NextCommon.cs

using Mirror;
using Steamworks;
using Steamworks.Data;
using System;
using System.Runtime.InteropServices;
using UnityEngine;

public abstract class NextCommon
{
  protected const int MAX_MESSAGES = 256;

  protected Result SendSocket(Connection conn, byte[] data, int channelId)
  {    
    Array.Resize(ref data, data.Length + 1);
    data[data.Length - 1] = (byte)channelId;

    GCHandle pinnedArray = GCHandle.Alloc(data, GCHandleType.Pinned);
    IntPtr pData = pinnedArray.AddrOfPinnedObject();
    SendType sendFlag = channelId == Channels.Unreliable ? SendType.Unreliable : SendType.Reliable;
    Result res = conn.SendMessage( pData, data.Length, sendFlag);
    if(res != Result.OK)
    {
      Debug.LogWarning($"Send issue: {res}");
    }

    pinnedArray.Free();
    return res;
  } 

  protected (byte[], int) ProcessMessage(IntPtr ptrs, int size)
  { 
    byte[] managedArray = new byte[size];    
    Marshal.Copy(ptrs, managedArray, 0, size);
    int channel = managedArray[managedArray.Length - 1];
    Array.Resize(ref managedArray, managedArray.Length - 1);
    return (managedArray, channel);
  }
}