BackgroundWorker backgroundWorkerA, backgroundWorkerB;
        System.Threading.ManualResetEvent manualReset;
        List<string> _shardList = new List<string>(0);

        public UserControl1()
        {

            InitializeComponent();
            manualReset = new System.Threading.ManualResetEvent(false);
            backgroundWorkerA = new BackgroundWorker();
            backgroundWorkerA.WorkerSupportsCancellation = true;
            backgroundWorkerA.DoWork += BackgroundWorkerA_DoWork;

            backgroundWorkerB = new BackgroundWorker();
            backgroundWorkerB.WorkerSupportsCancellation = true;
            backgroundWorkerB.DoWork += BackgroundWorkerB_DoWork;


            this.HandleCreated += UserControl1_HandleCreated;

        }


        private void UserControl1_HandleCreated(object sender, EventArgs e)
        {
            backgroundWorkerA.RunWorkerAsync(_shardList);
            backgroundWorkerB.RunWorkerAsync(_shardList);
            manualReset.Set();
        }

        private void BackgroundWorkerB_DoWork(object sender, DoWorkEventArgs e)
        {
            List<string> _shardList = (List<string>)e.Argument;
            manualReset.WaitOne();
            int _i = 0;
            while(!this.backgroundWorkerB.CancellationPending)
            {
                _shardList.Add("b" + _i++.ToString());
                System.Diagnostics.Debug.WriteLine("b is running");

            }

            thread2.Invoke(new MethodInvoker(delegate { thread2.Text = string.Join(System.Environment.NewLine, _shardList.ToArray()); }));
        }

        private void button1_Click(object sender, EventArgs e)
        {
            backgroundWorkerA.CancelAsync();
            backgroundWorkerB.CancelAsync();
        }

        private void BackgroundWorkerA_DoWork(object sender, DoWorkEventArgs e)
        {
            List<string> _shardList = (List<string>)e.Argument;
            manualReset.WaitOne();
            int _i = 0;
            while (!this.backgroundWorkerA.CancellationPending)
            {
                _shardList.Add("a" + _i++.ToString());
                System.Diagnostics.Debug.WriteLine("a is running");

            }

            thread1.Invoke(new MethodInvoker(delegate { thread1.Text = string.Join(System.Environment.NewLine, _shardList.ToArray()); }));
        }