using System;

using System.Runtime.InteropServices;

using System.Collections.Generic;

using System.Diagnostics;

​

public class Program

{

    private static List<ArraySegment<byte>> _buffers = new List<ArraySegment<byte>>();

    private static ArraySegment<byte> _buffer;

    [StructLayout(LayoutKind.Explicit)]

    public struct ValBytes

    {

        [FieldOffset(0)]

        public long LongVal;

​

        [FieldOffset(0)]

        public byte B0;

        [FieldOffset(1)]

        public byte B1;

        [FieldOffset(2)]

        public byte B2;

        [FieldOffset(3)]

        public byte B3;

        [FieldOffset(4)]

        public byte B4;

        [FieldOffset(5)]

        public byte B5;

        [FieldOffset(6)]

        public byte B6;

        [FieldOffset(7)]

        public byte B7;

    }

    private static ValBytes _valBytes = new ValBytes();

    private static Memory<byte> data = new byte[8];

    static void WriteLong(long v)

    {

        Span<byte> data = stackalloc byte[8];

        MemoryMarshal.Write<long>(data, ref v);

        _buffer = _buffers[0];

        _buffer[2] = data[0];

        _buffer[3] = data[1];

        _buffer = _buffers[1];

        _buffer[0] = data[2];

        _buffer[1] = data[3];

        _buffer[2] = data[4];

        _buffer[3] = data[5];

        _buffer[4] = data[6];

        _buffer[5] = data[7];

    }

    static void WriteLongVal(long v)

    {

        _valBytes.LongVal = v;

        _buffer = _buffers[0];

        _buffer[2] = _valBytes.B0;

        _buffer[3] = _valBytes.B1;

        _buffer = _buffers[1];

        _buffer[0] = _valBytes.B2;

        _buffer[1] = _valBytes.B3;

        _buffer[2] = _valBytes.B4;

        _buffer[3] = _valBytes.B5;

        _buffer[4] = _valBytes.B6;

        _buffer[5] = _valBytes.B7;

    }

​

    public static int Main()

    {

        _buffer = new byte[4];

        _buffers.Add(_buffer);

        _buffers.Add(new byte[8]);

        int iterations = 10_000_000;

        var watch = new Stopwatch();

        watch.Start();

        for(int i = 0; i < iterations; ++i)

        {

            WriteLongVal(128);

        }

        watch.Stop();

        Console.WriteLine($"WriteLongVal elapsed: {watch.ElapsedMilliseconds}");

        watch.Reset();

        watch.Start();

        for(int i = 0; i < iterations; ++i)

        {

            WriteLong(128);

        }

        watch.Stop();

        Console.WriteLine($"WriteLong elapsed: {watch.ElapsedMilliseconds}");

​

        return 0;   

    }

}