The pattern builders for days, weeks and months in turn allow you to configure them as desired:

• Daily patterns can be provided with:
  • A reference date to determine what day the pattern starts in case if an interval greater than 1
  • The option to include weekends, skip weekends or move any date that falls in a weekend to the following Monday or preceding Friday
• Weekly patterns can be provided with:
  • A reference date and first day of the week to determine what week the pattern starts in case if an interval greater than 1
  • The days of the week that are valid
  • If no days of the week are provided the day of the week of the reference date will be used
• Monthly patterns can be provided with:
  • A reference date to determine what month the pattern starts in case if an interval greater than 1
  • The days of the month that are valid
  • The ordinal days of the week in the month that are valid (e.g. last Friday of the month)
  • The last days of the month that are valid (e.g. second last day of the month)
  • If no days are provided, the day of the month of the reference date will be used
  • The option to enable caching to speed up this relatively slow pattern at a cost of increased memory usage

The filter builders also allow some configuration after adding them:

• A date filter can be provided with addditional dates to filter
• A date range filter can be provided with a new start and end date

It's simple to chain calls to the above methods to set the limits and add multiple patterns for a single recurrence.

Examples

// Build a recurrence that repeats every 9 days from January 1st 2022
var every9days = Recurs
    .Every(9).Days()
    .From(new DateTime(2022, 1, 1))
    .Build();

// Get all valid days for February 2022: 2022-02-06, 2022-02-15 and 2022-02-24
var dates = every9days.GetDates(new DateTime(2022, 2, 1), new DateTime(2022, 2, 28));


// Build a recurrence for the year 2022 with 2 patterns:
// - Every 2 weeks on Monday and Tuesday, determining the ongoing week using Sunday first day of the week
// - Every 1st and 3rd day of the month
var doublePattern = Recurs
    .From(new DateTime(2022, 1, 1))
    .Until(new DateTime(2022, 12, 31))
    .Every(2).Weeks().On(DayOfWeek.Monday, DayOfWeek.Tuesday).UsingFirstDayOfWeek(DayOfWeek.Sunday)
    .Monthly().On(1, 3)
    .Build();

// Get all valid days for February 2022: 2022-02-01, 2022-02-03, 2022-02-07, 2022-02-08, 2022-02-22 and 2022-02-23
var dates = doublePattern.GetDates(new DateTime(2022, 2, 1), new DateTime(2022, 2, 28));


// Build a recurrence that repeats every last Friday of the month for 3 occurrences, starting in April 2022
var fridays = Recurs
    .Monthly().On(DayOfWeekInMonth.Last, DayOfWeek.Friday)
    .From(new DateTime(2022, 4, 1))
    .StopAfter(3)
    .Build();

// Get all valid dates: 2022-04-29, 2022-05-27 and 2022-06-24
var dates = fridays.GetDates();


// Build a recurrence that repeats every weekday in December 2022, except for Christmas
var workingDays = Recurs
    .Daily()
    .SkipWeekends()
    .From(new DateTime(2022, 12, 1))
    .Until(new DateTime(2022, 12, 31))
    .ExceptOn(new DateTime(2022, 12, 25), new DateTime(2022, 12, 26))
    .Build();

// Get all valid dates: all week days in December except Christmas
var dates = workingDays.GetDates();

Manual setup

Though the builder syntax makes it easy to write understandable recurrences, it is also possible to write recurrences and patterns by constructing the recurrence objects directly.

Example

// Create a recurrence that repeats every 9 days from January 1st 2022
var every9days = new Recurrence(
    startDate: new DateTime(2022, 1, 1),
    endDate: DateTime.MaxValue,
    occurrences: null,
    patterns: new[] { new DailyRecurrencePattern(interval: 9, referenceDate: new DateTime(2022, 1, 1), weekendHandling: null) },
    filters: Enumerable.Empty<IFilter>(),
    cacheDates: false
);

// Get all valid days for February 2022: 2022-02-06, 2022-02-15 and 2022-02-24
var dates = every9days.GetDates(new DateTime(2022, 2, 1), new DateTime(2022, 2, 28));


// Create a recurrence for the year 2022 with 2 patterns:
// - Every 2 weeks on Monday and Tuesday, determining the ongoing week using Sunday first day of the week
// - Every 1st and 3rd day of the month
var doublePattern = new Recurrence(
    startDate: new DateTime(2022, 1, 1),
    endDate: new DateTime(2022, 12, 31),
    occurrences: null,
    patterns: new RecurrencePattern[] {
        new WeeklyRecurrencePattern(interval: 2, referenceDate: new DateTime(2022, 1, 1), firstDayOfWeek: DayOfWeek.Sunday, daysOfWeek: new[] { DayOfWeek.Monday, DayOfWeek.Tuesday }),
        new MonthlyRecurrencePattern(interval: 1, referenceDate: new DateTime(2022, 1, 1), daysOfMonth: new[] { 1, 3 })
    },
    filters: Enumerable.Empty<IFilter>(),
    cacheDates: false
);

// Get all valid days for February 2022: 2022-02-01, 2022-02-03, 2022-02-07, 2022-02-08, 2022-02-22 and 2022-02-23
var dates = doublePattern.GetDates(new DateTime(2022, 2, 1), new DateTime(2022, 2, 28));


// Create a recurrence that repeats every last Friday of the month for 3 occurrences, starting in April 2022
var fridays = new Recurrence(
    startDate: new DateTime(2022, 4, 1),
    endDate: DateTime.MaxValue,
    occurrences: 3,
    patterns: new[] { new MonthlyRecurrencePattern(interval: 1, referenceDate: new DateTime(2022, 4, 1), daysOfWeek: new[] { (DayOfWeekInMonth.Last, DayOfWeek.Friday) }) },
    filters: Enumerable.Empty<IFilter>(),
    cacheDates: false
);

// Get all valid dates: 2022-04-29, 2022-05-27 and 2022-06-24
var dates = fridays.GetDates();


// Create a recurrence that repeats every weekday in December 2022, except for Christmas
var workingDays = new Recurrence(
    startDate: new DateTime(2022, 12, 1),
    endDate: new DateTime(2022, 12, 31),
    occurrences: null,
    patterns: new[] { new DailyRecurrencePattern(interval: 1, weekendHandling: RecurrencePatternWeekendHandling.Skip) },
    filters: new[] { new DateFilter(new DateTime(2022, 12, 25), new DateTime(2022, 12, 26)) },
    cacheDates: false
);

// Get all valid dates: all week days in December except Christmas
var dates = workingDays.GetDates();

Custom patterns

If you have a need for adding custom patterns, you can create your own by inheriting from the base RecurrencePattern class. In addition, you can create your own builder by inheriting from the base RecurrencePatternBuilder<TBuilder> and adding extension methods to the RecurrenceBuilder interface and RecurrencePatternBuilderStart class.

Unfortunately compiler restrictions do not allow you to extend the static Recurs class, so you'll need to either manually instantiate the RecurrenceBuilder class, start with one of the existing starting methods on the Recurs class, or create your own starting point.

Example

public class AnnualRecurrencePattern : RecurrencePattern {
    public ImmutableHashSet<int> DaysOfYear { get; }

    public AnnualRecurrencePattern(int interval, DateTime? referenceDate, IEnumerable<int> daysOfYear) : base(interval, referenceDate) {
        DaysOfYear = ImmutableHashSet.CreateRange(daysOfYear);
    }

    public override bool IsValid(DateTime date)
        => DaysOfYear.Contains(date.DayOfYear) && (Interval == 1 || (date.Year - ReferenceDate.Year) % Interval == 0);
}

public class AnnualRecurrencePatternBuilder : RecurrencePatternBuilder<AnnualRecurrencePatternBuilder> {
    public List<int> DaysOfYear { get; }

    public AnnualRecurrencePatternBuilder(RecurrenceBuilder recurrenceBuilder, int interval) : base(recurrenceBuilder, interval) { }

    public AnnualRecurrencePatternBuilder On(params int[] days)
        => On(days.AsEnumerable());

    public AnnualRecurrencePatternBuilder On(IEnumerable<int> days) {
        DaysOfYear.AddRange(days);
        return this;
    }

    public override RecurrencePattern BuildPattern()
        => new AnnualRecurrencePattern(Interval, ReferenceDate ?? RecurrenceBuilder.StartDate, DaysOfYear);
}

public static class RecurrenceBuilderExtensions {
    public static AnnualRecurrencePatternBuilder Anually(this IRecurrenceBuilder recurrenceBuilder) {
        var builder = new AnnualRecurrencePatternBuilder(recurrenceBuilder.GetRecurrenceBuilder(), 1);
        recurrenceBuilder.GetRecurrenceBuilder().PatternBuilders.Add(builder);
        return builder;
    }
}

public static class RecurrencePatternBuilderStartExtensions {
    public static AnnualRecurrencePatternBuilder Years(this RecurrencePatternBuilderStart start) {
        var builder = new AnnualRecurrencePatternBuilder(start.RecurrenceBuilder, start.Interval);
        start.RecurrenceBuilder.PatternBuilders.Add(builder);
        return builder;
    }
}

// Create a recurrence that repeats every 2 years on day 100 and every year on day 300, starting in 2010
var recurrence = Recurs
    .Every(2).Years().On(100)
    .Anually().On(300)
    .From(new DateTime(2010, 1, 1))
    .Build();

// Get all valid days for the years 2010 to 2012; 2010-04-10, 2010-10-27, 2011-10-27, 2012-04-09 and 2012-10-26
var dates = recurrence.GetDates(new DateTime(2010, 1, 1), new DateTime(2012, 12, 31)).ToList();

Custom filters

If you need custom filtering functionality this is easily achieved by implementing the <code>IFilter</code> interface. To create your own filter builder you can inherit from the base <code>FilterBuilder</code> class and add extension methods to the <code>IRecurrenceBuilder</code> interface.

Unfortunately compiler restrictions do not allow you to extend the static Recurs class, so you'll need to either manually instantiate the RecurrenceBuilder class, start with one of the existing starting methods on the Recurs class, or create your own starting point.

Example

public class WeekdayFilter : IFilter {
    public ImmutableHashSet<DayOfWeek> DaysOfWeek { get; }

    public WeekdayFilter(IEnumerable<DayOfWeek> daysOfWeek) {
        DaysOfWeek = ImmutableHashSet.CreateRange(daysOfWeek);
    }

    public bool IsFiltered(DateTime date) => DaysOfWeek.Contains(date.DayOfWeek);
}

public class WeekdayFilterBuilder : FilterBuilder {
    public List<DayOfWeek> DaysOfWeek { get; set; } = new List<DayOfWeek>();

    public WeekdayFilterBuilder(RecurrenceBuilder recurrenceBuilder) : base(recurrenceBuilder) { }

    public WeekdayFilterBuilder On(params DayOfWeek[] days)
        => On(days.AsEnumerable());

    public WeekdayFilterBuilder On(IEnumerable<DayOfWeek> days) {
        DaysOfWeek.AddRange(days);
        return this;
    }

    public override IFilter BuildFilter() => new WeekdayFilter(DaysOfWeek);
}

public static class RecurrenceBuilderExtensions {
    public static WeekdayFilterBuilder ExceptOn(this IRecurrenceBuilder recurrenceBuilder, params DayOfWeek[] days)
        => recurrenceBuilder.ExceptOn(days.AsEnumerable());

    public static WeekdayFilterBuilder ExceptOn(this IRecurrenceBuilder recurrenceBuilder, IEnumerable<DayOfWeek> days) {
        var builder = new WeekdayFilterBuilder(recurrenceBuilder.GetRecurrenceBuilder()).On(days);
        recurrenceBuilder.GetRecurrenceBuilder().FilterBuilders.Add(builder);
        return builder;
    }
}

// Create a recurrence that repeats every day except on Wednesdays
var recurrence = Recurs
    .Daily()
    .ExceptOn(DayOfWeek.Wednesday)
    .Build();

// Get all valid days for the first week of 2023; 2022-01-01, 2022-01-02, 2022-01-03, 2022-01-05, 2022-01-06 and 2022-01-07
var dates = recurrence.GetDates(new DateTime(2010, 1, 1), new DateTime(2012, 12, 31)).ToList();