Turns each SharePoint calendar recurrence event into a series of individual events, taking into account recurrence exceptions
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SPCalendarRecurrenceExpander turns each SharePoint calendar recurrence event into a series of individual events, taking into account recurrence exceptions.

When to use it

SharePoint 2007, 2010, 2013, and Online comes with a calendar list type and web forms for creating either single events or recurrence events following a large number of patterns.

On-prem SharePoint comes with CAML query support for programmatic expansion of recurrence events (though the feature is known to be buggy). With SharePoint Online, however, the expansion feature has been disabled. Instead, the only out-of-the-box expansion option is to reverse engineer the internal and undocumented CalendarService.ashx web service used by the calendar web part.

SPCalendarRecurrenceExpander implements event recurrence expansion by working directly with the underlying calendar list items, i.e., it only requires access to the calendar app entries.

Use cases for SPCalendarRecurrenceExpander involve creating custom views on top of calendars, either presenting events from a single calendar or aggregating events across any number of calendars (the built-in SharePoint calendar supports aggregating up to four calendars). Another use case would be exposing the expanded recurrence events via a web service for JavaScript consumption.

How to get it

Download the package from NuGet:

Install-Package SPCalendarRecurrenceExpander

The NuGet package contains a .NET 4.5 assembly for use with SharePoint Online. For other .NET runtime versions (supporting older versions of SharePoint), currently you'd have to build the library yourself.

SPCalendarRecurrenceExpander is written in F# which means your project will have to reference fsharp.core.dll to consume the library. If you included installed Visual Studio with F# support, fsharp.core.dll is already installed. Otherwise, you can get the assembly by installing this NuGet package.

How to use it

The Examples folder contains complete C# and F# examples.

Here's an abbreviated example that makes use of the SharePoint CSOM API to read all calendar list items. These are then fed into the expander which returns a list of recurrence instances:

class Appointment {
    public int Id { get; set; }
    public string Title { get; set; }
    public DateTime Start { get; set; }
    public DateTime End { get; set; }
    // add any custom columns here

class Program {
    static void Main(string[] args) {
        var ctx = new ClientContext(web);
        var securePassword = new SecureString();
        ctx.Credentials = new SharePointOnlineCredentials(username, securePassword);
        var calendar = ctx.Web.Lists.GetByTitle(calendarTitle);
        var tz = ctx.Web.RegionalSettings.TimeZone;

        var query = new CamlQuery();
        var items = calendar.GetItems(query);

        var collapsedAppointments = items.ToList().Select(i => i.FieldValues).ToList();
        var expander = new CalendarRecurrenceExpander(
        var recurrenceInstances = expander.Expand(collapsedAppointments);

        Func<RecurrenceInstance, Appointment> toDomainObject = (ri => {
            var a = collapsedAppointments.First(i => int.Parse(i["ID"].ToString()) == ri.Id);
            return new Appointment {
                Id = ri.Id,
                Title = (string) a["Title"],
                Start = ri.Start,
                End = ri.End

        var expandedAppointments = recurrenceInstances.Select(toDomainObject).ToList();

Supported platforms

SPCalendarRecurrenceExpander doesn't depend on any SharePoint assembly and thus no specific SharePoint version. Provided you can access the raw calendar list items, the library will work. The library doesn't work with SharePoint's OData web service because it doesn't expose each item's FieldValues collection wherein the calendar metadata is stored.

How it works

When a user creates a recurrence event through the user interface, SharePoint transforms the event into a set of key/value properties and uses an XML-based domain specific language to describe recurrences.

SPCalendarRecurrenceExpander consists of a parser for these key/value properties and the recurrence description language. The output of the parser is a syntax tree representing the recurrence.

For instance, here's the output for a weekly recurrence event, repeating every week on Sundays and Thursdays for ten instances:

Weekly (EveryNthWeekOnDays (1, set [DayOfWeek.Sunday; DayOfWeek.Thursday]), RepeatInstances 10)

Another example is monthly recurrendes every third weekend day of the month, every second month for 999 instances (SharePoint's default number of instances when a user doesn't explicitly specify an end time):

Monthly (EveryQualifierOfKindOfDayEveryMthMonth (Third, WeekendDay, 2), NoExplicitEndRange)

These syntax trees show two of about 50 recurrence patterns supported by SharePoint. Each of these patterns is fed to a recurrence compiler which "executes" the recurrence program, effectively returning recurrence instances. Recurrence exceptions, such as deleted or modified instances, are special types of events which replace regular recurrence instances.

Please let me know if you find this package helpful.

See also

Internet Calendaring and Scheduling Core Object Specification Data elements and interchange formats – Information interchange – Representation of dates and times