C#

int cellvalue3 = int.Parse(my_table3.Rows[i][19].ToString());

System.FormatException: 'Input string was not in a correct format.'

namespace laptopWatcher {  
  public partial class clockCountDown: Window {  
    public string theCntDownTimerTime = "";  
  
    public clockCountDown() {  
      InitializeComponent();  
      //get countdown time  
      theCntDownTimerTime = getTimeToStart();  
    }  
  
    private void Window_Loaded(object sender, RoutedEventArgs e) {  
      var desktopWorkingArea = SystemParameters.WorkArea;  
      this.Left = desktopWorkingArea.Right - this.Width - 20;  
      this.Top = desktopWorkingArea.Top + this.Height - 30;  
      var ts = ((60 - DateTime.Now.Second) * 1000 - DateTime.Now.Millisecond);  
  
      clockTimer.Tick += new EventHandler(clockTimer_tick);  
      clockTimer.Interval = new TimeSpan(0, 0, 35);  
      clockTimer.Start();  
  
      //Populate the time once to show to user  
      bgTxt.Text = DateTime.Now.ToString("h:mm tt");  
      txt.Text = DateTime.Now.ToString("h:mm tt");  
  
      //check if time for count down  
      if (txt.Text.ToLower().Equals(theCntDownTimerTime.ToLower())) {  
        _timer.Stop();  
        TimeSpan time = TimeSpan.Parse(theCntDownTimerTime);  
        cdownTimer((_time).TotalSeconds);  
      }  
  
      _tws = new TinyWS();  
      Thread t = new Thread(new ThreadStart(_tws.startWS));  
  
      t.Start();  
    }  
  
    private void clockTimer_tick(object sender, EventArgs e) {  
      bgTxt.Text = DateTime.Now.ToString("h:mm tt");  
      txt.Text = DateTime.Now.ToString("h:mm tt");  
      Console.WriteLine(txt.Text.ToLower() + " vs " + theCntDownTimerTime.ToLower());  
    }  
  
    public string getTimeToStart() {  
      string line;  
  
      using(StreamReader sr = new StreamReader(Environment.CurrentDirectory + "\\timestart.txt")) {  
        line = sr.ReadLine();  
      }  
  
      return line.ToLower();  
    }  
  
    // etc etc....  
  }  
}

namespace laptopLogin {  
  class TinyWS: clockCountDown {  
    HttpListener listener = new HttpListener();  
    clockCountDown ccd = new clockCountDown();  
  
    public class receivedData {  
      public string changeCNT {  
        get;  
        set;  
      }  
      public int timeAdd {  
        get;  
        set;  
      }  
      public bool internet {  
        get;  
        set;  
      }  
      public bool shutdownNow {  
        get;  
        set;  
      }  
    }  
  
    public void startWS() {  
      var prefixes = new List < string > () {  
        "http://*:8888/"  
      };  
  
      foreach(string s in prefixes) {  
        listener.Prefixes.Add(s);  
      }  
  
      listener.Start();  
      Console.WriteLine("Listening...");  
  
      while (true) {  
        HttpListenerContext context = listener.GetContext();  
        HttpListenerRequest request = context.Request;  
  
        string documentContents;  
  
        using(Stream receiveStream = request.InputStream) {  
          using(StreamReader readStream = new StreamReader(receiveStream, Encoding.UTF8)) {  
            documentContents = readStream.ReadToEnd();  
          }  
        }  
  
        Console.WriteLine($"Recived request for {request.Url}");  
        Console.WriteLine(documentContents);  
        HttpListenerResponse response = context.Response;  
  
        //Check out the response  
        receivedData jsonSent = JsonConvert.DeserializeObject < receivedData > (documentContents);  
  
        if (jsonSent.changeCNT.ToLower() != ccd.getTimeToStart()) {  
          //The default shutdown time has changed so update the txt file.  
          using(StreamWriter sw = new StreamWriter(Environment.CurrentDirectory + "\\timestart.txt", false)) {  
            sw.WriteLine(jsonSent.changeCNT);  
          }  
  
          ccd.theCntDownTimerTime = jsonSent.changeCNT;  
        }  
  
        //Reply  
        string responseString = "hit";  
        byte[] buffer = System.Text.Encoding.UTF8.GetBytes(responseString);  
        response.ContentLength64 = buffer.Length;  
        Stream output = response.OutputStream;  
        output.Write(buffer, 0, buffer.Length);  
        output.Close();  
      }  
    }  
  
    public void stopWS() {  
      listener.Stop();  
    }  
  }  
}  

public string theCntDownTimerTime = "";  
public string _newCNT {  
  get {  
    return theCntDownTimerTime;  
  }  z
  set {  
    theCntDownTimerTime = value;  
  }  
}

System.InvalidOperationException: 'Collection was modified; enumeration operation may not execute.'

foreach (Form frm in fc)

FormCollection fc = Application.OpenForms;
            foreach (Form frm in fc)
            {
                if (frm.Name == "Chart")
                {
                    MessageBox.Show("The form is open","error",MessageBoxButtons.OK,MessageBoxIcon.Error);
                }
                else
                {
                    Chart myCharts = new Chart();
                    myCharts.Show();
                }
            }