C# Threads und Anzeige in der GUI

//Ein Button der den Thread startet:
private void buttonTest_Click(object sender, EventArgs e)
{
            _TmfStart = DateTime.Now;
            textBox1.Text = "Start loading...";
            textBox1.AppendText(Environment.NewLine);        
            textBox1.AppendText("Time: " + _TmfStart.ToString("mm ss f"));

            var thread = new Thread(new ThreadStart(getLongRun));
            thread.Start();
 }


private void getLongRun()
        {
            using (var db = new DatabaseContext())
            {
                var opt = db.artikeldatenbank.Take(50).ToList();
                this.Invoke((MethodInvoker)delegate
                {
                    //here can change GUI without freeze
                    dataGridView1.DataSource = opt;

                    _TmfEnd = DateTime.Now;
                    textBox1.AppendText(Environment.NewLine);
                    textBox1.AppendText("Time: " + _TmfEnd.ToString("ss f"));
                });
            }
        }

int ActivThreads = 0;
static CancellationTokenSource cancellationTokenSource = new CancellationTokenSource();
static CancellationToken token = cancellationTokenSource.Token;

private void button1ThreadStart_Click(object sender, EventArgs e)
{            
    TestBackgroundThread(token);
}

private void InvokeGUIThread(Action action)
{
    Invoke(action);
}

void TestBackgroundThread(CancellationToken cancelToken, bool bg = true)
{
    var thread = new Thread((ThreadStart)delegate ()
    {
        long count = 0;
        while (true)
        {
            if (cancelToken.IsCancellationRequested)
            {
                return;
            }
            Thread.Sleep(Convert.ToInt32(textBox2.Text));
            count++;
            InvokeGUIThread(() =>
            {
                textBox1.AppendText(count.ToString() + Environment.NewLine);
            });

        }
    });

    // Choose one option:
    thread.IsBackground = bg; // <--- This will make the thread run in background
    /*thread.IsBackground = false;*/ // <--- This will delay program termination

    thread.Start();
    ActivThreads++;           
}

private void button1StopAll_Click(object sender, EventArgs e)
{
    cancellationTokenSource.Cancel();
    ActivThreads = 0;
    cancellationTokenSource = new CancellationTokenSource();
    token = cancellationTokenSource.Token;
}