diff --git a/Ical.Net.Tests/CalendarEventTest.cs b/Ical.Net.Tests/CalendarEventTest.cs
index 01157066..61f5ade7 100644
--- a/Ical.Net.Tests/CalendarEventTest.cs
+++ b/Ical.Net.Tests/CalendarEventTest.cs
@@ -417,8 +417,8 @@ public void EventResourcesCanBeZeroedOut()
             Assert.That(e.Resources.Any(r => resources.Contains(r)), Is.False);
         });
 
-        e.Resources = null;
-        //See https://github.com/rianjs/ical.net/issues/208 -- this should be changed later so the collection is really null
+        //See https://github.com/rianjs/ical.net/issues/208
+        e.Resources = Array.Empty<string>();
         Assert.That(e.Resources?.Count, Is.EqualTo(0));
     }
 
diff --git a/Ical.Net.Tests/ComponentTest.cs b/Ical.Net.Tests/ComponentTest.cs
index 8dcf0b90..33090c7d 100644
--- a/Ical.Net.Tests/ComponentTest.cs
+++ b/Ical.Net.Tests/ComponentTest.cs
@@ -3,9 +3,8 @@
 // Licensed under the MIT license.
 //
 
-using System;
+#nullable enable
 using Ical.Net.CalendarComponents;
-using Ical.Net.DataTypes;
 using NUnit.Framework;
 
 namespace Ical.Net.Tests;
@@ -18,8 +17,11 @@ public void UniqueComponent1()
         var iCal = new Calendar();
         var evt = iCal.Create<CalendarEvent>();
 
-        Assert.That(evt.Uid, Is.Not.Null);
-        Assert.That(evt.Created, Is.Null); // We don't want this to be set automatically
-        Assert.That(evt.DtStamp, Is.Not.Null);
+        Assert.Multiple(() =>
+        {
+            Assert.That(evt.Uid, Is.Not.Null);
+            Assert.That(evt.Created, Is.Null); // We don't want this to be set automatically
+            Assert.That(evt.DtStamp, Is.Not.Null);
+        });
     }
 }
diff --git a/Ical.Net.Tests/CopyComponentTests.cs b/Ical.Net.Tests/CopyComponentTests.cs
index c4df1d63..1be5799f 100644
--- a/Ical.Net.Tests/CopyComponentTests.cs
+++ b/Ical.Net.Tests/CopyComponentTests.cs
@@ -23,7 +23,7 @@ public class CopyComponentTests
     [Test, TestCaseSource(nameof(CopyCalendarTest_TestCases)), Category("Copy tests")]
     public void CopyCalendarTest(string calendarString)
     {
-        var iCal1 = Calendar.Load(calendarString);
+        var iCal1 = Calendar.Load(calendarString)!;
         var iCal2 = iCal1.Copy<Calendar>();
         SerializationTests.CompareCalendars(iCal1, iCal2);
     }
diff --git a/Ical.Net.Tests/FreeBusyTest.cs b/Ical.Net.Tests/FreeBusyTest.cs
index 2371c8ab..261a7ee5 100644
--- a/Ical.Net.Tests/FreeBusyTest.cs
+++ b/Ical.Net.Tests/FreeBusyTest.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using Ical.Net.CalendarComponents;
 using Ical.Net.DataTypes;
 using NUnit.Framework;
@@ -18,14 +19,14 @@ public class FreeBusyTest
     [Test, Category("FreeBusy")]
     public void GetFreeBusyStatus1()
     {
-        Calendar cal = new Calendar();
+        var cal = new Calendar();
 
-        CalendarEvent evt = cal.Create<CalendarEvent>();
+        var evt = cal.Create<CalendarEvent>();
         evt.Summary = "Test event";
         evt.Start = new CalDateTime(2010, 10, 1, 8, 0, 0);
         evt.End = new CalDateTime(2010, 10, 1, 9, 0, 0);
 
-        var freeBusy = cal.GetFreeBusy(new CalDateTime(2010, 10, 1, 0, 0, 0), new CalDateTime(2010, 10, 7, 11, 59, 59));
+        var freeBusy = cal.GetFreeBusy(new CalDateTime(2010, 10, 1, 0, 0, 0), new CalDateTime(2010, 10, 7, 11, 59, 59))!;
         Assert.Multiple(() =>
         {
             Assert.That(freeBusy.GetFreeBusyStatus(new CalDateTime(2010, 10, 1, 7, 59, 59)), Is.EqualTo(FreeBusyStatus.Free));
@@ -34,4 +35,4 @@ public void GetFreeBusyStatus1()
             Assert.That(freeBusy.GetFreeBusyStatus(new CalDateTime(2010, 10, 1, 9, 0, 0)), Is.EqualTo(FreeBusyStatus.Free));
         });
     }
-}
\ No newline at end of file
+}
diff --git a/Ical.Net.Tests/PeriodListWrapperTests.cs b/Ical.Net.Tests/PeriodListWrapperTests.cs
index 4d506235..d6f5c8a0 100644
--- a/Ical.Net.Tests/PeriodListWrapperTests.cs
+++ b/Ical.Net.Tests/PeriodListWrapperTests.cs
@@ -187,14 +187,14 @@ public void AddRPeriod_ShouldCreate_DedicatePeriodList()
         var serializer = new CalendarSerializer(cal);
         var serialized = serializer.SerializeToString();
         // Assign the deserialized event
-        cal = Calendar.Load(serialized);
+        cal = Calendar.Load(serialized)!;
         evt = cal.Events[0];
 
         // Assert the serialized string and the deserialized event
         Assert.Multiple(() =>
         {
             // 2 dedicate PeriodList objects
-            Assert.That(evt.RecurrenceDatesPeriodLists, Has.Count.EqualTo(3));
+            Assert.That(evt!.RecurrenceDatesPeriodLists, Has.Count.EqualTo(3));
 
             // First PeriodList has date-only periods
             Assert.That(evt.RecurrenceDatesPeriodLists[0], Has.Count.EqualTo(3));
diff --git a/Ical.Net.Tests/ProgramTest.cs b/Ical.Net.Tests/ProgramTest.cs
index b15c6108..4cac77a9 100644
--- a/Ical.Net.Tests/ProgramTest.cs
+++ b/Ical.Net.Tests/ProgramTest.cs
@@ -40,11 +40,11 @@ public static void TestCal(Calendar cal)
     [Test]
     public void Merge1()
     {
-        var iCal1 = Calendar.Load(IcsFiles.MonthlyCountByMonthDay3);
-        var iCal2 = Calendar.Load(IcsFiles.MonthlyByDay1);
+        var iCal1 = Calendar.Load(IcsFiles.MonthlyCountByMonthDay3)!;
+        var iCal2 = Calendar.Load(IcsFiles.MonthlyByDay1)!;
 
         // Change the UID of the 2nd event to make sure it's different
-        iCal2.Events[iCal1.Events[0].Uid].Uid = "1234567890";
+        iCal2.Events[iCal1.Events[0].Uid!].Uid = "1234567890";
         iCal1.MergeWith(iCal2);
 
         var evt1 = iCal1.Events.First();
diff --git a/Ical.Net.Tests/RecurrenceTests.cs b/Ical.Net.Tests/RecurrenceTests.cs
index 0b5a4900..10c5ce25 100644
--- a/Ical.Net.Tests/RecurrenceTests.cs
+++ b/Ical.Net.Tests/RecurrenceTests.cs
@@ -107,7 +107,7 @@ public void EventOccurrenceTest(
             END:VCALENDAR
             """;
 
-        var cal = Calendar.Load(calendarIcalStr);
+        var cal = Calendar.Load(calendarIcalStr)!;
         var tzid = cal.Events.Single().Start!.TzId;
 
         var periodSerializer = new PeriodSerializer();
@@ -128,7 +128,7 @@ p.Duration is null
     [Test, Category("Recurrence")]
     public void YearlyComplex1()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyComplex1);
+        var iCal = Calendar.Load(IcsFiles.YearlyComplex1)!;
         ProgramTest.TestCal(iCal);
         var evt = iCal.Events.First();
         var occurrences = evt.GetOccurrences(
@@ -164,7 +164,7 @@ public void YearlyComplex1()
     [Test, Category("Recurrence")]
     public void DailyCount1()
     {
-        var iCal = Calendar.Load(IcsFiles.DailyCount1);
+        var iCal = Calendar.Load(IcsFiles.DailyCount1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2006, 7, 1),
@@ -191,7 +191,7 @@ public void DailyCount1()
     [Test, Category("Recurrence")]
     public void DailyUntil1()
     {
-        var iCal = Calendar.Load(IcsFiles.DailyUntil1);
+        var iCal = Calendar.Load(IcsFiles.DailyUntil1)!;
         ProgramTest.TestCal(iCal);
         var evt = iCal.Events.First();
 
@@ -228,7 +228,7 @@ public void DailyUntil1()
     [Test, Category("Recurrence")]
     public void Daily1()
     {
-        var iCal = Calendar.Load(IcsFiles.Daily1);
+        var iCal = Calendar.Load(IcsFiles.Daily1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1997, 9, 1),
@@ -341,7 +341,7 @@ public void Daily1()
     [Test, Category("Recurrence")]
     public void DailyCount2()
     {
-        var iCal = Calendar.Load(IcsFiles.DailyCount2);
+        var iCal = Calendar.Load(IcsFiles.DailyCount2)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1997, 9, 1),
@@ -363,7 +363,7 @@ public void DailyCount2()
     [Test, Category("Recurrence")]
     public void ByMonth1()
     {
-        var iCal = Calendar.Load(IcsFiles.ByMonth1);
+        var iCal = Calendar.Load(IcsFiles.ByMonth1)!;
         ProgramTest.TestCal(iCal);
         var evt = iCal.Events.First();
 
@@ -398,8 +398,8 @@ public void ByMonth1()
     [Test, Category("Recurrence")]
     public void ByMonth2()
     {
-        var iCal1 = Calendar.Load(IcsFiles.ByMonth1);
-        var iCal2 = Calendar.Load(IcsFiles.ByMonth2);
+        var iCal1 = Calendar.Load(IcsFiles.ByMonth1)!;
+        var iCal2 = Calendar.Load(IcsFiles.ByMonth2)!;
         ProgramTest.TestCal(iCal1);
         ProgramTest.TestCal(iCal2);
         var evt1 = iCal1.Events.First();
@@ -418,7 +418,7 @@ public void ByMonth2()
     [Test, Category("Recurrence")]
     public void WeeklyCount1()
     {
-        var iCal = Calendar.Load(IcsFiles.WeeklyCount1);
+        var iCal = Calendar.Load(IcsFiles.WeeklyCount1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1997, 9, 1),
@@ -457,7 +457,7 @@ public void WeeklyCount1()
     [Test, Category("Recurrence")]
     public void WeeklyUntil1()
     {
-        var iCal = Calendar.Load(IcsFiles.WeeklyUntil1);
+        var iCal = Calendar.Load(IcsFiles.WeeklyUntil1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1997, 9, 1),
@@ -510,7 +510,7 @@ public void WeeklyUntil1()
     [Test, Category("Recurrence")]
     public void WeeklyWkst1()
     {
-        var iCal = Calendar.Load(IcsFiles.WeeklyWkst1);
+        var iCal = Calendar.Load(IcsFiles.WeeklyWkst1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1997, 9, 1),
@@ -552,7 +552,7 @@ public void WeeklyWkst1()
     [Test, Category("Recurrence")]
     public void WeeklyUntilWkst1()
     {
-        var iCal = Calendar.Load(IcsFiles.WeeklyUntilWkst1);
+        var iCal = Calendar.Load(IcsFiles.WeeklyUntilWkst1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1997, 9, 1),
@@ -580,8 +580,8 @@ public void WeeklyUntilWkst1()
     [Test, Category("Recurrence")]
     public void WeeklyCountWkst1()
     {
-        var iCal1 = Calendar.Load(IcsFiles.WeeklyUntilWkst1);
-        var iCal2 = Calendar.Load(IcsFiles.WeeklyCountWkst1);
+        var iCal1 = Calendar.Load(IcsFiles.WeeklyUntilWkst1)!;
+        var iCal2 = Calendar.Load(IcsFiles.WeeklyCountWkst1)!;
         ProgramTest.TestCal(iCal1);
         ProgramTest.TestCal(iCal2);
         var evt1 = iCal1.Events.First();
@@ -602,7 +602,7 @@ public void WeeklyCountWkst1()
     [Test, Category("Recurrence")]
     public void WeeklyUntilWkst2()
     {
-        var iCal = Calendar.Load(IcsFiles.WeeklyUntilWkst2);
+        var iCal = Calendar.Load(IcsFiles.WeeklyUntilWkst2)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -671,7 +671,7 @@ public void WeeklyUntilWkst2()
     [Test, Category("Recurrence")]
     public void WeeklyUntilWkst2_1()
     {
-        var iCal = Calendar.Load(IcsFiles.WeeklyUntilWkst2);
+        var iCal = Calendar.Load(IcsFiles.WeeklyUntilWkst2)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1997, 9, 9),
@@ -735,7 +735,7 @@ public void WeeklyUntilWkst2_1()
     [Test, Category("Recurrence")]
     public void WeeklyCountWkst2()
     {
-        var iCal = Calendar.Load(IcsFiles.WeeklyCountWkst2);
+        var iCal = Calendar.Load(IcsFiles.WeeklyCountWkst2)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -761,7 +761,7 @@ public void WeeklyCountWkst2()
     [Test, Category("Recurrence")]
     public void MonthlyCountByDay1()
     {
-        var iCal = Calendar.Load(IcsFiles.MonthlyCountByDay1);
+        var iCal = Calendar.Load(IcsFiles.MonthlyCountByDay1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -801,7 +801,7 @@ public void MonthlyCountByDay1()
     [Test, Category("Recurrence")]
     public void MonthlyUntilByDay1()
     {
-        var iCal = Calendar.Load(IcsFiles.MonthlyUntilByDay1);
+        var iCal = Calendar.Load(IcsFiles.MonthlyUntilByDay1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -829,7 +829,7 @@ public void MonthlyUntilByDay1()
     [Test, Category("Recurrence")]
     public void MonthlyCountByDay2()
     {
-        var iCal = Calendar.Load(IcsFiles.MonthlyCountByDay2);
+        var iCal = Calendar.Load(IcsFiles.MonthlyCountByDay2)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -869,7 +869,7 @@ public void MonthlyCountByDay2()
     [Test, Category("Recurrence")]
     public void MonthlyCountByDay3()
     {
-        var iCal = Calendar.Load(IcsFiles.MonthlyCountByDay3);
+        var iCal = Calendar.Load(IcsFiles.MonthlyCountByDay3)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -901,7 +901,7 @@ public void MonthlyCountByDay3()
     [Test, Category("Recurrence")]
     public void ByMonthDay1()
     {
-        var iCal = Calendar.Load(IcsFiles.ByMonthDay1);
+        var iCal = Calendar.Load(IcsFiles.ByMonthDay1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -933,7 +933,7 @@ public void ByMonthDay1()
     [Test, Category("Recurrence")]
     public void MonthlyCountByMonthDay1()
     {
-        var iCal = Calendar.Load(IcsFiles.MonthlyCountByMonthDay1);
+        var iCal = Calendar.Load(IcsFiles.MonthlyCountByMonthDay1)!;
 
         EventOccurrenceTest(
             iCal,
@@ -974,7 +974,7 @@ public void MonthlyCountByMonthDay1()
     [Test, Category("Recurrence")]
     public void MonthlyCountByMonthDay2()
     {
-        var iCal = Calendar.Load(IcsFiles.MonthlyCountByMonthDay2);
+        var iCal = Calendar.Load(IcsFiles.MonthlyCountByMonthDay2)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1014,7 +1014,7 @@ public void MonthlyCountByMonthDay2()
     [Test, Category("Recurrence")]
     public void MonthlyCountByMonthDay3()
     {
-        var iCal = Calendar.Load(IcsFiles.MonthlyCountByMonthDay3);
+        var iCal = Calendar.Load(IcsFiles.MonthlyCountByMonthDay3)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1054,7 +1054,7 @@ public void MonthlyCountByMonthDay3()
     [Test, Category("Recurrence")]
     public void MonthlyByDay1()
     {
-        var iCal = Calendar.Load(IcsFiles.MonthlyByDay1);
+        var iCal = Calendar.Load(IcsFiles.MonthlyByDay1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1110,7 +1110,7 @@ public void MonthlyByDay1()
     [Test, Category("Recurrence")]
     public void YearlyByMonth1()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyByMonth1);
+        var iCal = Calendar.Load(IcsFiles.YearlyByMonth1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1138,7 +1138,7 @@ public void YearlyByMonth1()
     [Test, Category("Recurrence")]
     public void YearlyCountByMonth1()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyCountByMonth1);
+        var iCal = Calendar.Load(IcsFiles.YearlyCountByMonth1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1166,7 +1166,7 @@ public void YearlyCountByMonth1()
     [Test, Category("Recurrence")]
     public void YearlyCountByYearDay1()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyCountByYearDay1);
+        var iCal = Calendar.Load(IcsFiles.YearlyCountByYearDay1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1206,7 +1206,7 @@ public void YearlyCountByYearDay1()
     [Test, Category("Recurrence")]
     public void YearlyByDay1()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyByDay1);
+        var iCal = Calendar.Load(IcsFiles.YearlyByDay1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1244,7 +1244,7 @@ public void WeekNoOrderingShouldNotMatter()
     [Test, Category("Recurrence")]
     public void YearlyByWeekNo1()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyByWeekNo1);
+        var iCal = Calendar.Load(IcsFiles.YearlyByWeekNo1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1270,7 +1270,7 @@ public void YearlyByWeekNo1()
     [Test, Category("Recurrence")]
     public void YearlyByWeekNo2()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyByWeekNo2);
+        var iCal = Calendar.Load(IcsFiles.YearlyByWeekNo2)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1295,7 +1295,7 @@ public void YearlyByWeekNo2()
     [Test, Category("Recurrence")]
     public void YearlyByWeekNo3()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyByWeekNo3);
+        var iCal = Calendar.Load(IcsFiles.YearlyByWeekNo3)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2001, 1, 1),
@@ -1318,7 +1318,7 @@ public void YearlyByWeekNo3()
     [Test, Category("Recurrence")]
     public void YearlyByWeekNo4()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyByWeekNo4);
+        var iCal = Calendar.Load(IcsFiles.YearlyByWeekNo4)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1362,7 +1362,7 @@ public void YearlyByWeekNo4()
     [Test, Category("Recurrence")]
     public void YearlyByWeekNo5()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyByWeekNo5);
+        var iCal = Calendar.Load(IcsFiles.YearlyByWeekNo5)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2001, 1, 1),
@@ -1393,7 +1393,7 @@ public void YearlyByWeekNo5()
     [Test, Category("Recurrence")]
     public void YearlyByMonth2()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyByMonth2);
+        var iCal = Calendar.Load(IcsFiles.YearlyByMonth2)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1422,7 +1422,7 @@ public void YearlyByMonth2()
     [Test, Category("Recurrence")]
     public void YearlyByMonth3()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyByMonth3);
+        var iCal = Calendar.Load(IcsFiles.YearlyByMonth3)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1481,7 +1481,7 @@ public void YearlyByMonth3()
     [Test, Category("Recurrence")]
     public void MonthlyByMonthDay1()
     {
-        var iCal = Calendar.Load(IcsFiles.MonthlyByMonthDay1);
+        var iCal = Calendar.Load(IcsFiles.MonthlyByMonthDay1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1511,7 +1511,7 @@ public void MonthlyByMonthDay1()
     [Test, Category("Recurrence")]
     public void MonthlyByMonthDay2()
     {
-        var iCal = Calendar.Load(IcsFiles.MonthlyByMonthDay2);
+        var iCal = Calendar.Load(IcsFiles.MonthlyByMonthDay2)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1551,7 +1551,7 @@ public void MonthlyByMonthDay2()
     [Test, Category("Recurrence")]
     public void YearlyByMonthDay1()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyByMonthDay1);
+        var iCal = Calendar.Load(IcsFiles.YearlyByMonthDay1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1572,7 +1572,7 @@ public void YearlyByMonthDay1()
     [Test, Category("Recurrence")]
     public void MonthlyBySetPos1()
     {
-        var iCal = Calendar.Load(IcsFiles.MonthlyBySetPos1);
+        var iCal = Calendar.Load(IcsFiles.MonthlyBySetPos1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1598,7 +1598,7 @@ public void MonthlyBySetPos1()
     [Test, Category("Recurrence")]
     public void MonthlyBySetPos2()
     {
-        var iCal = Calendar.Load(IcsFiles.MonthlyBySetPos2);
+        var iCal = Calendar.Load(IcsFiles.MonthlyBySetPos2)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1634,7 +1634,7 @@ public void MonthlyBySetPos2()
     [Test, Category("Recurrence")]
     public void HourlyUntil1()
     {
-        var iCal = Calendar.Load(IcsFiles.HourlyUntil1);
+        var iCal = Calendar.Load(IcsFiles.HourlyUntil1)!;
         EventOccurrenceTest(
             iCal,
             fromDate: new CalDateTime(1996, 1, 1),
@@ -1656,7 +1656,7 @@ public void HourlyUntil1()
     [Test, Category("Recurrence")]
     public void MinutelyCount1()
     {
-        var iCal = Calendar.Load(IcsFiles.MinutelyCount1);
+        var iCal = Calendar.Load(IcsFiles.MinutelyCount1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1997, 9, 2),
@@ -1680,7 +1680,7 @@ public void MinutelyCount1()
     [Test, Category("Recurrence")]
     public void MinutelyCount2()
     {
-        var iCal = Calendar.Load(IcsFiles.MinutelyCount2);
+        var iCal = Calendar.Load(IcsFiles.MinutelyCount2)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1702,7 +1702,7 @@ public void MinutelyCount2()
     [Test, Category("Recurrence")]
     public void MinutelyCount3()
     {
-        var iCal = Calendar.Load(IcsFiles.MinutelyCount3);
+        var iCal = Calendar.Load(IcsFiles.MinutelyCount3)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2010, 8, 27),
@@ -1730,7 +1730,7 @@ public void MinutelyCount3()
     [Test, Category("Recurrence")]
     public void MinutelyCount4()
     {
-        var iCal = Calendar.Load(IcsFiles.MinutelyCount4);
+        var iCal = Calendar.Load(IcsFiles.MinutelyCount4)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2010, 8, 27),
@@ -1758,7 +1758,7 @@ public void MinutelyCount4()
     [Test, Category("Recurrence")]
     public void DailyByHourMinute1()
     {
-        var iCal = Calendar.Load(IcsFiles.DailyByHourMinute1);
+        var iCal = Calendar.Load(IcsFiles.DailyByHourMinute1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1997, 9, 2),
@@ -1824,8 +1824,8 @@ public void DailyByHourMinute1()
     [Test, Category("Recurrence")]
     public void MinutelyByHour1()
     {
-        var iCal1 = Calendar.Load(IcsFiles.DailyByHourMinute1);
-        var iCal2 = Calendar.Load(IcsFiles.MinutelyByHour1);
+        var iCal1 = Calendar.Load(IcsFiles.DailyByHourMinute1)!;
+        var iCal2 = Calendar.Load(IcsFiles.MinutelyByHour1)!;
         ProgramTest.TestCal(iCal1);
         ProgramTest.TestCal(iCal2);
         var evt1 = iCal1.Events.First();
@@ -1844,7 +1844,7 @@ public void MinutelyByHour1()
     [Test, Category("Recurrence")]
     public void WeeklyCountWkst3()
     {
-        var iCal = Calendar.Load(IcsFiles.WeeklyCountWkst3);
+        var iCal = Calendar.Load(IcsFiles.WeeklyCountWkst3)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1867,7 +1867,7 @@ public void WeeklyCountWkst3()
     [Test, Category("Recurrence")]
     public void WeeklyCountWkst4()
     {
-        var iCal = Calendar.Load(IcsFiles.WeeklyCountWkst4);
+        var iCal = Calendar.Load(IcsFiles.WeeklyCountWkst4)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(1996, 1, 1),
@@ -1890,7 +1890,7 @@ public void WeeklyCountWkst4()
     [Test, Category("Recurrence")]
     public void Bug1741093()
     {
-        var iCal = Calendar.Load(IcsFiles.Bug1741093);
+        var iCal = Calendar.Load(IcsFiles.Bug1741093)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2007, 7, 1),
@@ -1917,7 +1917,7 @@ public void Bug1741093()
     [Test, Category("Recurrence")]
     public void Secondly_DefinedNumberOfOccurrences_ShouldSucceed()
     {
-        var iCal = Calendar.Load(IcsFiles.Secondly1);
+        var iCal = Calendar.Load(IcsFiles.Secondly1)!;
 
         var start = new CalDateTime(2007, 6, 21, 8, 0, 0, _tzid);
         var end = new CalDateTime(2007, 6, 21, 8, 1, 0, _tzid); // End period is exclusive, not inclusive.
@@ -1934,7 +1934,7 @@ public void Secondly_DefinedNumberOfOccurrences_ShouldSucceed()
     [Test, Category("Recurrence")]
     public void Minutely_DefinedNumberOfOccurrences_ShouldSucceed()
     {
-        var iCal = Calendar.Load(IcsFiles.Minutely1);
+        var iCal = Calendar.Load(IcsFiles.Minutely1)!;
 
         var start = new CalDateTime(2007, 6, 21, 8, 0, 0, _tzid);
         var end = new CalDateTime(2007, 6, 21, 12, 0, 1, _tzid); // End period is exclusive, not inclusive.
@@ -1951,7 +1951,7 @@ public void Minutely_DefinedNumberOfOccurrences_ShouldSucceed()
     [Test, Category("Recurrence")]
     public void Hourly_DefinedNumberOfOccurrences_ShouldSucceed()
     {
-        var iCal = Calendar.Load(IcsFiles.Hourly1);
+        var iCal = Calendar.Load(IcsFiles.Hourly1)!;
 
         var start = new CalDateTime(2007, 6, 21, 8, 0, 0, _tzid);
         var end = new CalDateTime(2007, 6, 25, 8, 0, 1, _tzid); // End period is exclusive, not inclusive.
@@ -1971,7 +1971,7 @@ public void Hourly_DefinedNumberOfOccurrences_ShouldSucceed()
     [Test, Category("Recurrence")]
     public void MonthlyInterval1()
     {
-        var iCal = Calendar.Load(IcsFiles.MonthlyInterval1);
+        var iCal = Calendar.Load(IcsFiles.MonthlyInterval1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2008, 1, 1, 7, 0, 0, _tzid),
@@ -1991,7 +1991,7 @@ public void MonthlyInterval1()
     [Test, Category("Recurrence")]
     public void YearlyInterval1()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyInterval1);
+        var iCal = Calendar.Load(IcsFiles.YearlyInterval1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2006, 1, 1, 7, 0, 0, _tzid),
@@ -2011,7 +2011,7 @@ public void YearlyInterval1()
     [Test, Category("Recurrence")]
     public void DailyInterval1()
     {
-        var iCal = Calendar.Load(IcsFiles.DailyInterval1);
+        var iCal = Calendar.Load(IcsFiles.DailyInterval1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2007, 4, 11, 7, 0, 0, _tzid),
@@ -2031,7 +2031,7 @@ public void DailyInterval1()
     [Test, Category("Recurrence")]
     public void HourlyInterval1()
     {
-        var iCal = Calendar.Load(IcsFiles.HourlyInterval1);
+        var iCal = Calendar.Load(IcsFiles.HourlyInterval1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2007, 4, 9, 10, 0, 0, _tzid),
@@ -2058,7 +2058,7 @@ public void HourlyInterval1()
     [Test, Category("Recurrence")]
     public void YearlyBySetPos1()
     {
-        var iCal = Calendar.Load(IcsFiles.YearlyBySetPos1);
+        var iCal = Calendar.Load(IcsFiles.YearlyBySetPos1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2009, 1, 1, 0, 0, 0, _tzid),
@@ -2087,7 +2087,7 @@ public void YearlyBySetPos1()
     [Test, Category("Recurrence")]
     public void Empty1()
     {
-        var iCal = Calendar.Load(IcsFiles.Empty1);
+        var iCal = Calendar.Load(IcsFiles.Empty1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2009, 1, 1, 0, 0, 0, _tzid),
@@ -2106,7 +2106,7 @@ public void Empty1()
     [Test, Category("Recurrence")]
     public void HourlyInterval2()
     {
-        var iCal = Calendar.Load(IcsFiles.HourlyInterval2);
+        var iCal = Calendar.Load(IcsFiles.HourlyInterval2)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2007, 4, 9, 7, 0, 0),
@@ -2135,7 +2135,7 @@ public void HourlyInterval2()
     [Test, Category("Recurrence")]
     public void MinutelyInterval1()
     {
-        var iCal = Calendar.Load(IcsFiles.MinutelyInterval1);
+        var iCal = Calendar.Load(IcsFiles.MinutelyInterval1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2007, 4, 9, 7, 0, 0),
@@ -2164,7 +2164,7 @@ public void MinutelyInterval1()
     [Test, Category("Recurrence")]
     public void DailyInterval2()
     {
-        var iCal = Calendar.Load(IcsFiles.DailyInterval2);
+        var iCal = Calendar.Load(IcsFiles.DailyInterval2)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2007, 4, 9, 7, 0, 0),
@@ -2192,7 +2192,7 @@ public void DailyInterval2()
     [Test, Category("Recurrence")]
     public void DailyByDay1()
     {
-        var iCal = Calendar.Load(IcsFiles.DailyByDay1);
+        var iCal = Calendar.Load(IcsFiles.DailyByDay1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2007, 9, 10, 7, 0, 0),
@@ -2216,7 +2216,7 @@ public void DailyByDay1()
     [Test, Category("Recurrence")]
     public void WeeklyWeekStartsLastYear()
     {
-        var iCal = Calendar.Load(IcsFiles.WeeklyWeekStartsLastYear);
+        var iCal = Calendar.Load(IcsFiles.WeeklyWeekStartsLastYear)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2012, 1, 1, 7, 0, 0),
@@ -2244,7 +2244,7 @@ public void WeeklyWeekStartsLastYear()
     [Test, Category("Recurrence")]
     public void WeeklyInterval1()
     {
-        var iCal = Calendar.Load(IcsFiles.WeeklyInterval1);
+        var iCal = Calendar.Load(IcsFiles.WeeklyInterval1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2007, 9, 10, 7, 0, 0),
@@ -2271,7 +2271,7 @@ public void WeeklyInterval1()
     [Test, Category("Recurrence")]
     public void Monthly1()
     {
-        var iCal = Calendar.Load(IcsFiles.Monthly1);
+        var iCal = Calendar.Load(IcsFiles.Monthly1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2007, 9, 10, 7, 0, 0),
@@ -2302,7 +2302,7 @@ public void Monthly1()
     [Test, Category("Recurrence")]
     public void Yearly1()
     {
-        var iCal = Calendar.Load(IcsFiles.Yearly1);
+        var iCal = Calendar.Load(IcsFiles.Yearly1)!;
         EventOccurrenceTest(
             iCal,
             new CalDateTime(2007, 9, 10, 7, 0, 0),
@@ -2336,7 +2336,7 @@ public void Yearly1()
     [Test, Category("Recurrence")]
     public void Bug2912657()
     {
-        var iCal = Calendar.Load(IcsFiles.Bug2912657);
+        var iCal = Calendar.Load(IcsFiles.Bug2912657)!;
         var localTzid = iCal.Events.First().Start!.TzId;
 
         // Daily recurrence
@@ -2394,8 +2394,8 @@ public void Bug2912657()
     [Test, Category("Recurrence")]
     public void Bug2916581()
     {
-        var iCal = Calendar.Load(IcsFiles.Bug2916581);
-        var localTzid = iCal.TimeZones[0].TzId;
+        var iCal = Calendar.Load(IcsFiles.Bug2916581)!;
+        var localTzid = iCal.TimeZones[0]!.TzId!;
 
         // Weekly across year boundary
         EventOccurrenceTest(
@@ -2434,8 +2434,8 @@ public void Bug2916581()
     [Test, Category("Recurrence")]
     public void Bug2959692()
     {
-        var iCal = Calendar.Load(IcsFiles.Bug2959692);
-        var localTzid = iCal.TimeZones[0].TzId;
+        var iCal = Calendar.Load(IcsFiles.Bug2959692)!;
+        var localTzid = iCal.TimeZones[0]!.TzId!;
 
         EventOccurrenceTest(
             iCal,
@@ -2464,8 +2464,8 @@ public void Bug2959692()
     [Test, Category("Recurrence")]
     public void Bug2966236()
     {
-        var iCal = Calendar.Load(IcsFiles.Bug2966236);
-        var localTzid = iCal.TimeZones[0].TzId;
+        var iCal = Calendar.Load(IcsFiles.Bug2966236)!;
+        var localTzid = iCal.TimeZones[0]!.TzId;
 
         EventOccurrenceTest(
             iCal,
@@ -2508,7 +2508,7 @@ public void Bug2966236()
     [Test, Category("Recurrence")]
     public void Bug3007244()
     {
-        var iCal = Calendar.Load(IcsFiles.Bug3007244);
+        var iCal = Calendar.Load(IcsFiles.Bug3007244)!;
 
         // date only cannot have a time zone
         EventOccurrenceTest(
@@ -2588,7 +2588,7 @@ public void DurationOfRecurrencesOverDst(string dtStart, string dtEnd, string? d
             RRULE:FREQ=WEEKLY;COUNT=3
             END:VEVENT
             END:VCALENDAR
-            """);
+            """)!;
 
         var start = iCal.Events.First().Start;
 
@@ -3104,7 +3104,7 @@ public void ExDateShouldFilterOutAllPeriods()
 TRANSP:TRANSPARENT
 END:VEVENT
 END:VCALENDAR";
-        var calendar = Calendar.Load(ical);
+        var calendar = Calendar.Load(ical)!;
         var firstEvent = calendar.Events.First();
         var startSearch = new CalDateTime(2010, 1, 1);
         var endSearch = new CalDateTime(2016, 12, 31);
@@ -3133,7 +3133,7 @@ public void RDateShouldBeUnionedWithRecurrenceSet()
 END:VEVENT
 END:VCALENDAR";
 
-        var calendar = Calendar.Load(ical);
+        var calendar = Calendar.Load(ical)!;
         var firstEvent = calendar.Events.First();
         var startSearch = new CalDateTime(DateTime.Parse("2015-08-28T07:00:00"), _tzid);
         var endSearch = new CalDateTime(DateTime.Parse("2016-08-28T07:00:00").AddDays(7), _tzid);
@@ -3188,7 +3188,7 @@ public void OccurrenceMustBeCompletelyContainedWithinSearchRange()
             Uid = "abab717c-1786-4efc-87dd-6859c2b48eb6",
         };
 
-        var deserializedCalendar = Calendar.Load(ical);
+        var deserializedCalendar = Calendar.Load(ical)!;
         var firstEvent = deserializedCalendar.Events.First();
         var calendar = new Calendar();
         calendar.Events.Add(e);
@@ -3276,7 +3276,7 @@ public void EventsWithShareUidsShouldGenerateASingleRecurrenceSet()
             END:VCALENDAR
             """;
 
-        var calendar = Calendar.Load(ical);
+        var calendar = Calendar.Load(ical)!;
 
         //The API should be something like:
         var orderedOccurrences = calendar.GetOccurrences()
@@ -3519,10 +3519,10 @@ public void RecurrenceRuleTests()
                END:VCALENDAR
                """;
 
-        var simpleA = Calendar.Load(aString);
-        var normalA = Calendar.Load(aString);
-        var simpleB = Calendar.Load(bString);
-        var normalB = Calendar.Load(bString);
+        var simpleA = Calendar.Load(aString)!;
+        var normalA = Calendar.Load(aString)!;
+        var simpleB = Calendar.Load(bString)!;
+        var normalB = Calendar.Load(bString)!;
 
         var calendarList = new List<Calendar> { simpleA, normalA, simpleB, normalB };
         var eventList = new List<CalendarEvent>
@@ -3665,7 +3665,7 @@ public void InclusiveRruleUntil()
             END:VCALENDAR
             """;
         const string timeZoneId = @"Eastern Standard Time";
-        var calendar = Calendar.Load(icalText);
+        var calendar = Calendar.Load(icalText)!;
         var firstEvent = calendar.Events.First();
         var startSearch = new CalDateTime(DateTime.Parse("2017-07-01T00:00:00"), timeZoneId);
         var endSearch = new CalDateTime(DateTime.Parse("2018-07-01T00:00:00"), timeZoneId);
@@ -3848,7 +3848,7 @@ public void GetOccurrenceShouldExcludeDtEndFloating()
                    END:VCALENDAR
                    """;
 
-        var calendar = Calendar.Load(ical);
+        var calendar = Calendar.Load(ical)!;
         // Set start date for occurrences to search to the end date of the event
         var occurrences = calendar.GetOccurrences(new CalDateTime(2024, 12, 2), new CalDateTime(2024, 12, 3));
 
@@ -3871,7 +3871,7 @@ public void TestGetOccurrenceIndefinite()
             END:VCALENDAR
             """;
 
-        var calendar = Calendar.Load(ical);
+        var calendar = Calendar.Load(ical)!;
 
         // Although the occurrences are unbounded, we can still call GetOccurrences without
         // specifying bounds, because the instances are only generated on enumeration.
@@ -3901,7 +3901,7 @@ public void TestDtStartTimezone(string? tzId)
             END:VCALENDAR
             """;
 
-        var cal = Calendar.Load(icalText);
+        var cal = Calendar.Load(icalText)!;
         var evt = cal.Events.First();
         var ev = new EventEvaluator(evt);
         var occurrences = ev.Evaluate(evt.DtStart!, evt.DtStart!.ToTimeZone(tzId), evt.DtStart.AddMinutes(61).ToTimeZone(tzId), null);
diff --git a/Ical.Net.Tests/RecurrenceTests_From_Issues.cs b/Ical.Net.Tests/RecurrenceTests_From_Issues.cs
index b38f92c5..774325f9 100644
--- a/Ical.Net.Tests/RecurrenceTests_From_Issues.cs
+++ b/Ical.Net.Tests/RecurrenceTests_From_Issues.cs
@@ -46,7 +46,7 @@ public void GetOccurrence_DtEnd_ShouldBeExcluded()
              END:VCALENDAR
              """;
 
-        var calendar = Calendar.Load(ical);
+        var calendar = Calendar.Load(ical)!;
         // Event ends on 2024-10-27, at 02:00:00 GMT (when DST ends). The end time is excluded by RFC 5545 definition.
         var occurrences = calendar.GetOccurrences(endDate, new CalDateTime("20250101T000000", "Europe/London")).ToList();
 
diff --git a/Ical.Net.Tests/RecurrenceWithExDateTests.cs b/Ical.Net.Tests/RecurrenceWithExDateTests.cs
index 92dca0d3..160bfce0 100644
--- a/Ical.Net.Tests/RecurrenceWithExDateTests.cs
+++ b/Ical.Net.Tests/RecurrenceWithExDateTests.cs
@@ -57,7 +57,7 @@ public void ShouldNotOccurOnLocalExceptionDate(bool useExDateWithTime)
         var serializer = new CalendarSerializer();
         var ics = serializer.SerializeToString(calendar);
 
-        var deserializedCalendar = Calendar.Load(ics);
+        var deserializedCalendar = Calendar.Load(ics)!;
         var occurrences = deserializedCalendar.GetOccurrences<CalendarEvent>().ToList();
 
         Assert.Multiple(() =>
@@ -96,13 +96,13 @@ public void ShouldNotOccurOnUtcExceptionDate()
                   END:VCALENDAR
                   """;
 
-        var cal = Calendar.Load(ics);
+        var cal = Calendar.Load(ics)!;
         var occurrences = cal.GetOccurrences<CalendarEvent>().ToList();
 
         var serializer = new CalendarSerializer();
         ics = serializer.SerializeToString(cal);
         // serialize and deserialize to ensure the exclusion dates de/serialized
-        cal = Calendar.Load(new CalendarSerializer(cal).SerializeToString());
+        cal = Calendar.Load(new CalendarSerializer(cal).SerializeToString())!;
 
         // Start date: 2024-10-19 at 18:00 (GMT Standard Time)
         // Recurrence: Every hour, 4 occurrences
@@ -118,7 +118,7 @@ public void ShouldNotOccurOnUtcExceptionDate()
             Assert.That(
                 occurrences.All(
                     o => !cal
-                        .Events[0]
+                        .Events[0]!
                         .ExceptionDates.GetAllDates()
                         .Any(ex => ex.Equals(o.Period.StartTime))), Is.True);
             Assert.That(ics, Does.Contain("EXDATE:20241019T190000Z"));
@@ -144,13 +144,13 @@ public void MultipleExclusionDatesSameTimeZoneShouldBeExcluded()
                   END:VCALENDAR
                   """;
 
-        var cal = Calendar.Load(ics);
+        var cal = Calendar.Load(ics)!;
         var occurrences = cal.GetOccurrences<CalendarEvent>().ToList();
 
         var serializer = new CalendarSerializer();
         ics = serializer.SerializeToString(cal);
         // serialize and deserialize to ensure the exclusion dates de/serialized
-        cal = Calendar.Load(new CalendarSerializer(cal).SerializeToString());
+        cal = Calendar.Load(new CalendarSerializer(cal).SerializeToString())!;
 
         // Occurrences:
         // 2023-10-25 09:00 (UTC Offset: +0200)
@@ -173,11 +173,11 @@ public void MultipleExclusionDatesSameTimeZoneShouldBeExcluded()
         Assert.Multiple(() =>
         {
             Assert.That(occurrences.Count, Is.EqualTo(10));
-            Assert.That(cal.Events[0].ExceptionDates.GetAllDates().Count(), Is.EqualTo(3));
+            Assert.That(cal.Events[0]!.ExceptionDates.GetAllDates().Count(), Is.EqualTo(3));
             Assert.That(
                 occurrences.All(
                     o => !cal
-                        .Events[0]
+                        .Events[0]!
                         .ExceptionDates.GetAllDates()
                         .Any(ex => ex.Equals(o.Period.StartTime))), Is.True);
             Assert.That(ics, Does.Contain("EXDATE;TZID=Europe/Berlin:20231029T090000,20231105T090000,20231112T090000"));
@@ -204,9 +204,9 @@ public void MultipleExclusionDatesDifferentZoneShouldBeExcluded()
                   END:VCALENDAR
                   """;
 
-        var cal = Calendar.Load(ics);
+        var cal = Calendar.Load(ics)!;
         // serialize and deserialize to ensure the exclusion dates de/serialized
-        cal = Calendar.Load(new CalendarSerializer(cal).SerializeToString());
+        cal = Calendar.Load(new CalendarSerializer(cal).SerializeToString())!;
         var occurrences = cal.GetOccurrences<CalendarEvent>().ToList();
 
         // Occurrences:
@@ -226,11 +226,11 @@ public void MultipleExclusionDatesDifferentZoneShouldBeExcluded()
         Assert.Multiple(() =>
         {
             Assert.That(occurrences.Count, Is.EqualTo(9));
-            Assert.That(cal.Events[0].ExceptionDates.GetAllDates().Count(), Is.EqualTo(2));
+            Assert.That(cal.Events[0]!.ExceptionDates.GetAllDates().Count(), Is.EqualTo(2));
             Assert.That(
                 occurrences.All(
                     o => !cal
-                        .Events[0]
+                        .Events[0]!
                         .ExceptionDates.GetAllDates()
                         .Any(ex => ex.Equals(o.Period.StartTime))), Is.True);
         });
diff --git a/Ical.Net.Tests/RecurrenceWithRDateTests.cs b/Ical.Net.Tests/RecurrenceWithRDateTests.cs
index 25785a1d..fe072e98 100644
--- a/Ical.Net.Tests/RecurrenceWithRDateTests.cs
+++ b/Ical.Net.Tests/RecurrenceWithRDateTests.cs
@@ -158,7 +158,7 @@ public void RDate_PeriodsWithTimezone_AreProcessedCorrectly()
 
         // Deserialization
 
-        cal = Calendar.Load(ics);
+        cal = Calendar.Load(ics)!;
         occurrences = cal.Events.First().GetOccurrences().ToList();
 
         Assert.Multiple(() =>
diff --git a/Ical.Net/Calendar.cs b/Ical.Net/Calendar.cs
index f19d010c..5644fdf2 100644
--- a/Ical.Net/Calendar.cs
+++ b/Ical.Net/Calendar.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -20,7 +21,7 @@ namespace Ical.Net;
 
 public class Calendar : CalendarComponent, IGetOccurrencesTyped, IGetFreeBusy, IMergeable
 {
-    public static Calendar Load(string iCalendarString)
+    public static Calendar? Load(string iCalendarString)
         => CalendarCollection.Load(new StringReader(iCalendarString)).SingleOrDefault();
 
     /// <summary>
@@ -28,11 +29,11 @@ public static Calendar Load(string iCalendarString)
     /// </summary>
     /// <param name="s">The stream from which to load the <see cref="Calendar"/> object</param>
     /// <returns>An <see cref="Calendar"/> object</returns>
-    public static Calendar Load(Stream s)
+    public static Calendar? Load(Stream s)
         => CalendarCollection.Load(new StreamReader(s, Encoding.UTF8)).SingleOrDefault();
 
-    public static Calendar Load(TextReader tr)
-        => CalendarCollection.Load(tr)?.SingleOrDefault();
+    public static Calendar? Load(TextReader tr)
+        => CalendarCollection.Load(tr).SingleOrDefault();
 
     public static IList<T> Load<T>(Stream s, Encoding e)
         => Load<T>(new StreamReader(s, e));
@@ -65,6 +66,14 @@ public Calendar()
         ProductId = LibraryMetadata.ProdId;
         Version = LibraryMetadata.Version;
 
+        // Initialize the members to make the compiler happy.
+        _mUniqueComponents = null!;
+        _mEvents = null!;
+        _mTodos = null!;
+        _mJournals = null!;
+        _mFreeBusy = null!;
+        _mTimeZones = null!;
+
         Initialize();
     }
 
@@ -95,7 +104,7 @@ protected bool Equals(Calendar other)
            && CollectionHelpers.Equals(FreeBusy, other.FreeBusy)
            && CollectionHelpers.Equals(TimeZones, other.TimeZones);
 
-    public override bool Equals(object obj)
+    public override bool Equals(object? obj)
     {
         if (ReferenceEquals(null, obj))
         {
@@ -163,7 +172,7 @@ public override int GetHashCode()
     /// <para/>
     /// The default value does not apply to deserialized objects.
     /// </summary>
-    public virtual string Version
+    public virtual string? Version
     {
         get => Properties.Get<string>("VERSION");
         set => Properties.Set("VERSION", value);
@@ -178,19 +187,19 @@ public virtual string Version
     /// <para/>
     /// The default value does not apply to deserialized objects.
     /// </summary>
-    public virtual string ProductId
+    public virtual string? ProductId
     {
         get => Properties.Get<string>("PRODID");
         set => Properties.Set("PRODID", value);
     }
 
-    public virtual string Scale
+    public virtual string? Scale
     {
         get => Properties.Get<string>("CALSCALE");
         set => Properties.Set("CALSCALE", value);
     }
 
-    public virtual string Method
+    public virtual string? Method
     {
         get => Properties.Get<string>("METHOD");
         set => Properties.Set("METHOD", value);
@@ -214,8 +223,9 @@ public VTimeZone AddTimeZone(VTimeZone tz)
     /// </summary>
     /// <param name="startTime">The beginning date/time of the range.</param>
     /// <param name="endTime">The end date/time of the range.</param>
+    /// <param name="options"></param>
     /// <returns>A list of occurrences that fall between the date/time arguments provided.</returns>
-    public virtual IEnumerable<Occurrence> GetOccurrences(CalDateTime startTime = null, CalDateTime endTime = null, EvaluationOptions options = default)
+    public virtual IEnumerable<Occurrence> GetOccurrences(CalDateTime? startTime = null, CalDateTime? endTime = null, EvaluationOptions? options = null)
         => GetOccurrences<IRecurringComponent>(startTime, endTime, options);
 
     /// <summary>
@@ -225,12 +235,13 @@ public virtual IEnumerable<Occurrence> GetOccurrences(CalDateTime startTime = nu
     /// </summary>
     /// <param name="startTime">The starting date range</param>
     /// <param name="endTime">The ending date range</param>
-    public virtual IEnumerable<Occurrence> GetOccurrences<T>(CalDateTime startTime = null, CalDateTime endTime = null, EvaluationOptions options = default) where T : IRecurringComponent
+    /// <param name="options"></param>
+    public virtual IEnumerable<Occurrence> GetOccurrences<T>(CalDateTime? startTime = null, CalDateTime? endTime = null, EvaluationOptions? options = null) where T : IRecurringComponent
     {
         // These are the UID/RECURRENCE-ID combinations that replace other occurrences.
         var recurrenceIdsAndUids = this.Children.OfType<IRecurrable>()
             .Where(r => r.RecurrenceId != null)
-            .Select(r => new { (r as IUniqueComponent)?.Uid, Dt = r.RecurrenceId.Value })
+            .Select(r => new { (r as IUniqueComponent).Uid, Dt = r.RecurrenceId!.Value })
             .Where(r => r.Uid != null)
             .ToDictionary(x => x);
 
@@ -283,37 +294,33 @@ public T Create<T>() where T : ICalendarComponent
             this.AddChild(cal);
             return (T) cal;
         }
-        return default(T);
+        throw new ArgumentException($"Creating {typeof(T).FullName} failed.");
     }
 
     public virtual void MergeWith(IMergeable obj)
     {
-        var c = obj as Calendar;
-        if (c == null)
+        if (obj is not Calendar c)
         {
             return;
         }
 
-        if (Name == null)
-        {
-            Name = c.Name;
-        }
+        Name ??= c.Name;
 
         Method = c.Method;
         Version = c.Version;
         ProductId = c.ProductId;
         Scale = c.Scale;
 
-        foreach (var p in c.Properties.Where(p => !Properties.ContainsKey(p.Name)))
+        foreach (var p in c.Properties.Where(p => p.Name != null && !Properties.ContainsKey(p.Name)))
         {
             Properties.Add(p);
         }
 
         foreach (var child in c.Children)
         {
-            if (child is IUniqueComponent)
+            if (child is IUniqueComponent component)
             {
-                if (!UniqueComponents.ContainsKey(((IUniqueComponent) child).Uid))
+                if (component.Uid != null && !UniqueComponents.ContainsKey(component.Uid))
                 {
                     this.AddChild(child);
                 }
@@ -325,20 +332,20 @@ public virtual void MergeWith(IMergeable obj)
         }
     }
 
-    public virtual FreeBusy GetFreeBusy(FreeBusy freeBusyRequest) => CalendarComponents.FreeBusy.Create(this, freeBusyRequest);
+    public virtual FreeBusy? GetFreeBusy(FreeBusy freeBusyRequest) => CalendarComponents.FreeBusy.Create(this, freeBusyRequest);
 
-    public virtual FreeBusy GetFreeBusy(CalDateTime fromInclusive, CalDateTime toExclusive)
+    public virtual FreeBusy? GetFreeBusy(CalDateTime fromInclusive, CalDateTime toExclusive)
         => CalendarComponents.FreeBusy.Create(this, CalendarComponents.FreeBusy.CreateRequest(fromInclusive, toExclusive, null, null));
 
-    public virtual FreeBusy GetFreeBusy(Organizer organizer, IEnumerable<Attendee> contacts, CalDateTime fromInclusive, CalDateTime toExclusive)
+    public virtual FreeBusy? GetFreeBusy(Organizer organizer, IEnumerable<Attendee> contacts, CalDateTime fromInclusive, CalDateTime toExclusive)
         => CalendarComponents.FreeBusy.Create(this, CalendarComponents.FreeBusy.CreateRequest(fromInclusive, toExclusive, organizer, contacts));
 
     /// <summary>
-    /// Adds a system time zone to the iCalendar.  This time zone may
+    /// Adds a system time zone to the iCalendar. This time zone may
     /// then be used in date/time objects contained in the
     /// calendar.
     /// </summary>
-    /// <param name="tzi">A System.TimeZoneInfo object to add to the calendar.</param>
+    /// <param name="tzi">A <see cref="TimeZoneInfo"/> object to add to the calendar.</param>
     /// <returns>The time zone added to the calendar.</returns>
     public VTimeZone AddTimeZone(TimeZoneInfo tzi)
     {
diff --git a/Ical.Net/CalendarCollection.cs b/Ical.Net/CalendarCollection.cs
index a54e24b9..ed7b1285 100644
--- a/Ical.Net/CalendarCollection.cs
+++ b/Ical.Net/CalendarCollection.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.IO;
@@ -13,7 +14,6 @@
 using Ical.Net.Evaluation;
 using Ical.Net.Serialization;
 using Ical.Net.Utility;
-using static NodaTime.TimeZones.ZoneEqualityComparer;
 
 namespace Ical.Net;
 
@@ -57,38 +57,41 @@ private IEnumerable<Occurrence> GetOccurrences(Func<Calendar, IEnumerable<Occurr
         // being ordered to avoid full enumeration.
         .OrderedMergeMany();
 
-    public IEnumerable<Occurrence> GetOccurrences(CalDateTime startTime = null, CalDateTime endTime = null, EvaluationOptions options = default)
+    public IEnumerable<Occurrence> GetOccurrences(CalDateTime? startTime = null, CalDateTime? endTime = null, EvaluationOptions? options = null)
         => GetOccurrences(iCal => iCal.GetOccurrences(startTime, endTime, options));
 
-    public IEnumerable<Occurrence> GetOccurrences<T>(CalDateTime startTime = null, CalDateTime endTime = null, EvaluationOptions options = default) where T : IRecurringComponent
+    public IEnumerable<Occurrence> GetOccurrences<T>(CalDateTime? startTime = null, CalDateTime? endTime = null, EvaluationOptions? options = null) where T : IRecurringComponent
         => GetOccurrences(iCal => iCal.GetOccurrences<T>(startTime, endTime, options));
 
-    private FreeBusy CombineFreeBusy(FreeBusy main, FreeBusy current)
+    private FreeBusy CombineFreeBusy(FreeBusy? main, FreeBusy current)
     {
         main?.MergeWith(current);
         return current;
     }
 
-    public FreeBusy GetFreeBusy(FreeBusy freeBusyRequest)
+    public FreeBusy? GetFreeBusy(FreeBusy freeBusyRequest)
     {
-        return this.Aggregate<Calendar, FreeBusy>(null, (current, iCal) => CombineFreeBusy(current, iCal.GetFreeBusy(freeBusyRequest)));
+        return this.Aggregate<Calendar, FreeBusy?>(null, (current, iCal) =>
+        {
+            var freeBusy = iCal.GetFreeBusy(freeBusyRequest);
+            return current is null ? freeBusy : CombineFreeBusy(current, freeBusy);
+        });
     }
 
-    public FreeBusy GetFreeBusy(CalDateTime fromInclusive, CalDateTime toExclusive)
+    public FreeBusy? GetFreeBusy(Organizer organizer, IEnumerable<Attendee> contacts, CalDateTime fromInclusive, CalDateTime toExclusive)
     {
-        return this.Aggregate<Calendar, FreeBusy>(null, (current, iCal) => CombineFreeBusy(current, iCal.GetFreeBusy(fromInclusive, toExclusive)));
-    }
-
-    public FreeBusy GetFreeBusy(Organizer organizer, IEnumerable<Attendee> contacts, CalDateTime fromInclusive, CalDateTime toExclusive)
-    {
-        return this.Aggregate<Calendar, FreeBusy>(null, (current, iCal) => CombineFreeBusy(current, iCal.GetFreeBusy(organizer, contacts, fromInclusive, toExclusive)));
+        return this.Aggregate<Calendar, FreeBusy?>(null, (current, iCal) =>
+        {
+            var freeBusy = iCal.GetFreeBusy(organizer, contacts, fromInclusive, toExclusive);
+            return current is null ? freeBusy : CombineFreeBusy(current, freeBusy);
+        });
     }
 
     public override int GetHashCode() => CollectionHelpers.GetHashCode(this);
 
     protected bool Equals(CalendarCollection obj) => CollectionHelpers.Equals(this, obj);
 
-    public override bool Equals(object obj)
+    public override bool Equals(object? obj)
     {
         if (ReferenceEquals(null, obj)) return false;
         if (ReferenceEquals(this, obj)) return true;
diff --git a/Ical.Net/CalendarComponents/FreeBusy.cs b/Ical.Net/CalendarComponents/FreeBusy.cs
index 9142edcd..6e02bd05 100644
--- a/Ical.Net/CalendarComponents/FreeBusy.cs
+++ b/Ical.Net/CalendarComponents/FreeBusy.cs
@@ -65,8 +65,9 @@ public class FreeBusy : UniqueComponent, IMergeable
                 var participatingAttendeeQuery = uc.Attendees
                     .Where(attendee =>
                         attendee.Value != null
+                        && attendee.ParticipationStatus != null
                         && contacts.Contains(attendee.Value.OriginalString.Trim()))
-                    .Select(pa => pa.ParticipationStatus.ToUpperInvariant());
+                    .Select(pa => pa.ParticipationStatus!.ToUpperInvariant());
 
                 foreach (var participatingAttendee in participatingAttendeeQuery)
                 {
diff --git a/Ical.Net/CalendarExtensions.cs b/Ical.Net/CalendarExtensions.cs
index 1da2c3ae..6f936c86 100644
--- a/Ical.Net/CalendarExtensions.cs
+++ b/Ical.Net/CalendarExtensions.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.Globalization;
 using Ical.Net.DataTypes;
diff --git a/Ical.Net/CalendarObject.cs b/Ical.Net/CalendarObject.cs
index 0ec2fe0f..fb639a28 100644
--- a/Ical.Net/CalendarObject.cs
+++ b/Ical.Net/CalendarObject.cs
@@ -20,6 +20,7 @@ public class CalendarObject : CalendarObjectBase, ICalendarObject
 
     internal CalendarObject()
     {
+        Name = string.Empty;
         Initialize();
     }
 
@@ -61,7 +62,7 @@ public override bool Equals(object? obj)
         return obj.GetType() == GetType() && Equals((CalendarObject) obj);
     }
 
-    public override int GetHashCode() => Name?.GetHashCode() ?? 0;
+    public override int GetHashCode() => Name.GetHashCode();
 
     /// <inheritdoc/>
     public override void CopyFrom(ICopyable c)
@@ -82,7 +83,7 @@ public override void CopyFrom(ICopyable c)
         foreach (var child in obj.Children)
         {
             // Add a deep copy of the child instead of the child itself
-            this.AddChild(child.Copy<ICalendarObject>());
+            this.AddChild(child.Copy<ICalendarObject>()!);
         }
     }
 
@@ -97,9 +98,9 @@ public override void CopyFrom(ICopyable c)
     public virtual ICalendarObjectList<ICalendarObject> Children => _children;
 
     /// <summary>
-    /// Gets or sets the name of the iCalObject.  For iCalendar components, this is the RFC 5545 name of the component.
+    /// Gets or sets the name of the iCalObject. For iCalendar components, this is the RFC 5545 name of the component.
     /// </summary>
-    public virtual string? Name { get; set; }
+    public virtual string Name { get; set; }
 
     /// <summary>
     /// Gets the <see cref="Net.Calendar"/> object.
@@ -124,7 +125,7 @@ public virtual Calendar? Calendar
 
     public virtual int Column { get; set; }
 
-    public virtual string? Group
+    public virtual string Group
     {
         get => Name;
         set => Name = value;
diff --git a/Ical.Net/CalendarObjectExtensions.cs b/Ical.Net/CalendarObjectExtensions.cs
index 2c7d50ca..acf9670e 100644
--- a/Ical.Net/CalendarObjectExtensions.cs
+++ b/Ical.Net/CalendarObjectExtensions.cs
@@ -3,17 +3,14 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 namespace Ical.Net;
 
 public static class CalendarObjectExtensions
 {
     public static void AddChild<TItem>(this ICalendarObject obj, TItem child) where TItem : ICalendarObject
-    {
-        obj.Children.Add(child);
-    }
+        => obj.Children.Add(child);
 
     public static void RemoveChild<TItem>(this ICalendarObject obj, TItem child) where TItem : ICalendarObject
-    {
-        obj.Children.Remove(child);
-    }
-}
\ No newline at end of file
+        => obj.Children.Remove(child);
+}
diff --git a/Ical.Net/CalendarObjectList.cs b/Ical.Net/CalendarObjectList.cs
index 165cc9b8..d15300e7 100644
--- a/Ical.Net/CalendarObjectList.cs
+++ b/Ical.Net/CalendarObjectList.cs
@@ -3,7 +3,7 @@
 // Licensed under the MIT license.
 //
 
-using System.Diagnostics.CodeAnalysis;
+#nullable enable
 using Ical.Net.Collections;
 
 namespace Ical.Net;
@@ -11,7 +11,5 @@ namespace Ical.Net;
 /// <summary>
 /// A collection of calendar objects.
 /// </summary>
-[ExcludeFromCodeCoverage]
 public class CalendarObjectList : GroupedList<string, ICalendarObject>, ICalendarObjectList<ICalendarObject>
-{
-}
\ No newline at end of file
+{}
diff --git a/Ical.Net/CalendarParameter.cs b/Ical.Net/CalendarParameter.cs
index 9cfa9ea4..46189d65 100644
--- a/Ical.Net/CalendarParameter.cs
+++ b/Ical.Net/CalendarParameter.cs
@@ -16,7 +16,7 @@ namespace Ical.Net;
 [DebuggerDisplay("{Name}={string.Join(\",\", Values)}")]
 public class CalendarParameter : CalendarObject, IValueObject<string>
 {
-    private HashSet<string> _values = new();
+    private HashSet<string?> _values = new();
 
     public CalendarParameter()
     {
@@ -34,7 +34,7 @@ public CalendarParameter(string name, string? value) : base(name)
         AddValue(value);
     }
 
-    public CalendarParameter(string name, IEnumerable<string> values) : base(name)
+    public CalendarParameter(string name, IEnumerable<string?> values) : base(name)
     {
         Initialize();
         foreach (var v in values)
@@ -45,7 +45,7 @@ public CalendarParameter(string name, IEnumerable<string> values) : base(name)
 
     private void Initialize()
     {
-        _values = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+        _values = new HashSet<string?>(StringComparer.OrdinalIgnoreCase);
     }
 
     protected override void OnDeserializing(StreamingContext context)
@@ -66,22 +66,22 @@ public override void CopyFrom(ICopyable c)
             return;
         }
 
-        _values = new HashSet<string>(p.Values.Where(IsValidValue), StringComparer.OrdinalIgnoreCase);
+        _values = new HashSet<string?>(p.Values.Where(IsValidValue), StringComparer.OrdinalIgnoreCase);
     }
 
-    public virtual IEnumerable<string> Values => _values;
+    public virtual IEnumerable<string?> Values => _values;
 
     public virtual bool ContainsValue(string value) => _values.Contains(value);
 
     public virtual int ValueCount => _values.Count;
 
-    public virtual void SetValue(string value)
+    public virtual void SetValue(string? value)
     {
         _values.Clear();
         _values.Add(value);
     }
 
-    public virtual void SetValue(IEnumerable<string> values)
+    public virtual void SetValue(IEnumerable<string?> values)
     {
         // Remove all previous values
         _values.Clear();
@@ -90,16 +90,16 @@ public virtual void SetValue(IEnumerable<string> values)
 
     private static bool IsValidValue(string? value) => !string.IsNullOrWhiteSpace(value);
 
-    public virtual void AddValue(string? value)
+    public virtual bool AddValue(string? value)
     {
         if (!IsValidValue(value))
         {
-            return;
+            return false;
         }
-        _values.Add(value!);
+        return _values.Add(value!);
     }
 
-    public virtual void RemoveValue(string value) => _values.Remove(value);
+    public virtual bool RemoveValue(string? value) => _values.Remove(value);
 
     public virtual string? Value
     {
diff --git a/Ical.Net/CalendarProperty.cs b/Ical.Net/CalendarProperty.cs
index 5f939c4f..a9025ef3 100644
--- a/Ical.Net/CalendarProperty.cs
+++ b/Ical.Net/CalendarProperty.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -31,7 +32,7 @@ namespace Ical.Net;
 [DebuggerDisplay("{Name}:{Value}")]
 public class CalendarProperty : CalendarObject, ICalendarProperty
 {
-    private List<object> _values = new List<object>();
+    private readonly List<object?> _values = new List<object?>();
 
     /// <summary>
     /// Returns a list of parameters that are associated with the iCalendar object.
@@ -77,36 +78,40 @@ public override void CopyFrom(ICopyable obj)
         SetValue(p.Values);
     }
 
-    public virtual IEnumerable<object> Values => _values;
+    public virtual IEnumerable<object?> Values => _values;
 
-    public object Value
+    /// <summary>
+    /// Gets or sets the first value in the list of values.
+    /// Using <see langword="null"/> for <paramref name="value"/> will clear all values.
+    /// </summary>
+    public object? Value
     {
-        get => _values?.FirstOrDefault();
+        get => _values.FirstOrDefault();
         set
         {
             if (value == null)
             {
-                _values = null;
+                _values.Clear();
                 return;
             }
 
-            if (_values != null && _values.Count > 0)
+            if (_values.Count > 0)
             {
                 _values[0] = value;
             }
             else
             {
-                _values?.Clear();
-                _values?.Add(value);
+                // collection is known to be empty here
+                _values.Add(value);
             }
         }
     }
 
-    public virtual bool ContainsValue(object value) => _values.Contains(value);
+    public virtual bool ContainsValue(object? value) => _values.Contains(value);
 
-    public virtual int ValueCount => _values?.Count ?? 0;
+    public virtual int ValueCount => _values.Count;
 
-    public virtual void SetValue(object value)
+    public virtual void SetValue(object? value)
     {
         if (_values.Count == 0)
         {
@@ -114,7 +119,7 @@ public virtual void SetValue(object value)
         }
         else if (value != null)
         {
-            // Our list contains values.  Let's set the first value!
+            // Our list contains values. Let's set the first value!
             _values[0] = value;
         }
         else
@@ -123,32 +128,26 @@ public virtual void SetValue(object value)
         }
     }
 
-    public virtual void SetValue(IEnumerable<object> values)
+    /// <summary>
+    /// Sets the value of the property to the specified <paramref name="values"/>.
+    /// Using <see langword="null"/> for <paramref name="values"/> will clear all values.
+    /// </summary>
+    /// <param name="values"></param>
+    public virtual void SetValue(IEnumerable<object?>? values)
     {
         // Remove all previous values
         _values.Clear();
         // If the values are ICopyable, create a deep copy of each value,
         // otherwise just add the value
-        var toAdd = values?.Select(x => (x as ICopyable)?.Copy<object>() ?? x) ?? Enumerable.Empty<object>();
+        var toAdd = values?.Select(x => (x as ICopyable)?.Copy<object?>() ?? x) ?? Enumerable.Empty<object?>();
         _values.AddRange(toAdd);
     }
 
-    public virtual void AddValue(object value)
+    public virtual bool AddValue(object? value)
     {
-        if (value == null)
-        {
-            return;
-        }
-
         _values.Add(value);
+        return true;
     }
 
-    public virtual void RemoveValue(object value)
-    {
-        if (value == null)
-        {
-            return;
-        }
-        _values.Remove(value);
-    }
+    public virtual bool RemoveValue(object? value) => _values.Remove(value);
 }
diff --git a/Ical.Net/CalendarPropertyList.cs b/Ical.Net/CalendarPropertyList.cs
index c93cbb84..cc8e4389 100644
--- a/Ical.Net/CalendarPropertyList.cs
+++ b/Ical.Net/CalendarPropertyList.cs
@@ -3,6 +3,8 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
+using System;
 using System.Linq;
 using Ical.Net.Collections;
 
@@ -10,7 +12,7 @@ namespace Ical.Net;
 
 public class CalendarPropertyList : GroupedValueList<string, ICalendarProperty, CalendarProperty, object>
 {
-    private readonly ICalendarObject _mParent;
+    private readonly ICalendarObject? _mParent;
 
     public CalendarPropertyList() { }
 
@@ -20,12 +22,10 @@ public CalendarPropertyList(ICalendarObject parent)
         ItemAdded += CalendarPropertyList_ItemAdded;
     }
 
-    private void CalendarPropertyList_ItemAdded(object sender, ObjectEventArgs<ICalendarProperty, int> e)
-    {
-        e.First.Parent = _mParent;
-    }
+    private void CalendarPropertyList_ItemAdded(object? sender, ObjectEventArgs<ICalendarProperty, int> e)
+        => e.First.Parent = _mParent;
 
-    public ICalendarProperty this[string name] => ContainsKey(name)
+    public ICalendarProperty? this[string name] => ContainsKey(name)
         ? AllOf(name).FirstOrDefault()
         : null;
-}
\ No newline at end of file
+}
diff --git a/Ical.Net/Collections/GroupedList.cs b/Ical.Net/Collections/GroupedList.cs
index e4158b7e..d9c74ad7 100644
--- a/Ical.Net/Collections/GroupedList.cs
+++ b/Ical.Net/Collections/GroupedList.cs
@@ -22,7 +22,7 @@ public class GroupedList<TGroup, TItem> :
     private readonly List<IMultiLinkedList<TItem>> _lists = new();
     private readonly Dictionary<TGroup, IMultiLinkedList<TItem>> _dictionary = new();
 
-    private IMultiLinkedList<TItem>? EnsureList(TGroup group)
+    private IMultiLinkedList<TItem> EnsureList(TGroup group)
     {
         if (_dictionary.TryGetValue(group, out var existingList))
         {
@@ -62,10 +62,6 @@ public virtual void Add(TItem? item)
 
         var group = item.Group;
         var list = EnsureList(group);
-        if (list == null)
-        {
-            return;
-        }
 
         var index = list.Count;
         list.Add(item);
@@ -184,12 +180,8 @@ public virtual void Insert(int index, TItem? item)
     public virtual void RemoveAt(int index)
     {
         var list = ListForIndex(index, out var relativeIndex);
-        if (list == null)
-        {
-            return;
-        }
 
-        list.RemoveAt(relativeIndex);
+        list?.RemoveAt(relativeIndex);
     }
 
     /// <summary>
diff --git a/Ical.Net/Collections/GroupedValueList.cs b/Ical.Net/Collections/GroupedValueList.cs
index 3815a7e0..d46890ac 100644
--- a/Ical.Net/Collections/GroupedValueList.cs
+++ b/Ical.Net/Collections/GroupedValueList.cs
@@ -18,16 +18,14 @@ public class GroupedValueList<TGroup, TInterface, TItem, TValueType> :
     where TInterface : class, IGroupedObject<TGroup>, IValueObject<TValueType>
     where TItem : new()
 {
-    public virtual void Set(TGroup group, TValueType value)
-    {
-        Set(group, new[] { value });
-    }
+    public virtual void Set(TGroup group, TValueType? value)
+        => Set(group, new[] { value });
 
-    public virtual void Set(TGroup group, IEnumerable<TValueType> values)
+    public virtual void Set(TGroup group, IEnumerable<TValueType?> values)
     {
         if (ContainsKey(group))
         {
-            AllOf(group)?.FirstOrDefault()?.SetValue(values);
+            AllOf(group).FirstOrDefault()?.SetValue(values);
             return;
         }
 
diff --git a/Ical.Net/Collections/Interfaces/IValueObject.cs b/Ical.Net/Collections/Interfaces/IValueObject.cs
index 65725626..d4d398e9 100644
--- a/Ical.Net/Collections/Interfaces/IValueObject.cs
+++ b/Ical.Net/Collections/Interfaces/IValueObject.cs
@@ -10,12 +10,12 @@ namespace Ical.Net.Collections.Interfaces;
 
 public interface IValueObject<T>
 {
-    IEnumerable<T>? Values { get; }
+    IEnumerable<T?> Values { get; }
 
     bool ContainsValue(T value);
-    void SetValue(T value);
-    void SetValue(IEnumerable<T> values);
-    void AddValue(T value);
-    void RemoveValue(T value);
+    void SetValue(T? value);
+    void SetValue(IEnumerable<T?> values);
+    bool AddValue(T? value);
+    bool RemoveValue(T? value);
     int ValueCount { get; }
 }
diff --git a/Ical.Net/DataTypes/Attachment.cs b/Ical.Net/DataTypes/Attachment.cs
index 22c56734..6cb6ba93 100644
--- a/Ical.Net/DataTypes/Attachment.cs
+++ b/Ical.Net/DataTypes/Attachment.cs
@@ -23,20 +23,13 @@ public class Attachment : EncodableDataType
     public virtual byte[]? Data { get; private set; } // private set for CopyFrom
 
     private Encoding _valueEncoding = System.Text.Encoding.UTF8;
-    public virtual Encoding ValueEncoding
+    public virtual Encoding ValueEncoding //NOSONAR
     {
         get => _valueEncoding;
-        set
-        {
-            if (value == null)
-            {
-                return;
-            }
-            _valueEncoding = value;
-        }
+        set => _valueEncoding = value;
     }
 
-    public virtual string FormatType
+    public virtual string? FormatType
     {
         get => Parameters.Get("FMTTYPE");
         set => Parameters.Set("FMTTYPE", value);
@@ -66,7 +59,7 @@ public Attachment(string value) : this()
             throw new ArgumentException($"{value} is not a valid ATTACH component");
         }
 
-        ValueEncoding = a.ValueEncoding;
+        _valueEncoding = a.ValueEncoding;
 
         Data = a.Data;
         Uri = a.Uri;
diff --git a/Ical.Net/DataTypes/Attendee.cs b/Ical.Net/DataTypes/Attendee.cs
index f4a1b4a9..1c45f060 100644
--- a/Ical.Net/DataTypes/Attendee.cs
+++ b/Ical.Net/DataTypes/Attendee.cs
@@ -40,8 +40,10 @@ public virtual Uri? SentBy
     }
 
     private string? _commonName;
-    /// <summary> CN: to show the common or displayable name associated with the calendar address </summary>
-    public virtual string CommonName
+    /// <summary>
+    /// CN: to show the common or displayable name associated with the calendar address.
+    /// </summary>
+    public virtual string? CommonName
     {
         get
         {
@@ -63,7 +65,9 @@ public virtual string CommonName
     }
 
     private Uri? _directoryEntry;
-    /// <summary> DIR, to indicate the URI that points to the directory information corresponding to the attendee </summary>
+    /// <summary>
+    /// DIR, to indicate the URI that points to the directory information corresponding to the attendee.
+    /// </summary>
     public virtual Uri? DirectoryEntry
     {
         get
@@ -89,8 +93,10 @@ public virtual Uri? DirectoryEntry
     }
 
     private string? _type;
-    /// <summary> CUTYPE: the type of calendar user </summary>
-    public virtual string Type
+    /// <summary>
+    /// CUTYPE: the type of calendar user.
+    /// </summary>
+    public virtual string? Type
     {
         get
         {
@@ -113,7 +119,9 @@ public virtual string Type
     }
 
     private List<string>? _members;
-    /// <summary> MEMBER: the groups the user belongs to </summary>
+    /// <summary>
+    /// MEMBER: the groups the user belongs to.
+    /// </summary>
     public virtual IList<string> Members
     {
         get => _members ?? (_members = new List<string>(Parameters.GetMany("MEMBER")));
@@ -125,8 +133,10 @@ public virtual IList<string> Members
     }
 
     private string? _role;
-    /// <summary> ROLE: the intended role the attendee will have </summary>
-    public virtual string Role
+    /// <summary>
+    /// ROLE: the intended role the attendee will have.
+    /// </summary>
+    public virtual string? Role
     {
         get
         {
@@ -148,7 +158,7 @@ public virtual string Role
     }
 
     private string? _participationStatus;
-    public virtual string ParticipationStatus
+    public virtual string? ParticipationStatus
     {
         get
         {
@@ -170,7 +180,9 @@ public virtual string ParticipationStatus
     }
 
     private bool? _rsvp;
-    /// <summary> RSVP, to indicate whether a reply is requested </summary>
+    /// <summary>
+    /// RSVP, to indicate whether a reply is requested.
+    /// </summary>
     public virtual bool Rsvp
     {
         get
@@ -197,7 +209,9 @@ public virtual bool Rsvp
     }
 
     private List<string>? _delegatedTo;
-    /// <summary> DELEGATED-TO, to indicate the calendar users that the original request was delegated to </summary>
+    /// <summary>
+    /// DELEGATED-TO, to indicate the calendar users that the original request was delegated to.
+    /// </summary>
     public virtual IList<string> DelegatedTo
     {
         get => _delegatedTo ?? (_delegatedTo = new List<string>(Parameters.GetMany("DELEGATED-TO")));
@@ -209,7 +223,9 @@ public virtual IList<string> DelegatedTo
     }
 
     private List<string>? _delegatedFrom;
-    /// <summary> DELEGATED-FROM, to indicate whom the request was delegated from </summary>
+    /// <summary>
+    /// DELEGATED-FROM, to indicate whom the request was delegated from.
+    /// </summary>
     public virtual IList<string> DelegatedFrom
     {
         get => _delegatedFrom ?? (_delegatedFrom = new List<string>(Parameters.GetMany("DELEGATED-FROM")));
@@ -220,7 +236,9 @@ public virtual IList<string> DelegatedFrom
         }
     }
 
-    /// <summary> Uri associated with the attendee, typically an email address </summary>
+    /// <summary>
+    /// Uri associated with the attendee, typically an email address.
+    /// </summary>
     public virtual Uri? Value { get; set; }
 
     public Attendee() { }
@@ -259,17 +277,20 @@ public override void CopyFrom(ICopyable obj)
         DirectoryEntry = atn.DirectoryEntry;
     }
 
-    protected bool Equals(Attendee other) => Equals(SentBy, other.SentBy)
-                                             && string.Equals(CommonName, other.CommonName, StringComparison.OrdinalIgnoreCase)
-                                             && Equals(DirectoryEntry, other.DirectoryEntry)
-                                             && string.Equals(Type, other.Type, StringComparison.OrdinalIgnoreCase)
-                                             && string.Equals(Role, other.Role)
-                                             && string.Equals(ParticipationStatus, other.ParticipationStatus, StringComparison.OrdinalIgnoreCase)
-                                             && Rsvp == other.Rsvp
-                                             && Equals(Value, other.Value)
-                                             && Members.SequenceEqual(other.Members)
-                                             && DelegatedTo.SequenceEqual(other.DelegatedTo)
-                                             && DelegatedFrom.SequenceEqual(other.DelegatedFrom);
+    protected bool Equals(Attendee other)
+        => Equals(SentBy, other.SentBy)
+           && string.Equals(CommonName, other.CommonName,
+               StringComparison.OrdinalIgnoreCase)
+           && Equals(DirectoryEntry, other.DirectoryEntry)
+           && string.Equals(Type, other.Type, StringComparison.OrdinalIgnoreCase)
+           && string.Equals(Role, other.Role)
+           && string.Equals(ParticipationStatus, other.ParticipationStatus,
+               StringComparison.OrdinalIgnoreCase)
+           && Rsvp == other.Rsvp
+           && Equals(Value, other.Value)
+           && Members.SequenceEqual(other.Members)
+           && DelegatedTo.SequenceEqual(other.DelegatedTo)
+           && DelegatedFrom.SequenceEqual(other.DelegatedFrom);
     public override bool Equals(object? obj)
     {
         if (ReferenceEquals(null, obj)) return false;
diff --git a/Ical.Net/DataTypes/Organizer.cs b/Ical.Net/DataTypes/Organizer.cs
index 4273eb65..52137d73 100644
--- a/Ical.Net/DataTypes/Organizer.cs
+++ b/Ical.Net/DataTypes/Organizer.cs
@@ -41,7 +41,7 @@ public virtual Uri? SentBy
         }
     }
 
-    public virtual string CommonName
+    public virtual string? CommonName
     {
         get => Parameters.Get("CN");
         set => Parameters.Set("CN", value);
@@ -51,7 +51,7 @@ public virtual Uri? DirectoryEntry
     {
         get
         {
-            string dir = Parameters.Get("DIR");
+            var dir = Parameters.Get("DIR");
             if (!string.IsNullOrWhiteSpace(dir))
             {
                 return new Uri(dir);
diff --git a/Ical.Net/DefaultTimeZoneResolver.cs b/Ical.Net/DefaultTimeZoneResolver.cs
index e78bde3c..ed90b05a 100644
--- a/Ical.Net/DefaultTimeZoneResolver.cs
+++ b/Ical.Net/DefaultTimeZoneResolver.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -30,7 +31,7 @@ private static readonly Lazy<Dictionary<string, string>> _windowsMapping
     /// <param name="tzId">A BCL, IANA, or serialization time zone identifier</param>
     /// <exception cref="ArgumentException">Processing failed</exception>
     /// <remarks>The DateTimeZone if found or null otherwise.</remarks>
-    public static DateTimeZone GetZone(string tzId)
+    public static DateTimeZone? GetZone(string? tzId)
     {
         var exMsg = $"Unrecognized time zone id {tzId}";
 
diff --git a/Ical.Net/ICalendarObject.cs b/Ical.Net/ICalendarObject.cs
index 9d84e4e3..853ef10b 100644
--- a/Ical.Net/ICalendarObject.cs
+++ b/Ical.Net/ICalendarObject.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using Ical.Net.Collections;
 
 namespace Ical.Net;
@@ -19,7 +20,7 @@ public interface ICalendarObject : IGroupedObject<string>, ILoadable, ICopyable
     /// <summary>
     /// Returns the parent of this object.
     /// </summary>
-    ICalendarObject Parent { get; set; }
+    ICalendarObject? Parent { get; set; }
 
     /// <summary>
     /// Returns a collection of children of this object.
@@ -30,7 +31,7 @@ public interface ICalendarObject : IGroupedObject<string>, ILoadable, ICopyable
     /// Returns the iCalendar that this object
     /// is associated with.
     /// </summary>
-    Calendar Calendar { get; }
+    Calendar? Calendar { get; }
 
     /// <summary>
     /// Returns the line number where this calendar
diff --git a/Ical.Net/ICalendarObjectList.cs b/Ical.Net/ICalendarObjectList.cs
index 26f775a5..20e5b634 100644
--- a/Ical.Net/ICalendarObjectList.cs
+++ b/Ical.Net/ICalendarObjectList.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using Ical.Net.Collections;
 
 namespace Ical.Net;
@@ -10,5 +11,5 @@ namespace Ical.Net;
 public interface ICalendarObjectList<TType> :
     IGroupedCollection<string, TType> where TType : class, ICalendarObject
 {
-    TType this[int index] { get; }
-}
\ No newline at end of file
+    TType? this[int index] { get; }
+}
diff --git a/Ical.Net/ICalendarProperty.cs b/Ical.Net/ICalendarProperty.cs
index 968962d6..be9556b4 100644
--- a/Ical.Net/ICalendarProperty.cs
+++ b/Ical.Net/ICalendarProperty.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using Ical.Net.Collections.Interfaces;
 using Ical.Net.DataTypes;
 
@@ -10,5 +11,5 @@ namespace Ical.Net;
 
 public interface ICalendarProperty : ICalendarParameterCollectionContainer, ICalendarObject, IValueObject<object>
 {
-    object Value { get; set; }
-}
\ No newline at end of file
+    object? Value { get; set; }
+}
diff --git a/Ical.Net/ICalendarPropertyListContainer.cs b/Ical.Net/ICalendarPropertyListContainer.cs
index cfa4e6f1..6c263a31 100644
--- a/Ical.Net/ICalendarPropertyListContainer.cs
+++ b/Ical.Net/ICalendarPropertyListContainer.cs
@@ -3,9 +3,10 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 namespace Ical.Net;
 
 public interface ICalendarPropertyListContainer : ICalendarObject
 {
     CalendarPropertyList Properties { get; }
-}
\ No newline at end of file
+}
diff --git a/Ical.Net/ICopyable.cs b/Ical.Net/ICopyable.cs
index f93dc366..01e5767e 100644
--- a/Ical.Net/ICopyable.cs
+++ b/Ical.Net/ICopyable.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 namespace Ical.Net;
 
 public interface ICopyable
@@ -21,5 +22,5 @@ public interface ICopyable
     /// of the object when it is overridden, otherwise is used the implementation of the base class.
     /// This is necessary when working with mutable reference types.
     /// </summary>
-    T Copy<T>();
-}
\ No newline at end of file
+    T? Copy<T>();
+}
diff --git a/Ical.Net/IGetFreeBusy.cs b/Ical.Net/IGetFreeBusy.cs
index 67cae4df..0ed497cc 100644
--- a/Ical.Net/IGetFreeBusy.cs
+++ b/Ical.Net/IGetFreeBusy.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System.Collections.Generic;
 using Ical.Net.CalendarComponents;
 using Ical.Net.DataTypes;
@@ -11,7 +12,7 @@ namespace Ical.Net;
 
 public interface IGetFreeBusy
 {
-    FreeBusy GetFreeBusy(FreeBusy freeBusyRequest);
-    FreeBusy GetFreeBusy(CalDateTime fromInclusive, CalDateTime toExclusive);
-    FreeBusy GetFreeBusy(Organizer organizer, IEnumerable<Attendee> contacts, CalDateTime fromInclusive, CalDateTime toExclusive);
-}
\ No newline at end of file
+    FreeBusy? GetFreeBusy(FreeBusy freeBusyRequest);
+    FreeBusy? GetFreeBusy(CalDateTime fromInclusive, CalDateTime toExclusive);
+    FreeBusy? GetFreeBusy(Organizer organizer, IEnumerable<Attendee> contacts, CalDateTime fromInclusive, CalDateTime toExclusive);
+}
diff --git a/Ical.Net/ILoadable.cs b/Ical.Net/ILoadable.cs
index da19f509..9c17ecd8 100644
--- a/Ical.Net/ILoadable.cs
+++ b/Ical.Net/ILoadable.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 
 namespace Ical.Net;
diff --git a/Ical.Net/IMergeable.cs b/Ical.Net/IMergeable.cs
index 61aab5be..a395a5de 100644
--- a/Ical.Net/IMergeable.cs
+++ b/Ical.Net/IMergeable.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 namespace Ical.Net;
 
 public interface IMergeable
@@ -11,4 +12,4 @@ public interface IMergeable
     /// Merges this object with another.
     /// </summary>
     void MergeWith(IMergeable obj);
-}
\ No newline at end of file
+}
diff --git a/Ical.Net/IParameterCollection.cs b/Ical.Net/IParameterCollection.cs
index f23edc94..c1fc1bc2 100644
--- a/Ical.Net/IParameterCollection.cs
+++ b/Ical.Net/IParameterCollection.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System.Collections.Generic;
 using Ical.Net.Collections;
 
@@ -12,8 +13,8 @@ public interface IParameterCollection : IGroupedList<string, CalendarParameter>
 {
     void SetParent(ICalendarObject parent);
     void Add(string name, string value);
-    string Get(string name);
+    string? Get(string name);
     IList<string> GetMany(string name);
-    void Set(string name, string value);
-    void Set(string name, IEnumerable<string> values);
-}
\ No newline at end of file
+    void Set(string name, string? value);
+    void Set(string name, IEnumerable<string?> values);
+}
diff --git a/Ical.Net/ParameterList.cs b/Ical.Net/ParameterList.cs
index b9afb90d..f306640f 100644
--- a/Ical.Net/ParameterList.cs
+++ b/Ical.Net/ParameterList.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System.Collections.Generic;
 using Ical.Net.Collections;
 
@@ -10,7 +11,7 @@ namespace Ical.Net;
 
 public class ParameterList : GroupedValueList<string, CalendarParameter, CalendarParameter, string>, IParameterCollection
 {
-    public virtual void SetParent(ICalendarObject parent)
+    public virtual void SetParent(ICalendarObject? parent)
     {
         foreach (var parameter in this)
         {
@@ -19,11 +20,9 @@ public virtual void SetParent(ICalendarObject parent)
     }
 
     public virtual void Add(string name, string value)
-    {
-        Add(new CalendarParameter(name, value));
-    }
+        => Add(new CalendarParameter(name, value));
 
-    public virtual string Get(string name) => Get<string>(name);
+    public virtual string? Get(string name) => Get<string>(name);
 
     public virtual IList<string> GetMany(string name) => GetMany<string>(name);
-}
\ No newline at end of file
+}
diff --git a/Ical.Net/Proxies/CalendarObjectListProxy.cs b/Ical.Net/Proxies/CalendarObjectListProxy.cs
index 8218f551..5cf5916d 100644
--- a/Ical.Net/Proxies/CalendarObjectListProxy.cs
+++ b/Ical.Net/Proxies/CalendarObjectListProxy.cs
@@ -31,5 +31,5 @@ public CalendarObjectListProxy(IGroupedCollection<string, ICalendarObject> list)
     /// <param name="index"></param>
     /// <returns></returns>
     /// <exception cref="ArgumentOutOfRangeException"></exception>
-    TType ICalendarObjectList<TType>.this[int index] => this[index] ?? throw new ArgumentOutOfRangeException(nameof(index));
+    TType ICalendarObjectList<TType>.this[int index] => this.Skip(index).FirstOrDefault() ?? throw new ArgumentOutOfRangeException(nameof(index));
 }
diff --git a/Ical.Net/Proxies/ParameterCollectionProxy.cs b/Ical.Net/Proxies/ParameterCollectionProxy.cs
index ad06b15e..98dbc2a3 100644
--- a/Ical.Net/Proxies/ParameterCollectionProxy.cs
+++ b/Ical.Net/Proxies/ParameterCollectionProxy.cs
@@ -39,7 +39,7 @@ public virtual void Add(string name, string value)
 
     public virtual IList<string> GetMany(string name) => new GroupedValueListProxy<string, CalendarParameter, CalendarParameter, string, string>(Parameters, name);
 
-    public virtual void Set(string name, string value)
+    public virtual void Set(string name, string? value)
     {
         var parameter = RealObject.FirstOrDefault(o => string.Equals(o.Name, name, StringComparison.Ordinal));
 
@@ -53,7 +53,7 @@ public virtual void Set(string name, string value)
         }
     }
 
-    public virtual void Set(string name, IEnumerable<string> values)
+    public virtual void Set(string name, IEnumerable<string?> values)
     {
         var parameter = RealObject.FirstOrDefault(o => string.Equals(o.Name, name, StringComparison.Ordinal));
 
diff --git a/Ical.Net/Serialization/PropertySerializer.cs b/Ical.Net/Serialization/PropertySerializer.cs
index d426f6ab..ba56c828 100644
--- a/Ical.Net/Serialization/PropertySerializer.cs
+++ b/Ical.Net/Serialization/PropertySerializer.cs
@@ -40,7 +40,7 @@ public PropertySerializer(SerializationContext ctx) : base(ctx) { }
         var result = new StringBuilder();
         foreach (var v in prop.Values.Where(value => value != null))
         {
-            SerializeValue(result, prop, v, sf);
+            SerializeValue(result, prop, v!, sf);
         }
 
         // Pop the object off the serialization context.
diff --git a/Ical.Net/ServiceProvider.cs b/Ical.Net/ServiceProvider.cs
index 05136327..7a7542ec 100644
--- a/Ical.Net/ServiceProvider.cs
+++ b/Ical.Net/ServiceProvider.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -14,43 +15,41 @@ public class ServiceProvider
     private readonly IDictionary<Type, object> _mTypedServices = new Dictionary<Type, object>();
     private readonly IDictionary<string, object> _mNamedServices = new Dictionary<string, object>();
 
-    public virtual object GetService(Type serviceType)
+    public virtual object? GetService(Type serviceType)
     {
-        object service;
-        _mTypedServices.TryGetValue(serviceType, out service);
+        _mTypedServices.TryGetValue(serviceType, out var service);
         return service;
     }
 
-    public virtual object GetService(string name)
+    public virtual object? GetService(string name)
     {
-        object service;
-        _mNamedServices.TryGetValue(name, out service);
+        _mNamedServices.TryGetValue(name, out var service);
         return service;
     }
 
-    public virtual T GetService<T>()
+    public virtual T? GetService<T>()
     {
         var service = GetService(typeof(T));
-        if (service is T)
+        if (service is T svc)
         {
-            return (T) service;
+            return svc;
         }
         return default(T);
     }
 
-    public virtual T GetService<T>(string name)
+    public virtual T? GetService<T>(string name)
     {
         var service = GetService(name);
-        if (service is T)
+        if (service is T svc)
         {
-            return (T) service;
+            return svc;
         }
         return default(T);
     }
 
     public virtual void SetService(string name, object obj)
     {
-        if (!string.IsNullOrEmpty(name) && obj != null)
+        if (!string.IsNullOrEmpty(name))
         {
             _mNamedServices[name] = obj;
         }
@@ -58,33 +57,26 @@ public virtual void SetService(string name, object obj)
 
     public virtual void SetService(object obj)
     {
-        if (obj != null)
-        {
-            var type = obj.GetType();
-            _mTypedServices[type] = obj;
+        var type = obj.GetType();
+        _mTypedServices[type] = obj;
 
-            // Get interfaces for the given type
-            foreach (var iface in type.GetInterfaces())
-            {
-                _mTypedServices[iface] = obj;
-            }
+        // Get interfaces for the given type
+        foreach (var interfaceType in type.GetInterfaces())
+        {
+            _mTypedServices[interfaceType] = obj;
         }
     }
 
     public virtual void RemoveService(Type type)
     {
-        if (type != null)
+        if (_mTypedServices.ContainsKey(type))
         {
-            if (_mTypedServices.ContainsKey(type))
-            {
-                _mTypedServices.Remove(type);
-            }
+            _mTypedServices.Remove(type);
+        }
 
-            // Get interfaces for the given type
-            foreach (var iface in type.GetInterfaces().Where(iface => _mTypedServices.ContainsKey(iface)))
-            {
-                _mTypedServices.Remove(iface);
-            }
+        foreach (var interfaceType in type.GetInterfaces().Where(interfaceType => _mTypedServices.ContainsKey(interfaceType)))
+        {
+            _mTypedServices.Remove(interfaceType);
         }
     }
 
@@ -95,4 +87,4 @@ public virtual void RemoveService(string name)
             _mNamedServices.Remove(name);
         }
     }
-}
\ No newline at end of file
+}
diff --git a/Ical.Net/TimeZoneResolvers.cs b/Ical.Net/TimeZoneResolvers.cs
index 669e37d2..a64b2383 100644
--- a/Ical.Net/TimeZoneResolvers.cs
+++ b/Ical.Net/TimeZoneResolvers.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using NodaTime;
 
@@ -13,7 +14,8 @@ public static class TimeZoneResolvers
     /// <summary>
     /// The default time zone resolver.
     /// </summary>
-    public static Func<string, DateTimeZone> Default => tzId => DefaultTimeZoneResolver.GetZone(tzId);
+    public static Func<string, DateTimeZone> Default
+        => tzId => DefaultTimeZoneResolver.GetZone(tzId) ?? throw new ArgumentException($"Unrecognized time zone id {tzId}", nameof(tzId));
 
     /// <summary>
     /// Gets or sets a function that returns the NodaTime DateTimeZone for the given TZID.
diff --git a/Ical.Net/Utility/DateUtil.cs b/Ical.Net/Utility/DateUtil.cs
index 7bf3ab48..984de63f 100644
--- a/Ical.Net/Utility/DateUtil.cs
+++ b/Ical.Net/Utility/DateUtil.cs
@@ -44,8 +44,7 @@ public static CalDateTime FirstDayOfWeek(CalDateTime dt, DayOfWeek firstDayOfWee
     /// <param name="tzId">A time zone identifier</param>
     /// <exception cref="ArgumentException">Unrecognized time zone id</exception>
     public static DateTimeZone GetZone(string tzId)
-        => (TimeZoneResolvers.TimeZoneResolver ?? throw new InvalidOperationException())(tzId)
-            ?? throw new ArgumentException($"Unrecognized time zone id {tzId}");
+        => TimeZoneResolvers.TimeZoneResolver(tzId) ?? throw new ArgumentException($"Unrecognized time zone id {tzId}", nameof(tzId));
 
     public static ZonedDateTime AddYears(ZonedDateTime zonedDateTime, int years)
     {