using System;

using System.Runtime.InteropServices;

using System.Runtime.CompilerServices;

public class Program

{

    public static void Main()

    {

        var points = new[]

        {

            new Point() { X = 1, Y = 2, Z = 3 },

            new Point() { X = 4, Y = 5, Z = 6 },

        };

        var bytes = new byte[points.Length * 3 * sizeof(double)];

        var doubles = MemoryMarshal.Cast<byte, double>(bytes);

        for (int i = 0; i < points.Length; i++)

        {

            doubles[i * 3 + 0] = points[i].X;   

            doubles[i * 3 + 1] = points[i].Y;

            doubles[i * 3 + 2] = points[i].Z;

        }

        Console.WriteLine(BitConverter.ToString(bytes));

    }

}

​

class Point

{

    public double X { get; set; }

    public double Y { get; set; }

    public double Z { get; set; }

}