Unity ディレクティブは必要ありません。

前提・実現したいこと

Visual Studio でusing System.IO.Ports;にエラー（文字の下に赤線）が出ないようにしたいです。

発生している問題・エラーメッセージ

エラーメッセージ

該当のソースコード

using UnityEngine;
using System.Collections;
using System.IO.Ports; //←using ディレクティブは必要ありません。と出る
using System.Threading;
using System;

public class Serialhandler : MonoBehaviour
{
    public delegate void SerialDataReceivedEventHandler(string message);
    public event SerialDataReceivedEventHandler OnDataReceived;

    public string portName = "COM3";
    public int baudRate = 9600;

    private SerialPort serialPort_;
    private Thread thread_;
    private bool isRunning_ = false;

    private string message_;
    private bool isNewMessageReceived_ = false;

    private int[] swIn = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
    private int intmes;


    void Awake()
    {
        Open();
    }

    void Update()
    {
        if (isNewMessageReceived_)
        {
            OnDataReceived(message_);
        }
        isNewMessageReceived_ = false;


    }

    public void OnDataKeyInput(string a)
    {
        OnDataReceived(a);
    }


    void OnDestroy()
    {
        Close();
    }

    private void Open()
    {
        serialPort_ = new SerialPort(portName, baudRate, Parity.None, 8, StopBits.One);
        
        serialPort_.Open();

        isRunning_ = true;

        thread_ = new Thread(Read);
        thread_.Start();
    }

    private void Close()
    {
        isNewMessageReceived_ = false;
        isRunning_ = false;

        if (thread_ != null && thread_.IsAlive)
        {
            thread_.Join();
        }

        if (serialPort_ != null && serialPort_.IsOpen)
        {
            serialPort_.Close();
            serialPort_.Dispose();
        }
    }

    private void Read()
    {
        while (isRunning_ && serialPort_ != null && serialPort_.IsOpen)
        {
            try
            {
                message_ = serialPort_.ReadLine();
                intmes = Int32.Parse(message_);
                intmes--;

                if (swIn[intmes] == 0)
                {
                    swIn[intmes] = 1;
                    isNewMessageReceived_ = true;
                }
                else
                {
                    swIn[intmes] = 0;
                }


            }
            catch (System.Exception e)
            {
                Debug.LogWarning(e.Message);
            }
        }
    }

    public void Write(string message)
    {
        try
        {
            serialPort_.Write(message);
        }
        catch (System.Exception e)
        {
            Debug.LogWarning(e.Message);
        }
    }
}