask = 0xF000, Version7Value = 0x7000;

    Guid result = Guid.NewGuid();
    timestamp ??= DateTimeOffset.UtcNow;

    long unix_ts_ms = timestamp.Value.ToUnixTimeMilliseconds();
    ArgumentOutOfRangeException.ThrowIfNegative(unix_ts_ms, nameof(timestamp));


    Span<byte> bytes = stackalloc byte[16];
    MemoryMarshal.TryWrite(bytes, in result);

    //var a = MemoryMarshal.Read<int>(bytes[..4]);
    var a = (int)(unix_ts_ms >> 16);
    MemoryMarshal.Write(bytes[..4], a);

    //var b = MemoryMarshal.Read<short>(bytes[4..6]);
    var b = (short)(unix_ts_ms);
    MemoryMarshal.Write(bytes[4..6], b);


    var cBytes = bytes[6..8];
    var c = MemoryMarshal.Read<short>(cBytes);
    MemoryMarshal.Write(cBytes, (short)((c & ~VersionMask) | Version7Value));


    //var d = bytes[8];
    bytes[8] = (byte)((bytes[8] & ~Variant10xxMask) | Variant10xxValue);

    result = new Guid(bytes, false);

    return result;
}

UlidTest.csproj

<Project Sdk="Microsoft.NET.Sdk">

	<PropertyGroup>
		<OutputType>Exe</OutputType>
		<TargetFramework>net9.0</TargetFramework>
		<ImplicitUsings>enable</ImplicitUsings>
		<Nullable>enable</Nullable>
	</PropertyGroup>


	<ItemGroup>
		<PackageReference Include="Ulid" Version="1.3.4" />
	</ItemGroup>

</Project>