Asp.Net Core Hangfire Cronos expression to repeat every x weeks

Question

I'm working on a task system where you can create repeating tasks, and there is one case that I need help to figure out:

I'm using this library to figure out when to create the tasks and to project the time it will spend in the next 28 days. For example, every day at 9:

CronExpression expression = CronExpression.Parse("0 9 * * *");

But reading about the Cronos expression I still couldn't figure out how to get an expression That would repeat every 3 weeks. Which days of the week OK, but every x weeks I can't figure out.

Can you help me please? Thanks

You can't, this is not how the cron works. – Puchacz Oct 03 '18 at 12:30 — Puchacz, Oct 03 '18 at 12:30

score 0 · Answer 1 · answered Oct 03 '18 at 05:41

I dont think you can schedule a cron based on weeks, see below: I would suggest scheduling the same job for different days of the month.

┌───────────── minute (0 - 59)
│ ┌───────────── hour (0 - 23)
│ │ ┌───────────── day of the month (1 - 31)
│ │ │ ┌───────────── month (1 - 12)
│ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday;
│ │ │ │ │                                   7 is also Sunday on some systems)
│ │ │ │ │
│ │ │ │ │
* * * * * command to execute

https://en.wikipedia.org/wiki/Cron

score 0 · Answer 2 · answered Oct 03 '18 at 15:23

There is no way to do this with cron expressions. In my case, I needed the occurrences to make some cost calculation. This is the function I made that seems to be working fine:

        public List<DateTime> GetOccurrencesInWeeksInterval(DateTime start, DateTime end)
    {
        var ret = new List<DateTime>();
        DateTime? lastDate = null;
        int weekInterval = _rptConfig.Interval;

        CronExpression expression = CronExpression.Parse(this.GetCronExpression()); // this would return something like "0 9 * * 1,3,5"
        var dates = expression.GetOccurrences(start, end).ToList();

        if (weekInterval == 1) {
            return dates;
        }

        if (dates.Any())
        {
            foreach (var date in dates)
            {
                if (lastDate == null)
                {
                    ret.Add(date);
                    lastDate = date;
                }
                else
                {
                    var weekDiff = HelperFunctions.GetWeekDiff(lastDate.Value, date);
                    if (weekDiff == 0)
                    {
                        ret.Add(date);
                    }
                    else if (weekDiff > 0 && (weekDiff % weekInterval) == 0)
                    {
                        ret.Add(date);
                    }
                }
            }
        }

        return ret;
    }

This is the GetWeekDiff function:

        /// <summary>
    /// https://stackoverflow.com/questions/49924085/week-difference-between-2-dates-in-c-sharp#49924454
    /// </summary>
    /// <param name="dtStart"></param>
    /// <param name="dtEnd"></param>
    /// <returns></returns>
    public static int GetWeekDiff(DateTime dtStart, DateTime dtEnd, DayOfWeek startOfWeek = DayOfWeek.Monday) {

        //===============================================
        var diff = dtEnd.Subtract(dtStart);
        var weeks = (int)diff.Days / 7;

        // need to check if there's an extra week to count
        var remainingDays = diff.Days % 7;
        var cal = CultureInfo.InvariantCulture.Calendar;
        var d1WeekNo = cal.GetWeekOfYear(dtStart, CalendarWeekRule.FirstFullWeek, startOfWeek);
        var d1PlusRemainingWeekNo = cal.GetWeekOfYear(dtStart.AddDays(remainingDays), CalendarWeekRule.FirstFullWeek, startOfWeek);

        if (d1WeekNo != d1PlusRemainingWeekNo)
            weeks++;

        return weeks;

    }

Hope this helps someone because I realized this is a pretty common question.

Asp.Net Core Hangfire Cronos expression to repeat every x weeks

2 Answers2