Demonstration of Guid.CreateVersion7() behavior

SequentialGuid.cs

using Microsoft.Data.SqlClient;
using System.Runtime.InteropServices;
using System.Runtime.Intrinsics;

static class Program
{
    public static int Main()
    {
        var connStr = 
            new SqlConnectionStringBuilder(Environment.GetEnvironmentVariable("MSSqlConnStr")) 
            { 
                InitialCatalog = "Test" 
            };

        var conn = new SqlConnection(connStr.ConnectionString);
        conn.Open();

        var cmd = conn.CreateCommand();
        cmd.CommandText =
            """
            create table GuidOrder (
            guid uniqueidentifier not null,
            seq int not null,
            constraint PK_GuidOrder primary key (guid)
            )
            """;

       // cmd.ExecuteNonQuery();

        cmd.CommandText = "insert into GuidOrder (guid, seq) values (@guid, @seq)";
        var guidParam = cmd.CreateParameter();
        guidParam.ParameterName = "guid";
        guidParam.SqlDbType = System.Data.SqlDbType.UniqueIdentifier;

        var seqParam = cmd.CreateParameter();
        seqParam.ParameterName = "seq";
        seqParam.SqlDbType = System.Data.SqlDbType.Int;

        cmd.Parameters.Add(guidParam);
        cmd.Parameters.Add(seqParam);

        for (int i = 0; i < 1000; i++)
        {
            //guidParam.Value = Guid.CreateVersion7();
            guidParam.Value = Guid.NewSequentialGuid();
            seqParam.Value = i;
            cmd.ExecuteNonQuery();
        }

        cmd.Parameters.Clear();
        cmd.CommandText = "select guid, seq from GuidOrder order by guid";

        using var r = cmd.ExecuteReader();

        var last = int.MinValue;
        while (r.Read())
        {
            var id = r.GetInt32(1);
            if (id < last)
            {
                Console.WriteLine("FAILED");
                return -1;
            }
            last = id;
        }
        Console.WriteLine("SUCCESS");
        return 0;
    }
}

public static class GuidExtensions
{
    static readonly Vector128<byte> Shuffle = Vector128.Create((byte)3, 2, 1, 0, 5, 4, 7, 6, 9, 8, 16, 14, 13, 12, 11, 10);
    static unsafe Guid ShuffleFast(Guid g)
    {
        var v = Vector128.Load((byte*)&g);
        v = Vector128.Shuffle(v, Shuffle);

        var span = AsSpan(ref g);
        Vector128.CopyTo(v, span);
        return g;
    }

    static Span<byte> AsSpan(ref Guid g)
    {
        Span<Guid> valSpan = MemoryMarshal.CreateSpan(ref g, 1);
        return MemoryMarshal.AsBytes(valSpan);
    }

    [DllImport("rpcrt4.dll", SetLastError = true)]
    static extern int UuidCreateSequential(out Guid guid);

    extension(Guid)
    {
        public static Guid NewSequentialGuid()
        {
            Guid g;
            UuidCreateSequential(out g);

            if (Vector128<byte>.IsSupported)
            {
                return ShuffleFast(g);
            }
            else
            {
                var a = AsSpan(ref g);
                var t = default(Guid);
                var b = AsSpan(ref t);
                b[10] = a[15];
                b[11] = a[14];
                b[12] = a[13];
                b[13] = a[12];
                b[14] = a[11];
                b[15] = a[10];
                b[8] = a[9];
                b[9] = a[8];
                b[6] = a[7];
                b[7] = a[6];
                b[4] = a[5];
                b[5] = a[4];
                b[0] = a[3];
                b[1] = a[2];
                b[2] = a[1];
                b[3] = a[0];
                return t;
            }
        }
    }
}