Question

I use System.Threading.Timer to get accurate three minute intervals:

private System.Threading.Timer ReadTimer;
        private System.Threading.TimerCallback ReadTimerCallback;
        public Form1()
        {
            InitializeComponent();
        }

        #region Working with RichTextBox and system event log for logging
        public enum LogStyle
        {
            InformationStyle = 0,
            WarningStyle = 1,
            ErrorStyle = 3
        }
        private delegate void LogTextDelegate(LogStyle logStyle, string message);
        private int NewLinesCount(string Message)
        {
            string[] strings = { Environment.NewLine };
            return Message.Split(strings, StringSplitOptions.RemoveEmptyEntries).Count();
        }
        private void AddText(LogStyle logStyle, string message)
        {
            LogBox.Select(0, 0);
            string TextToAppend = DateTime.Now.ToString("dd.MM.yyyy HH:mm:ss.fff") + " " + message + Environment.NewLine;
            LogBox.AppendText(TextToAppend);
            LogBox.Select(LogBox.TextLength - TextToAppend.Length + NewLinesCount(TextToAppend), TextToAppend.Length);
            switch (logStyle)
            {
                case LogStyle.InformationStyle:
                    {
                        EventLog.WriteEntry(Process.GetCurrentProcess().ProcessName, TextToAppend, EventLogEntryType.Information);
                        LogBox.SelectionColor = Color.Green;
                        LogBox.Select(LogBox.TextLength, LogBox.TextLength);
                        break;
                    }
                case LogStyle.WarningStyle:
                    {
                        EventLog.WriteEntry(Process.GetCurrentProcess().ProcessName, TextToAppend, EventLogEntryType.Warning);
                        LogBox.SelectionColor = Color.DarkOrange;
                        LogBox.Select(LogBox.TextLength, LogBox.TextLength);
                        break;
                    }
                case LogStyle.ErrorStyle:
                    {
                        EventLog.WriteEntry(Process.GetCurrentProcess().ProcessName, TextToAppend, EventLogEntryType.Error);
                        LogBox.SelectionColor = Color.Red;
                        LogBox.Select(LogBox.TextLength, LogBox.TextLength);
                        break;
                    }
            }
        }
        public void LogText(LogStyle logStyle, string Message)
        {
            if (LogBox.InvokeRequired)
            {
                LogBox.Invoke(new LogTextDelegate(this.AddText), new object[] { logStyle, Message });
            }
            else
            {
                AddText(logStyle, Message);
            }
        }
        #endregion

        private void button1_Click(object sender, EventArgs e)
        {
            LogText(LogStyle.WarningStyle, "Current thread ID = " + System.Threading.Thread.CurrentThread.ManagedThreadId);
            LogText(LogStyle.WarningStyle, "Threads count = " + System.Diagnostics.Process.GetCurrentProcess().Threads.Count);
            AutoResetEvent ReadautoEvent = new AutoResetEvent(false);
            ReadTimerCallback = new TimerCallback(this.ShowText);
            ReadTimer = new System.Threading.Timer(ReadTimerCallback, ReadautoEvent, (long)(CurrentTime.AddMinutes(3) - DateTime.Now).TotalMilliseconds, 180000);

        }

        public void ShowText(object Object)
        {
            ReadTimer.Change((long)(CurrentTime.AddMinutes(3) - DateTime.Now).TotalMilliseconds, 180000);
            LogText(LogStyle.WarningStyle, "Now is " + DateTime.Now.ToString("dd.MM.yyyy HH:mm:ss.fff"));
            LogText(LogStyle.WarningStyle, "Now is " + "Current thread ID = " + System.Threading.Thread.CurrentThread.ManagedThreadId);
            LogText(LogStyle.WarningStyle, "Threads count = " + System.Diagnostics.Process.GetCurrentProcess().Threads.Count);
        }

        private DateTime CurrentTime
        {
            get
            {

                DateTime now = DateTime.Now;
                DateTime val;
                val = new DateTime(now.Year, now.Month, now.Day, now.Hour, now.Minute, 0)
                     .AddMinutes(((now.Minute) / 3) * 3 - now.Minute);
                return val;
            }
        }

But sometimes I see following:

14.03.2012 09:20:33.264 Current thread ID = 10
14.03.2012 09:20:33.361 Threads count = 13
14.03.2012 09:20:59.985 Now is 14.03.2012 09:20:59.977
14.03.2012 09:20:59.991 Now is Current thread ID = 12
14.03.2012 09:20:59.997 Threads count = 15
14.03.2012 09:21:00.010 Now is 14.03.2012 09:21:00.009
14.03.2012 09:21:00.019 Now is Current thread ID = 12
14.03.2012 09:21:00.025 Threads count = 17
14.03.2012 09:24:00.252 Now is 14.03.2012 09:24:00.252
14.03.2012 09:24:00.260 Now is Current thread ID = 8
14.03.2012 09:24:00.268 Threads count = 17
14.03.2012 09:27:00.331 Now is 14.03.2012 09:27:00.330
14.03.2012 09:27:00.337 Now is Current thread ID = 6
14.03.2012 09:27:00.343 Threads count = 14
14.03.2012 09:30:00.021 Now is 14.03.2012 09:30:00.021
14.03.2012 09:30:00.027 Now is Current thread ID = 8
14.03.2012 09:30:00.033 Threads count = 13
14.03.2012 09:32:59.962 Now is 14.03.2012 09:32:59.961
14.03.2012 09:32:59.968 Now is Current thread ID = 6
14.03.2012 09:32:59.974 Threads count = 13
14.03.2012 09:33:00.013 Now is 14.03.2012 09:33:00.013
14.03.2012 09:33:00.020 Now is Current thread ID = 6
14.03.2012 09:33:00.026 Threads count = 14
14.03.2012 09:35:59.891 Now is 14.03.2012 09:35:59.891
14.03.2012 09:35:59.898 Now is Current thread ID = 8
14.03.2012 09:35:59.904 Threads count = 13
14.03.2012 09:36:00.001 Now is 14.03.2012 09:35:59.999
14.03.2012 09:36:00.008 Now is 14.03.2012 09:36:00.004
14.03.2012 09:36:00.015 Now is Current thread ID = 8
14.03.2012 09:36:00.021 Now is Current thread ID = 6
14.03.2012 09:36:00.030 Threads count = 14
14.03.2012 09:36:00.035 Threads count = 14

Why does this execute twice in a three minute time interval : at 14.03.2012 09:20:59.985 and 14.03.2012 09:21:00.010?

Where I must fix this?

In CurrentTime I must add 30 seconds such

DateTime now = DateTime.Now.AddSeconds(30) or what?

Answer 1

There are a few issues here. The biggest one I see is your CurrentTime property:

DateTime now = DateTime.Now;
DateTime val;
val = new DateTime(now.Year, now.Month, now.Day, now.Hour, now.Minute, 0)
                 .AddMinutes(((now.Minute) / 3) * 3 - now.Minute);
return val;

Consider what happens when now.Minute is 59, 59/3 = 19 because this is an int. The rest of your expression 59/3 * 3 - 59 results in -2. You have effectively rewound the timer back two minutes. However, since you zeroed out the seconds when you created val, you’ve really set the timer back 3 minutes. 59 is not a unique case, but I’m pulling it from your set of data points. The code that uses your CurrentTime adds 3 minutes. So you’re pretty much telling the code to fire immediately. The reason you are seeing it raised twice is because of this line in ShowText:

`ReadTimer.Change((long)(CurrentTime.AddMinutes(3) - DateTime.Now).TotalMilliseconds, 180000);` 

When your delegate handles the timer, the event is raised immediately again because now.Minute is still 59, causing your overall CurrentTime calculation to be 3 minutes less than DateTime.Now.

If all you want to do is have a task run every three minutes, then the following is all you need:

ReadTimer = new System.Threading.Timer(ReadTimerCallback, ReadautoEvent, 0, 180000);