question

@UyenNguyen-4262, Welcome to Microsoft Q&A, Based on your description, you want to make a countdown timer and hope that it will contine when the form is closed.

I make some steps and code example. You could check if it works for you.

First, I make a countdown Timer from the answer.

Second, I add the following settings in the winform.

Third, I used FormClosing event to save some information to the settings.

   private void Form1_FormClosing(object sender, FormClosingEventArgs e)
         {
             if(timer.IsRunnign)
             {
                 Properties.Settings.Default.lefttime = label1.Text;
                 Properties.Settings.Default.Isrunning = true;
                 Properties.Settings.Default.closedtime=DateTime.Now;
                 Properties.Settings.Default.Save();
             }
         }
    
  Fourth, I used Form1_Load event to read the information.
    
   private void Form1_Load(object sender, EventArgs e)
         {
             if (Properties.Settings.Default.Isrunning== true)
             {
                 string lefttime = Properties.Settings.Default.lefttime;
                 TimeSpan time1=TimeSpan.ParseExact(lefttime, @"mm\:ss", CultureInfo.CurrentCulture);
                 DateTime closedtime = Properties.Settings.Default.closedtime;
                 TimeSpan passedtime = DateTime.Now - closedtime;
                 var t = time1.Subtract(passedtime);
                 label1.Text=t.ToString(@"mm\:ss");
                 timer.SetTime(t.Minutes, t.Seconds);
                 timer.Start();
                 timer.TimeChanged += () => { label1.Text = timer.TimeLeftStr; };
    
                 // show messageBox on timer = 00:00.000
                 timer.CountDownFinished += () => { timer.Stop(); MessageBox.Show("Timer finished the work!"); };
    
                 //timer step. By default is 1 second
                 timer.StepMs = 1; // for nice milliseconds time switch
             }
             else
             {
                 //set to 5 mins
                 timer.SetTime(5, 0);
    
                 //update label text
                 timer.TimeChanged += () => { label1.Text = timer.TimeLeftStr; };
    
                 // show messageBox on timer = 00:00.000
                 timer.CountDownFinished += () => { timer.Stop(); MessageBox.Show("Timer finished the work!"); };
    
                 //timer step. By default is 1 second
                 timer.StepMs = 1; // for nice milliseconds time switch
             }
               
         }

Finally, Please set the properties when we click the stop button.

Completed code:

  public partial class Form1 : Form
     {
         public Form1()
         {
             InitializeComponent();
         }
         CountDownTimer timer = new CountDownTimer();
         private void Form1_Load(object sender, EventArgs e)
         {
             if (Properties.Settings.Default.Isrunning== true)
             {
                 string lefttime = Properties.Settings.Default.lefttime;
                 TimeSpan time1=TimeSpan.ParseExact(lefttime, @"mm\:ss", CultureInfo.CurrentCulture);
                 DateTime closedtime = Properties.Settings.Default.closedtime;
                 TimeSpan passedtime = DateTime.Now - closedtime;
                 var t = time1.Subtract(passedtime);
                 label1.Text=t.ToString(@"mm\:ss");
                 timer.SetTime(t.Minutes, t.Seconds);
                 timer.Start();
                 timer.TimeChanged += () => { label1.Text = timer.TimeLeftStr; };
    
                 // show messageBox on timer = 00:00.000
                 timer.CountDownFinished += () => { timer.Stop(); MessageBox.Show("Timer finished the work!"); };
    
                 //timer step. By default is 1 second
                 timer.StepMs = 1; // for nice milliseconds time switch
             }
             else
             {
                 //set to 5 mins
                 timer.SetTime(5, 0);
    
                 //update label text
                 timer.TimeChanged += () => { label1.Text = timer.TimeLeftStr; };
    
                 // show messageBox on timer = 00:00.000
                 timer.CountDownFinished += () => { timer.Stop(); MessageBox.Show("Timer finished the work!"); };
    
                 //timer step. By default is 1 second
                 timer.StepMs = 1; // for nice milliseconds time switch
                 Console.WriteLine(Properties.Settings.Default.lefttime);
             }
               
         }
    
         private void button1_Click(object sender, EventArgs e)
         {
             timer.Start();
         }
    
         private void button2_Click(object sender, EventArgs e)
         {
             timer.Pause();
         }
    
         private void button3_Click(object sender, EventArgs e)
         {
             timer.Stop();
             timer.Dispose();
             Properties.Settings.Default.lefttime = "";
             Properties.Settings.Default.Isrunning = false;
             Properties.Settings.Default.closedtime = DateTime.Now;
             Properties.Settings.Default.Save();
         }
    
         private void Form1_FormClosing(object sender, FormClosingEventArgs e)
         {
             if(timer.IsRunnign)
             {
                 Properties.Settings.Default.lefttime = label1.Text;
                 Properties.Settings.Default.Isrunning = true;
                 Properties.Settings.Default.closedtime=DateTime.Now;
                 Properties.Settings.Default.Save();
             }
         }
     }
     public class CountDownTimer : IDisposable
     {
         public Stopwatch _stpWatch = new Stopwatch();
    
         public Action TimeChanged;
         public Action CountDownFinished;
    
         public bool IsRunnign => timer.Enabled;
    
         public int StepMs
         {
             get => timer.Interval;
             set => timer.Interval = value;
         }
    
         private Timer timer = new Timer();
    
         private TimeSpan _max = TimeSpan.FromMilliseconds(30000);
    
         public TimeSpan TimeLeft => (_max.TotalMilliseconds - _stpWatch.ElapsedMilliseconds) > 0 ? TimeSpan.FromMilliseconds(_max.TotalMilliseconds - _stpWatch.ElapsedMilliseconds) : TimeSpan.FromMilliseconds(0);
    
         private bool _mustStop => (_max.TotalMilliseconds - _stpWatch.ElapsedMilliseconds) < 0;
    
         public string TimeLeftStr => TimeLeft.ToString(@"mm\:ss");
    
         public string TimeLeftMsStr => TimeLeft.ToString(@"mm\:ss\.fff");
    
         private void TimerTick(object sender, EventArgs e)
         {
             TimeChanged?.Invoke();
    
             if (_mustStop)
             {
                 CountDownFinished?.Invoke();
                 _stpWatch.Stop();
                 timer.Enabled = false;
             }
         }
    
         public CountDownTimer(int min, int sec)
         {
             SetTime(min, sec);
             Init();
         }
    
         public CountDownTimer(TimeSpan ts)
         {
             SetTime(ts);
             Init();
         }
    
         public CountDownTimer()
         {
             Init();
         }
    
         private void Init()
         {
             StepMs = 1000;
             timer.Tick += new EventHandler(TimerTick);
         }
    
         public void SetTime(TimeSpan ts)
         {
             _max = ts;
             TimeChanged?.Invoke();
         }
    
         public void SetTime(int min, int sec = 0) => SetTime(TimeSpan.FromSeconds(min * 60 + sec));
    
         public void Start()
         {
             timer.Start();
             _stpWatch.Start();
         }
    
         public void Pause()
         {
             timer.Stop();
             _stpWatch.Stop();
         }
    
         public void Stop()
         {
             Reset();
             Pause();
         }
    
         public void Reset()
         {
             _stpWatch.Reset();
         }
    
         public void Restart()
         {
             _stpWatch.Reset();
             timer.Start();
         }
    
         public void Dispose() => timer.Dispose();
     }

Tested result:

Hope this could help you.

Best Regards,
Jack

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