public partial class Form1 : Form
    {
        CancellationTokenSource cts = new CancellationTokenSource();        
        public Form1()
        {
            InitializeComponent(); 
        }

        private void StartClick(object sender, EventArgs e)
        {             
            Task testTask = Task.Run(() =>
            {
                while (!cts.IsCancellationRequested)
                {
                    string str = $"{DateTime.Now.ToString("yyyyMMddHHmmssffff")},{Guid.NewGuid().ToString()}";
                    BeginInvoke(new Action(() =>
                    {
                        listBox1.Items.Add(str);
                    }));
                    Thread.Sleep(500);
                }
            }, cts.Token);
        }

        private void CancelClick(object sender, EventArgs e)
        {
            cts.Cancel();
            string str = "Task cancelled";
            listBox1.Items.Add(str);
        }
    }

public partial class Form1 : Form
    {
        CancellationTokenSource cts = new CancellationTokenSource();
        List<string> lbList = new List<string>();      
        public Form1()
        {
            InitializeComponent();
            listBox1.DataSource = lbList; 
        }

        private void StartClick(object sender, EventArgs e)
        {             
            Task testTask = Task.Run(() =>
            {
                while (!cts.IsCancellationRequested)
                {
                    string str = $"{DateTime.Now.ToString("yyyyMMddHHmmssffff")},{Guid.NewGuid().ToString()}";
                    BeginInvoke(new Action(() =>
                    {
                        lbList.Add(str);
                        listBox1.DataSource = null;
                        listBox1.DataSource = lbList;                                         
                    }));
                    Thread.Sleep(500);
                }
            }, cts.Token);
        }

        private void CancelClick(object sender, EventArgs e)
        {
            cts.Cancel();
            string str = "Task cancelled";
            BeginInvoke(new Action(() =>
            {
                lbList.Add(str);
                listBox1.DataSource = null;
                listBox1.DataSource = lbList;                                           
            }));
        }
    }

 public partial class Form1 : Form
    {
        CancellationTokenSource cts = new CancellationTokenSource();
        List<string> lbList = new List<string>();      
        public Form1()
        {
            InitializeComponent();
            listBox1.DataSource = lbList; 
        }

        private void StartClick(object sender, EventArgs e)
        {             
            Task testTask = Task.Run(() =>
            {
                while (!cts.IsCancellationRequested)
                {
                    AddContentToLBx();                    
                }
            }, cts.Token);
        }

        int i = 0;

        private void AddContentToLBx()
        {
            for(i=0;i<10000;i++)
            {
                string str = $"i={i},{DateTime.Now.ToString("yyyyMMddHHmmssffff")},{Guid.NewGuid().ToString()}";
                BeginInvoke(new Action(() =>
                {
                    lbList.Add(str);
                    listBox1.DataSource = null;
                    listBox1.DataSource = lbList;
                }));
                Thread.Sleep(500);
                if (cts.IsCancellationRequested)
                {
                    break;
                }
            }
        }

        private void CancelClick(object sender, EventArgs e)
        {
            cts.Cancel();
            string str = $"i={i},Task cancelled";
            BeginInvoke(new Action(() =>
            {
                lbList.Add(str);
                listBox1.DataSource = null;
                listBox1.DataSource = lbList;                                           
            }));
        }
    }

int i = 0;
        List<string> lbList = new List<string>();      
        private bool IsCancelled { get; set; }
        public Form1()
        {
            InitializeComponent();
            listBox1.DataSource = lbList; 
        }

        //Start or restart
        private void StartClick(object sender, EventArgs e)
        {
            IsCancelled = false;
            if(!IsCancelled)
            {
                Task testTask = Task.Run(() =>
                {
                    AddContentToLBx();
                });
            }           
        }        

        private void AddContentToLBx()
        {
            while (i < 10000)
            {
                if (!IsCancelled)
                {
                    string str = $"i={i},{DateTime.Now.ToString("yyyyMMddHHmmssffff")},{Guid.NewGuid().ToString()}";
                    BeginInvoke(new Action(() =>
                    {
                        lbList.Add(str);
                        listBox1.DataSource = null;
                        listBox1.DataSource = lbList;
                    }));
                    Thread.Sleep(500);
                }
                else
                {
                    break;
                }
                i++;
            }
        }

        private void CancelClick(object sender, EventArgs e)
        {            
            IsCancelled = true;
            string str = $"i={i},Task cancelled";
            BeginInvoke(new Action(() =>
            {
                lbList.Add(str);
                listBox1.DataSource = null;
                listBox1.DataSource = lbList;                                           
            }));
        }