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Recurring Appointments and Exceptions

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This document introduces the main concepts behind the creation and use of recurring appointments in your scheduling application. The XtraScheduler can process end-user appointments based on the daily, weekly, monthly, or yearly recurrence pattern, either with an end condition or without any end date. You're also able to create exceptions to these recurring series, and this makes using recurring appointments more useful in real scheduling applications.

Non-Recurring and Recurring Appointments

All appointments maintained by the XtraScheduler can be either simple (non-recurring) which occur only once on the specified time interval (represented via Appointment.Start and Appointment.End properties), or recurring which occur many times at the same time interval. Whether an appointment is recurring or not is identified by the Appointment.IsRecurring value.

Recurring appointments can be of different types. Note that the type of any particular appointment is accessed via its Appointment.Type property. The following image demonstrates the possible types of recurring appointments.


So, a recurring appointment can either be a base (a Pattern), or an occurrence. Occurring appointments can either be a simple Occurrence, or an exception (a ChangedOccurrence or a DeletedOccurrence). The following sections describe the differences between these recurring appointments.

Base Recurring Appointment and Recurrence Pattern

Every series of recurring appointment is specified by its base appointment (Pattern) which is returned by the Appointment.RecurrencePattern property of all other appointments in the series. The Pattern appointment contains information about a recurrence. Actually, it sets the start date and time of the recurrence series and specifies a rule according to which a series is repeated. This recurrence rule is accessed via the pattern's Appointment.RecurrenceInfo property.

The recurrence rule as a string defined in RFC 2445 (RRULE in iCalendar format) can be obtained using the static iCalendarHelper.ExtractRecurrenceRule method.

The Pattern appointment is stored in the SchedulerDataStorage.Appointments collection of the AppointmentStorage. All other Occurrence appointments (except for ChangedOccurrence and DeletedOccurrence) are not stored in any collection. They are automatically generated at runtime according to the RecurrenceInfo of the pattern appointment.


A series of recurring appointments can be either definitive or infinitive. This is the main reason why the Occurrence appointments are not stored in any collection, to avoid stack overflow for infinitive recurring series.

End-users can edit the recurrence pattern of any appointment at runtime via the AppointmentRecurrenceForm dialog. The image below demonstrates the common appearance of this dialog.


There are different types of recurrence patterns which can be used to specify the rule for recurring series. The following table describes what properties of the RecurrenceInfo is used for different recurrence patterns.

The RecurrenceInfo.Type property value Used properties
RecurrenceType.Daily RecurrenceInfo.Periodicity, RecurrenceInfo.WeekDays
RecurrenceType.Weekly RecurrenceInfo.Periodicity, RecurrenceInfo.WeekDays
RecurrenceType.Monthly RecurrenceInfo.DayNumber, RecurrenceInfo.Periodicity, RecurrenceInfo.WeekDays, RecurrenceInfo.WeekOfMonth
RecurrenceType.Yearly RecurrenceInfo.DayNumber, RecurrenceInfo.Month, RecurrenceInfo.WeekDays, RecurrenceInfo.WeekOfMonth

You can use the OccurrenceCalculator instance to calculate occurrences. The OccurrenceCalculator.CalcOccurrences method creates a sequence of appointments for the specified pattern within the specified time interval.

Recurrence Pattern Exceptions

As stated above, from all the recurring series only the Pattern appointment is stored in the SchedulerDataStorage.Appointments collection. Others (Occurrence) are usually generated at runtime, to avoid excessive use of system memory.

However, every Pattern appointment contains a collection of exceptions. Exceptions are the Occurrence appointments which have either been changed or deleted, and so no longer meet the common recurrence rule specified by the pattern appointment.



All occurrences within a recurring series are associated with the same resource. A changed occurrence can differ from its pattern by a subject, description, label, status, start time, end time, but not by a resource.

A collection of exceptions for a particular recurrence pattern is returned via the pattern's Appointment.GetExceptions method. This collection contains both appointments of the AppointmentType.ChangedOccurrence and AppointmentType.DeletedOccurrence types. Note that if an appointment is contained in this collection, then the corresponding Occurrence appointment will not be generated in this case.


To stop an appointment from being an exception, you should call the Appointment.RestoreOccurrence method for it.

See Also