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copied!<p>Here's the function that yields the intersection of the Recurrence points and a specified subinterval:</p> <pre><code>public class Recurrence { public int Id { get; protected set; } public DateTime StartDate { get; protected set; } public DateTime? EndDate { get; protected set; } public long? RecurrenceInterval { get; protected set; } // returns the set of DateTimes within [subStart, subEnd] that are // of the form StartDate + k*RecurrenceInterval, where k is an Integer public IEnumerable<DateTime> GetBetween(DateTime subStart, DateTime subEnd) { long stride = RecurrenceInterval ?? 1; if (stride < 1) throw new ArgumentException("Need a positive recurrence stride"); long realStart, realEnd; // figure out where we really need to start if (StartDate >= subStart) realStart = StartDate.Ticks; else { long rem = subStart.Ticks % stride; if (rem == 0) realStart = subStart.Ticks; else // break off the incomplete stride and add a full one realStart = subStart.Ticks - rem + stride; } // figure out where we really need to stop if (EndDate <= subEnd) // we know EndDate has a value. Null can't be "less than" something realEnd = EndDate.Value.Ticks; else { long rem = subEnd.Ticks % stride; // break off any incomplete stride realEnd = subEnd.Ticks - rem; } if (realEnd < realStart) yield break; // the intersection is empty // now yield all the results in the intersection of the sets for (long t = realStart; t <= realEnd; t += stride) yield return new DateTime(t); } } </code></pre>

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