Add abstraction layer to create EXDATE and RDATE

- Introduced `ExceptionDateCollection`, `RecurrencePeriodCollection` and PeriodCollectionBase` classes.
- Updated serializers to handle the modified `PeriodList` implementation.

These classes are an abstraction layer for creating `List<PeriodList>` objects.
`Period`s and `CalDateTime`s are accepted without having to care for RFC 5545 rules regarding serialization:

`ToRecurrenceDates` and `ToRecurrenceDates` methods aggregate and convert the exception or recurrence `CalDateTime` and `Period` objects into a list of `PeriodList` objects. Periods are grouped by their timezone IDs and period kinds in a way, that each `PeriodList` contains only distinct periods ready to serialize.

Ical.Net.Tests/PeriodCollectionTests.cs

@@ -0,0 +1,153 @@
+//
+// Copyright ical.net project maintainers and contributors.
+// Licensed under the MIT license.
+//
+
+#nullable enable
+using Ical.Net.Collections;
+using Ical.Net.DataTypes;
+using NUnit.Framework;
+
+namespace Ical.Net.Tests;
+
+[TestFixture]
+public class PeriodCollectionTests
+{
+    [Test]
+    public void RemovePeriod_ShouldDecreaseCount()
+    {
+        // Arrange
+        var recCollection = new RecurrencePeriodCollection();
+        var period = new Period(new CalDateTime(2023, 1, 1, 0, 0, 0), Duration.FromHours(1));
+        recCollection.Add(period);
+
+        // Act
+        recCollection.Remove(period);
+
+        // Assert
+        Assert.That(recCollection, Has.Count.EqualTo(0));
+    }
+
+    [Test]
+    public void GetSet_Period_ShouldReturnCorrectPeriod()
+    {
+        // Arrange
+        var recCollection = new RecurrencePeriodCollection();
+        var period1 = new Period(new CalDateTime(2025, 1, 1, 0, 0, 0), Duration.FromHours(1));
+        var period2 = new Period(new CalDateTime(2025, 2, 1, 0, 0, 0), Duration.FromHours(1));
+
+        recCollection.Add(period1);
+        recCollection.Add(period1);
+
+        // Act
+        var retrievedPeriod = recCollection[0];
+        recCollection[1] = period2;
+
+        // Assert
+        Assert.Multiple(() =>
+        {
+            Assert.That(period1, Is.EqualTo(retrievedPeriod));
+            Assert.That(recCollection.Contains(period1), Is.True);
+        });
+    }
+
+    [Test]
+    public void Clear_ShouldRemoveAll_PeriodsAdded()
+    {
+        // Arrange
+        var recCollection = new RecurrencePeriodCollection();
+        var pl = new PeriodList
+        {
+            new CalDateTime(2025, 1, 2),
+            new CalDateTime(2025, 1, 3)
+        };
+
+        recCollection.AddRange( [pl] );
+        recCollection.AddRange(
+        [
+            new CalDateTime(2025, 10, 1),
+            new CalDateTime(2025, 10, 2)
+        ]);
+
+        var count = recCollection.Count;
+
+        // Act
+        recCollection.Clear();
+
+        // Assert
+        Assert.Multiple(() =>
+        {
+            Assert.That(count, Is.EqualTo(4));
+            Assert.That(recCollection, Has.Count.EqualTo(0));
+            Assert.That(recCollection.IsReadOnly, Is.False);
+        });
+    }
+
+    [Test]
+    public void CopyToPeriod_ShouldCopyPeriodsCorrectly()
+    {
+        // Arrange
+        var recCollection = new RecurrencePeriodCollection([new CalDateTime(2025, 1, 2),
+            new CalDateTime(2025, 1, 3)]);
+
+        var array = new Period[2];
+
+        // Act
+        recCollection.CopyTo(array, 0);
+
+        // Assert
+        Assert.Multiple(() =>
+        {
+            Assert.That(array[0], Is.EqualTo(recCollection[0]));
+            Assert.That(array[1], Is.EqualTo(recCollection[1]));
+        });
+    }
+
+    [Test]
+    public void Create_RecurrencePeriodCollection_With_DateTime()
+    {
+        var recCollection = new RecurrencePeriodCollection(new CalDateTime(2025, 1, 2));
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(recCollection, Has.Count.EqualTo(1));
+        });
+    }
+
+    [Test]
+    public void Create_RecurrencePeriodCollection_With_PeriodEnumerable()
+    {
+        var recCollection = new RecurrencePeriodCollection([new Period(new CalDateTime(2025, 1, 2), Duration.FromDays(1))]);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(recCollection, Has.Count.EqualTo(1));
+            Assert.That(recCollection[0].Duration, Is.EqualTo(Duration.FromDays(1)));
+        });
+    }
+
+    [Test]
+    public void Create_ExceptionDateCollection_With_DateTimeEnumerable()
+    {
+        var exDateCollection = new ExceptionDateCollection([new CalDateTime(2025, 1, 2),
+            new CalDateTime(2025, 1, 3)]);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(exDateCollection, Has.Count.EqualTo(2));
+        });
+    }
+
+    [Test]
+    public void Create_ExceptionDateCollection_With_Period()
+    {
+        var exDateCollection = new ExceptionDateCollection();
+        exDateCollection.Add(new Period(new CalDateTime(2025, 1, 2), Duration.FromHours(1)));
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(exDateCollection, Has.Count.EqualTo(1));
+            Assert.That(exDateCollection[0].EffectiveDuration, Is.Null);
+        });
+    }
+}

Ical.Net.Tests/PeriodListTest.cs

@@ -7,6 +7,7 @@
 using System;
 using System.IO;
 using Ical.Net.DataTypes;
+using Ical.Net.Utility;
 using NUnit.Framework;
 
 namespace Ical.Net.Tests;
@@ -37,7 +38,8 @@ public void GetSet_Period_ShouldReturnCorrectPeriod()
         var period1 = new Period(new CalDateTime(2025, 1, 1, 0, 0, 0), Duration.FromHours(1));
         var period2 = new Period(new CalDateTime(2025, 2, 1, 0, 0, 0), Duration.FromHours(1));
 
-        periodList.AddPeriod(period1).AddPeriod(period1);
+        periodList.Add(period1);
+        periodList.Add(period1);
 
         // Act
         var retrievedPeriod = periodList[0];
@@ -57,11 +59,14 @@ public void Clear_ShouldRemoveAllPeriods()
     {
         // Arrange
         var periodList = new PeriodList();
-        var pl = PeriodList
-            .FromDateTime(new CalDateTime(2025, 1, 2))
-            .Add(new CalDateTime(2025, 1, 3));
+        var pl = new PeriodList
+        {
+            new CalDateTime(2025, 1, 2),
+            new CalDateTime(2025, 1, 3)
+        };
 
-        var count = pl.Count;
+        periodList.AddRange(pl);
+        var count = periodList.Count;
 
         // Act
         periodList.Clear();
@@ -104,7 +109,8 @@ public void InsertAt_ShouldInsertPeriodAtCorrectPosition()
         var period1 = new Period(new CalDateTime(2025, 1, 1, 0, 0, 0), Duration.FromHours(1));
         var period2 = new Period(new CalDateTime(2025, 1, 2, 0, 0, 0), Duration.FromHours(1));
         var period3 = new Period(new CalDateTime(2025, 1, 3, 0, 0, 0), Duration.FromHours(1));
-        periodList.AddPeriod(period1).AddPeriod(period3);
+        periodList.Add(period1);
+        periodList.Add(period3);
 
         // Act
         periodList.Insert(1, period2);
@@ -125,7 +131,9 @@ public void RemoveAt_ShouldRemovePeriodAtCorrectPosition()
         var period1 = new Period(new CalDateTime(2025, 1, 1, 0, 0, 0), Duration.FromHours(1));
         var period2 = new Period(new CalDateTime(2025, 1, 2, 0, 0, 0), Duration.FromHours(1));
         var period3 = new Period(new CalDateTime(2025, 1, 3, 0, 0, 0), Duration.FromHours(1));
-        periodList.AddPeriod(period1).AddPeriod(period2).AddPeriod(period3);
+        periodList.Add(period1);
+        periodList.Add(period2);
+        periodList.Add(period3);
 
         // Act
         periodList.RemoveAt(1);

Ical.Net.Tests/RecurrenceTests.cs

@@ -3201,10 +3201,10 @@ public void OccurrenceMustBeCompletelyContainedWithinSearchRange()
     /// Evaluate relevancy and validity of the request.
     /// Find a solution for issue #120 or close forever
     /// </summary>
-    [Test, Ignore("No solution for issue #120 yet", Until = "2024-12-31")]
+    [Test, Ignore("No solution for issue #120 yet", Until = "2025-02-28")]
     public void EventsWithShareUidsShouldGenerateASingleRecurrenceSet()
     {
-        //https://github.com/rianjs/ical.net/issues/120 dated Sep 5, 2016
+        //https://github.com/ical-org/ical.net/issues/120 dated Sep 5, 2016
         const string ical =
 @"BEGIN:VCALENDAR
 PRODID:-//Google Inc//Google Calendar 70.9054//EN

Ical.Net.Tests/RecurrenceWithExDateTests.cs

@@ -7,6 +7,7 @@
 using System;
 using System.Linq;
 using Ical.Net.CalendarComponents;
+using Ical.Net.Collections;
 using Ical.Net.DataTypes;
 using Ical.Net.Serialization;
 using NUnit.Framework;
@@ -29,10 +30,13 @@ public void ShouldNotOccurOnLocalExceptionDate(bool useExDateWithTime)
         const string timeZoneId = "Europe/London"; // IANA Time Zone ID
         var start = new CalDateTime(2024, 10, 19, 18, 0, 0, timeZoneId);
         var end = new CalDateTime(2024, 10, 19, 19, 0, 0, timeZoneId);
+
         var exceptionDate = useExDateWithTime
             ? new CalDateTime(2024, 10, 19, 21, 0, 0, timeZoneId)
             : new CalDateTime(2024, 10, 19);
 
+        var exDateCollection = new ExceptionDateCollection(exceptionDate);
+
         var recurrencePattern = new RecurrencePattern(FrequencyType.Hourly)
         {
             Count = 2,
@@ -47,7 +51,7 @@ public void ShouldNotOccurOnLocalExceptionDate(bool useExDateWithTime)
             End = end
         };
         recurringEvent.RecurrenceRules.Add(recurrencePattern);
-        recurringEvent.ExceptionDates.Add(PeriodList.FromDateTime(exceptionDate));
+        recurringEvent.ExceptionDates = exDateCollection.ToExceptionDates();
 
         var calendar = new Calendar();
         calendar.Events.Add(recurringEvent);