Calendar feeds

Coworkee.Application.Calendar writes iCalendar (RFC 5545) files: absences, appointments, deadlines, anything a calendar app should import or subscribe to. Results.Ics(...) in Coworkee.AspNetCore.Http serves one.

using Coworkee.Application.Calendar;
using Coworkee.AspNetCore.Http;

var calendar = new IcsCalendar { Name = "Team absences", RefreshInterval = TimeSpan.FromHours(1) }.Add(
    IcsEvent.AllDay($"{absence.Id}@my-app", absence.First, absence.Last, $"{name}: vacation") with { Transparent = true },
    new IcsEvent($"{talk.Id}@my-app", IcsTime.InZone(talk.StartsAt, "Europe/Berlin"), "Annual review")
    {
        End = IcsTime.InZone(talk.StartsAt.AddHours(1), "Europe/Berlin"),
        Location = "Room 2",
        Url = new Uri($"{appUrl}/talks/{talk.Id}"),
        Status = IcsStatus.Confirmed,
        Organizer = new IcsPerson(lead.Email, lead.Name),
        Attendees = [new IcsPerson(employee.Email, employee.Name)],
    });

return Results.Ics(calendar);              // text/calendar; charset=utf-8
return Results.Ics(calendar, "talks.ics"); // the same as a download

Part	What it writes
IcsTime	DateOnly → whole day (VALUE=DATE), DateTimeOffset → UTC (…Z), IcsTime.InZone(wallClock, "Europe/Berlin") → TZID= with the local time. Both convert implicitly.
IcsEvent	Uid (stable across feeds, so updates replace the event), Start, Summary, optional End (exclusive), Description, Location, Url, Status, Categories, Organizer, Attendees, Transparent (does not block the time), Stamp.
IcsEvent.AllDay(uid, first, last, summary)	Days from first to last, both included (DTEND is the day after last).
IcsCalendar	Name (X-WR-CALNAME), RefreshInterval (REFRESH-INTERVAL, X-PUBLISHED-TTL), ProductId, Stamp (default now, used as DTSTAMP), ToString().

Text is escaped (\, ;, , and line breaks), parameters with :;, are quoted, and lines are folded after 75 octets without splitting a UTF-8 character, so long German or emoji text stays valid.

Time zones

TZID is written without a VTIMEZONE block. Google Calendar, Outlook and Apple Calendar know IANA ids; for strict parsers use UTC (DateTimeOffset) instead.

Subscription feeds

Calendar apps subscribe to an address and fetch it every few hours. They send no cookie and no token header, so the address itself is the credential:

app.MapGet("/api/v1/absences/ics/{token}", async (string token, AbsenceFeed feed, HttpContext http, CancellationToken ct) =>
    {
        if (await feed.RenderAsync(token, ct) is not { } calendar)
        {
            return Results.NotFound();
        }

        http.Response.Headers.CacheControl = "private, max-age=900";
        http.Response.Headers["X-Robots-Tag"] = "noindex";
        return Results.Ics(calendar);
    })
    .AllowAnonymous();

• Token: 32 random bytes (RandomNumberGenerator.GetHexString(64)) per user and feed, stored with the user id. Renewing it revokes the old address. Look it up with IgnoreQueryFilters() and render as that user (CurrentUserScope.Begin(new ImpersonatedUser(userId, tenantId))), so the feed shows exactly what the user may see.
• Unknown token: 404, never 401 (calendar apps would ask for a password).
• Caching: private keeps shared caches and proxies from storing a URL that is a secret; max-age of 5 to 60 minutes is enough, the apps refresh on their own schedule (Google every few hours, whatever RefreshInterval says). Do not add no-store: Outlook then refetches on every open.
• Links: offer webcal://host/api/v1/absences/ics/{token} next to the https:// address; most apps open the subscribe dialog for webcal.
• Size: limit the range (for example two months back, a year ahead) instead of the whole history.