private void WorkGraded(IAsyncResult res) {
    WorkCompleted wc = (WorkCompleted)res.AsyncState;
    int grade = wc.EndInvoke(res);
    Console.WriteLine( "Worker grade= "  + grade);
    DoWork(); //доделка дабы зациклить
}

//скип using
namespace myApp
{
    public class Form1 : System.Windows.Forms.Form
    {
        //скип инициализация
        private void butNew_Click(object sender, System.EventArgs e)
        {
            Form1 f = new Form1();
            f.ShowDialog(this);
            f.Dispose();
        }
        //скип
    }

    public class Form2 : System.Windows.Forms.Form
    {
        private Scale sc;
        //поскипано инициализация
        public Form2()
        {
            InitializeComponent();
            this.sc = new Scale() ;
            this.sc.OnCheckCompleted += new CheckCompletedDelegate(this.CheckCompleted);
        }
        //скип
    
        private void Form2_Load(object sender, System.EventArgs e)
        {
            this.sc.Check();
        }

        public void CheckCompleted(string data) 
        {
            this.textVes.Text = data
        }
    }
    
    delegate void CheckCompletedDelegate(string Data);
    
    class Scale 
    {
        public void Check()
        {
            APScale.APScaleClass scale = new APScale.APScaleClass();
            scale.Connect( "COM1" );
            string data = scale.Data as string;
            scale.DisConnect();

            if( OnCheckCompleted != null ) 
            {
                foreach( CheckCompletedDelegate ccd in OnCheckCompleted.GetInvocationList() ) 
                {
                    ccd.BeginInvoke(data, new AsyncCallback(CheckCompletedCallback), ccd);
                }
            }
        }
        
        public event CheckCompletedDelegate OnCheckCompleted;
        
        private void CheckCompletedCallback(IAsyncResult res) 
        {
            CheckCompletedDelegate ccd = (CheckCompletedDelegate)res.AsyncState;
            ccd.EndInvoke(res);
            Check();
        }
    }
}

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;

namespace vesi
{
    public class Form2 : System.Windows.Forms.Form
    {
        Scale s = new Scale();
        private void Form2_Load(object sender, System.EventArgs e)
        {
            s.CheckComplete+=new CheckCompleteHandler(s_CheckComplete);
            s.BeginCheck();
        }
        private void s_CheckComplete(string data)
        {
            this.textBox.Text=data;
            s.BeginCheck();
        }
    }
    public class Scale
    {
        APScale.APScaleClass scale = new APScale.APScaleClass();
        bool terminate = false;

        public void BeginCheck()
        {
            CheckDelegate check = new CheckDelegate(Check);
            check.BeginInvoke(new AsyncCallback(EndCheck), check);
        }
        string Check()
        {
            scale.Connect( "COM1" );
                string data = this.scale.Data as string;
            scale.DisConnect();
            return data;
        }
        void EndCheck(IAsyncResult ar)
        {
            CheckDelegate check = (CheckDelegate)ar.AsyncState;
            string data = check.EndInvoke(ar);
            if (!terminate)
            {CheckComplete(data);}
        }
        public void Terminate()
        {
            terminate=true;
        }
        public event CheckCompleteHandler CheckComplete;
    }
    delegate string CheckDelegate();
    public delegate void CheckCompleteHandler(string data);
}