diff --git a/Ical.Net.Tests/RecurrenceTests.cs b/Ical.Net.Tests/RecurrenceTests.cs
index 10fc184cf..b3cd1b8a6 100644
--- a/Ical.Net.Tests/RecurrenceTests.cs
+++ b/Ical.Net.Tests/RecurrenceTests.cs
@@ -108,7 +108,7 @@ public void EventOccurrenceTest(
             """;
 
         var cal = Calendar.Load(calendarIcalStr);
-        var tzid = cal.Events.Single().Start.TzId;
+        var tzid = cal.Events.Single().Start?.TzId;
 
         var periodSerializer = new PeriodSerializer();
         var periods = expectedPeriods
@@ -140,7 +140,7 @@ public void YearlyComplex1()
 
         while (dt.Year < 2011)
         {
-            if (dt.GreaterThan(evt.Start) &&
+            if (dt.GreaterThan(evt.Start!) &&
                 (dt.Year % 2 == 1) && // Every-other year from 2005
                 (dt.Month == 1) &&
                 (dt.DayOfWeek == DayOfWeek.Sunday))
@@ -203,7 +203,7 @@ public void DailyUntil1()
         var i = 0;
         while (dt.Year < 1998)
         {
-            if (dt.GreaterThanOrEqual(evt.Start) &&
+            if (dt.GreaterThanOrEqual(evt.Start!) &&
                 dt.LessThan(new CalDateTime(1997, 12, 24, 0, 0, 0, _tzid)))
             {
                 Assert.Multiple(() =>
@@ -375,7 +375,7 @@ public void ByMonth1()
         var i = 0;
         while (dt.Year < 2001)
         {
-            if (dt.GreaterThanOrEqual(evt.Start) &&
+            if (dt.GreaterThanOrEqual(evt.Start!) &&
                 dt.Month == 1 &&
                 dt.LessThanOrEqual(new CalDateTime(2000, 1, 31, 9, 0, 0, _tzid)))
             {
@@ -2337,7 +2337,7 @@ public void Yearly1()
     public void Bug2912657()
     {
         var iCal = Calendar.Load(IcsFiles.Bug2912657);
-        var localTzid = iCal.Events.First().Start.TzId;
+        var localTzid = iCal.Events.First().Start?.TzId;
 
         // Daily recurrence
         EventOccurrenceTest(
@@ -2602,7 +2602,7 @@ public void DurationOfRecurrencesOverDst(string dtStart, string dtEnd, string? d
         for (var index = 0; index < expectedPeriods.Length; index++)
         {
             var p = expectedPeriods[index];
-            var newStart = p.StartTime.ToTimeZone(start.TzId);
+            var newStart = p.StartTime.ToTimeZone(start?.TzId);
             expectedPeriods[index] = Period.Create(newStart, end: newStart.Add(p.Duration!.Value));
         }
 
@@ -2866,8 +2866,8 @@ public void UsHolidays()
             Assert.That(evt, Is.Not.Null);
             Assert.Multiple(() =>
             {
-                Assert.That(items.ContainsKey(evt.Summary), Is.True, "Holiday text '" + evt.Summary + "' did not match known holidays.");
-                Assert.That(o.Period.StartTime, Is.EqualTo(items[evt.Summary]), "Date/time of holiday '" + evt.Summary + "' did not match.");
+                Assert.That(items.ContainsKey(evt.Summary!), Is.True, "Holiday text '" + evt.Summary + "' did not match known holidays.");
+                Assert.That(o.Period.StartTime, Is.EqualTo(items[evt.Summary!]), "Date/time of holiday '" + evt.Summary + "' did not match.");
             });
         }
     }
@@ -3904,7 +3904,7 @@ public void TestDtStartTimezone(string? tzId)
         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);
+        var occurrences = ev.Evaluate(evt.DtStart!, evt.DtStart!.ToTimeZone(tzId), evt.DtStart.AddMinutes(61).ToTimeZone(tzId), null);
         var occurrencesStartTimes = occurrences.Select(x => x.StartTime).Take(2).ToList();
 
         var expectedStartTimes = new[]
diff --git a/Ical.Net/CalendarComponents/Alarm.cs b/Ical.Net/CalendarComponents/Alarm.cs
index 83ac2f06c..d47aad584 100644
--- a/Ical.Net/CalendarComponents/Alarm.cs
+++ b/Ical.Net/CalendarComponents/Alarm.cs
@@ -91,7 +91,7 @@ public virtual IList<AlarmOccurrence> GetOccurrences(IRecurringComponent rc, Cal
             // Ensure that "FromDate" has already been set
             if (fromDate == null)
             {
-                fromDate = rc.Start.Copy();
+                fromDate = rc.Start?.Copy();
             }
 
             Duration? duration = null;
diff --git a/Ical.Net/CalendarComponents/CalendarComponent.cs b/Ical.Net/CalendarComponents/CalendarComponent.cs
index 408bb4d03..bc16da19a 100644
--- a/Ical.Net/CalendarComponents/CalendarComponent.cs
+++ b/Ical.Net/CalendarComponents/CalendarComponent.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System.Diagnostics;
 using System.Runtime.Serialization;
 
@@ -18,7 +19,7 @@ public class CalendarComponent : CalendarObject, ICalendarComponent
     /// <summary>
     /// Returns a list of properties that are associated with the iCalendar object.
     /// </summary>
-    public virtual CalendarPropertyList Properties { get; protected set; }
+    public virtual CalendarPropertyList Properties { get; protected set; } = null!;
 
     public CalendarComponent() : base()
     {
@@ -47,8 +48,7 @@ public override void CopyFrom(ICopyable obj)
     {
         base.CopyFrom(obj);
 
-        var c = obj as ICalendarComponent;
-        if (c == null)
+        if (obj is not ICalendarComponent c)
         {
             return;
         }
@@ -78,4 +78,4 @@ public virtual void AddProperty(ICalendarProperty p)
         p.Parent = this;
         Properties.Set(p.Name, p.Value);
     }
-}
\ No newline at end of file
+}
diff --git a/Ical.Net/CalendarComponents/CalendarEvent.cs b/Ical.Net/CalendarComponents/CalendarEvent.cs
index bc7f43845..e480e6b69 100644
--- a/Ical.Net/CalendarComponents/CalendarEvent.cs
+++ b/Ical.Net/CalendarComponents/CalendarEvent.cs
@@ -174,7 +174,7 @@ public virtual CalDateTime? End
     /// Returns <see langword="true"/> if the event is an all-day event,
     /// meaning the <see cref="RecurringComponent.Start"/> is a 'DATE' value type.
     /// </summary>
-    public virtual bool IsAllDay => !Start.HasTime;
+    public virtual bool IsAllDay => Start?.HasTime == false;
 
     /// <summary>
     /// The geographic location (lat/long) of the event.
@@ -346,16 +346,14 @@ public int CompareTo(CalendarEvent? other)
         {
             return 1;
         }
-        if (DtStart.Equals(other.DtStart))
+        if (DtStart is null)
         {
-            return 0;
+            return other.DtStart is null ? 0 : -1;
         }
-        if (DtStart.LessThan(other.DtStart))
+        if (other.DtStart is null)
         {
-            return -1;
+            return 1;
         }
-
-        // meaning DtStart is greater than other
-        return 1;
+        return DtStart.CompareTo(other.DtStart);
     }
 }
diff --git a/Ical.Net/CalendarComponents/FreeBusy.cs b/Ical.Net/CalendarComponents/FreeBusy.cs
index 5566986ae..6b7659cf0 100644
--- a/Ical.Net/CalendarComponents/FreeBusy.cs
+++ b/Ical.Net/CalendarComponents/FreeBusy.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -14,23 +15,23 @@ namespace Ical.Net.CalendarComponents;
 
 public class FreeBusy : UniqueComponent, IMergeable
 {
-    public static FreeBusy Create(ICalendarObject obj, FreeBusy freeBusyRequest, EvaluationOptions options = default)
+    public static FreeBusy? Create(ICalendarObject obj, FreeBusy freeBusyRequest, EvaluationOptions? options = null)
     {
-        if (!(obj is IGetOccurrencesTyped))
+        if ((obj is not IGetOccurrencesTyped occ))
         {
             return null;
         }
-        var getOccurrences = (IGetOccurrencesTyped) obj;
-        var occurrences = getOccurrences.GetOccurrences<CalendarEvent>(freeBusyRequest.Start, freeBusyRequest.End, options);
+
+        var occurrences = occ.GetOccurrences<CalendarEvent>(freeBusyRequest.Start, freeBusyRequest.End, options);
         var contacts = new List<string>();
         var isFilteredByAttendees = false;
 
-        if (freeBusyRequest.Attendees != null && freeBusyRequest.Attendees.Count > 0)
+        if (freeBusyRequest.Attendees.Count > 0)
         {
             isFilteredByAttendees = true;
             var attendees = freeBusyRequest.Attendees
                 .Where(a => a.Value != null)
-                .Select(a => a.Value.OriginalString.Trim());
+                .Select(a => a.Value!.OriginalString.Trim());
             contacts.AddRange(attendees);
         }
 
@@ -41,12 +42,8 @@ public static FreeBusy Create(ICalendarObject obj, FreeBusy freeBusyRequest, Eva
 
         foreach (var o in occurrences)
         {
-            var uc = o.Source as IUniqueComponent;
-
-            if (uc == null)
-            {
+            if (o.Source is not IUniqueComponent uc)
                 continue;
-            }
 
             var evt = uc as CalendarEvent;
             var accepted = false;
@@ -68,8 +65,7 @@ public static FreeBusy Create(ICalendarObject obj, FreeBusy freeBusyRequest, Eva
                 var participatingAttendeeQuery = uc.Attendees
                     .Where(attendee =>
                         attendee.Value != null
-                        && contacts.Contains(attendee.Value.OriginalString.Trim())
-                        && attendee.ParticipationStatus != null)
+                        && contacts.Contains(attendee.Value.OriginalString.Trim()))
                     .Select(pa => pa.ParticipationStatus.ToUpperInvariant());
 
                 foreach (var participatingAttendee in participatingAttendeeQuery)
@@ -98,7 +94,7 @@ public static FreeBusy Create(ICalendarObject obj, FreeBusy freeBusyRequest, Eva
         return fb;
     }
 
-    public static FreeBusy CreateRequest(CalDateTime fromInclusive, CalDateTime toExclusive, Organizer organizer, IEnumerable<Attendee> contacts)
+    public static FreeBusy CreateRequest(CalDateTime fromInclusive, CalDateTime toExclusive, Organizer? organizer, IEnumerable<Attendee>? contacts)
     {
         var fb = new FreeBusy
         {
@@ -158,7 +154,7 @@ public virtual CalDateTime End
         set => Properties.Set("DTEND", value);
     }
 
-    public virtual FreeBusyStatus GetFreeBusyStatus(Period period)
+    public virtual FreeBusyStatus GetFreeBusyStatus(Period? period)
     {
         var status = FreeBusyStatus.Free;
         if (period == null)
@@ -173,7 +169,7 @@ public virtual FreeBusyStatus GetFreeBusyStatus(Period period)
         return status;
     }
 
-    public virtual FreeBusyStatus GetFreeBusyStatus(CalDateTime dt)
+    public virtual FreeBusyStatus GetFreeBusyStatus(CalDateTime? dt)
     {
         var status = FreeBusyStatus.Free;
         if (dt == null)
@@ -190,7 +186,7 @@ public virtual FreeBusyStatus GetFreeBusyStatus(CalDateTime dt)
 
     public virtual void MergeWith(IMergeable obj)
     {
-        if (!(obj is FreeBusy fb))
+        if (obj is not FreeBusy fb)
         {
             return;
         }
diff --git a/Ical.Net/CalendarComponents/IAlarmContainer.cs b/Ical.Net/CalendarComponents/IAlarmContainer.cs
index 5e42f5796..222ab2204 100644
--- a/Ical.Net/CalendarComponents/IAlarmContainer.cs
+++ b/Ical.Net/CalendarComponents/IAlarmContainer.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System.Collections.Generic;
 using Ical.Net.DataTypes;
 
@@ -16,7 +17,7 @@ public interface IAlarmContainer
     ICalendarObjectList<Alarm> Alarms { get; }
 
     ///  <summary>
-    ///  Polls <see cref="Alarm"/>s for occurrences within the <see cref="Evaluate"/>d
+    ///  Polls <see cref="Alarm"/>s for occurrences within the <see cref="RecurringComponent.GetOccurrences"/>d
     ///  time frame of this <see cref="RecurringComponent"/>.  For each evaluated
     ///  occurrence if this component, each <see cref="Alarm"/> is polled for its
     ///  corresponding alarm occurrences.
@@ -24,5 +25,5 @@ public interface IAlarmContainer
     /// <param name="startTime">The earliest allowable alarm occurrence to poll, or <c>null</c>.</param>
     /// <param name="endTime"></param>
     /// <returns>A List of <see cref="AlarmOccurrence"/> objects, one for each occurrence of the <see cref="Alarm"/>.</returns>
-    IList<AlarmOccurrence> PollAlarms(CalDateTime startTime, CalDateTime endTime);
-}
\ No newline at end of file
+    IList<AlarmOccurrence> PollAlarms(CalDateTime? startTime, CalDateTime? endTime);
+}
diff --git a/Ical.Net/CalendarComponents/ICalendarComponent.cs b/Ical.Net/CalendarComponents/ICalendarComponent.cs
index 7df4820f2..227f24618 100644
--- a/Ical.Net/CalendarComponents/ICalendarComponent.cs
+++ b/Ical.Net/CalendarComponents/ICalendarComponent.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 namespace Ical.Net.CalendarComponents;
 
-public interface ICalendarComponent : ICalendarPropertyListContainer { }
\ No newline at end of file
+public interface ICalendarComponent : ICalendarPropertyListContainer { }
diff --git a/Ical.Net/CalendarComponents/IRecurrable.cs b/Ical.Net/CalendarComponents/IRecurrable.cs
index 1cb1a0865..05ff404a8 100644
--- a/Ical.Net/CalendarComponents/IRecurrable.cs
+++ b/Ical.Net/CalendarComponents/IRecurrable.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System.Collections.Generic;
 using Ical.Net.DataTypes;
 using Ical.Net.Evaluation;
@@ -14,12 +15,12 @@ public interface IRecurrable : IGetOccurrences
     /// <summary>
     /// Gets/sets the start date/time of the component.
     /// </summary>
-    CalDateTime Start { get; set; }
+    CalDateTime? Start { get; set; }
 
     ExceptionDates ExceptionDates { get; }
     IList<RecurrencePattern> ExceptionRules { get; set; }
     RecurrenceDates RecurrenceDates { get; }
     IList<RecurrencePattern> RecurrenceRules { get; set; }
-    CalDateTime RecurrenceId { get; set; }
-    IEvaluator Evaluator { get; }
+    CalDateTime? RecurrenceId { get; set; }
+    IEvaluator? Evaluator { get; }
 }
diff --git a/Ical.Net/CalendarComponents/IRecurringComponent.cs b/Ical.Net/CalendarComponents/IRecurringComponent.cs
index 597659397..9b5690b19 100644
--- a/Ical.Net/CalendarComponents/IRecurringComponent.cs
+++ b/Ical.Net/CalendarComponents/IRecurringComponent.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System.Collections.Generic;
 using Ical.Net.DataTypes;
 
@@ -12,13 +13,13 @@ public interface IRecurringComponent : IUniqueComponent, IRecurrable
 {
     IList<Attachment> Attachments { get; set; }
     IList<string> Categories { get; set; }
-    string Class { get; set; }
+    string? Class { get; set; }
     IList<string> Contacts { get; set; }
-    CalDateTime Created { get; set; }
-    string Description { get; set; }
-    CalDateTime LastModified { get; set; }
+    CalDateTime? Created { get; set; }
+    string? Description { get; set; }
+    CalDateTime? LastModified { get; set; }
     int Priority { get; set; }
     IList<string> RelatedComponents { get; set; }
     int Sequence { get; set; }
-    string Summary { get; set; }
-}
\ No newline at end of file
+    string? Summary { get; set; }
+}
diff --git a/Ical.Net/CalendarComponents/IUniqueComponent.cs b/Ical.Net/CalendarComponents/IUniqueComponent.cs
index e5004b0e8..6f5b3da82 100644
--- a/Ical.Net/CalendarComponents/IUniqueComponent.cs
+++ b/Ical.Net/CalendarComponents/IUniqueComponent.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.Collections.Generic;
 using Ical.Net.DataTypes;
@@ -11,12 +12,12 @@ namespace Ical.Net.CalendarComponents;
 
 public interface IUniqueComponent : ICalendarComponent
 {
-    string Uid { get; set; }
+    string? Uid { get; set; }
 
     IList<Attendee> Attendees { get; set; }
     IList<string> Comments { get; set; }
-    CalDateTime DtStamp { get; set; }
-    Organizer Organizer { get; set; }
+    CalDateTime? DtStamp { get; set; }
+    Organizer? Organizer { get; set; }
     IList<RequestStatus> RequestStatuses { get; set; }
-    Uri Url { get; set; }
-}
\ No newline at end of file
+    Uri? Url { get; set; }
+}
diff --git a/Ical.Net/CalendarComponents/Journal.cs b/Ical.Net/CalendarComponents/Journal.cs
index d6194bc8f..6128a85b3 100644
--- a/Ical.Net/CalendarComponents/Journal.cs
+++ b/Ical.Net/CalendarComponents/Journal.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System.Runtime.Serialization;
 using Ical.Net.Evaluation;
 
@@ -35,9 +36,17 @@ protected override void OnDeserializing(StreamingContext context)
         base.OnDeserializing(context);
     }
 
-    protected bool Equals(Journal other) => Start.Equals(other.Start) && Equals(other as RecurringComponent);
+    protected bool Equals(Journal? other)
+    {
+        if (Start == null || other?.Start == null)
+        {
+            return Start == other?.Start; // Both must be null to be considered equal
+        }
+
+        return Start.Equals(other.Start) && Equals(other as RecurringComponent);
+    }
 
-    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/RecurringComponent.cs b/Ical.Net/CalendarComponents/RecurringComponent.cs
index ca377dd58..24db107e2 100644
--- a/Ical.Net/CalendarComponents/RecurringComponent.cs
+++ b/Ical.Net/CalendarComponents/RecurringComponent.cs
@@ -41,7 +41,7 @@ public virtual IList<string> Categories
         set => Properties.Set("CATEGORIES", value);
     }
 
-    public virtual string Class
+    public virtual string? Class
     {
         get => Properties.Get<string>("CLASS");
         set => Properties.Set("CLASS", value);
@@ -53,13 +53,13 @@ public virtual IList<string> Contacts
         set => Properties.Set("CONTACT", value);
     }
 
-    public virtual CalDateTime Created
+    public virtual CalDateTime? Created
     {
         get => Properties.Get<CalDateTime>("CREATED");
         set => Properties.Set("CREATED", value);
     }
 
-    public virtual string Description
+    public virtual string? Description
     {
         get => Properties.Get<string>("DESCRIPTION");
         set => Properties.Set("DESCRIPTION", value);
@@ -68,7 +68,7 @@ public virtual string Description
     /// <summary>
     /// The start date/time of the component.
     /// </summary>
-    public virtual CalDateTime DtStart
+    public virtual CalDateTime? DtStart
     {
         get => Properties.Get<CalDateTime>("DTSTART");
         set => Properties.Set("DTSTART", value);
@@ -88,7 +88,7 @@ public virtual IList<RecurrencePattern> ExceptionRules
         set => Properties.Set("EXRULE", value);
     }
 
-    public virtual CalDateTime LastModified
+    public virtual CalDateTime? LastModified
     {
         get => Properties.Get<CalDateTime>("LAST-MODIFIED");
         set => Properties.Set("LAST-MODIFIED", value);
@@ -114,7 +114,7 @@ public virtual IList<RecurrencePattern> RecurrenceRules
         set => Properties.Set("RRULE", value);
     }
 
-    public virtual CalDateTime RecurrenceId
+    public virtual CalDateTime? RecurrenceId
     {
         get => Properties.Get<CalDateTime>("RECURRENCE-ID");
         set => Properties.Set("RECURRENCE-ID", value);
@@ -135,13 +135,13 @@ public virtual int Sequence
     /// <summary>
     /// An alias to the DTStart field (i.e. start date/time).
     /// </summary>
-    public virtual CalDateTime Start
+    public virtual CalDateTime? Start
     {
         get => DtStart;
         set => DtStart = value;
     }
 
-    public virtual string Summary
+    public virtual string? Summary
     {
         get => Properties.Get<string>("SUMMARY");
         set => Properties.Set("SUMMARY", value);
diff --git a/Ical.Net/CalendarComponents/Todo.cs b/Ical.Net/CalendarComponents/Todo.cs
index e39eb7e6a..243cf0d03 100644
--- a/Ical.Net/CalendarComponents/Todo.cs
+++ b/Ical.Net/CalendarComponents/Todo.cs
@@ -3,7 +3,7 @@
 // Licensed under the MIT license.
 //
 
-using System;
+#nullable enable
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -22,18 +22,18 @@ public class Todo : RecurringComponent, IAlarmContainer
     private readonly TodoEvaluator _mEvaluator;
 
     /// <summary>
-    /// The date/time the todo was completed.
+    /// The date/time when the item was completed.
     /// </summary>
-    public virtual CalDateTime Completed
+    public virtual CalDateTime? Completed
     {
         get => Properties.Get<CalDateTime>("COMPLETED");
         set => Properties.Set("COMPLETED", value);
     }
 
     /// <summary>
-    /// The due date of the todo item.
+    /// The due date of the item.
     /// </summary>
-    public virtual CalDateTime Due
+    public virtual CalDateTime? Due
     {
         get => Properties.Get<CalDateTime>("DUE");
         set
@@ -43,7 +43,7 @@ public virtual CalDateTime Due
     }
 
     /// <summary>
-    /// The duration of the todo item.
+    /// The duration of the item.
     /// </summary>
     // NOTE: Duration is not supported by all systems,
     // (i.e. iPhone) and cannot co-exist with Due.
@@ -64,13 +64,13 @@ public virtual Duration? Duration
         }
     }
 
-    public virtual GeographicLocation GeographicLocation
+    public virtual GeographicLocation? GeographicLocation
     {
         get => Properties.Get<GeographicLocation>("GEO");
         set => Properties.Set("GEO", value);
     }
 
-    public virtual string Location
+    public virtual string? Location
     {
         get => Properties.Get<string>("LOCATION");
         set => Properties.Set("LOCATION", value);
@@ -89,7 +89,7 @@ public virtual IList<string> Resources
     }
 
     /// <summary>
-    /// The status of the todo item.
+    /// The status of the item.
     /// </summary>
     public virtual string Status
     {
@@ -124,16 +124,16 @@ public Todo()
     }
 
     /// <summary>
-    /// Use this method to determine if a todo item has been completed.
+    /// Use this method to determine if an item has been completed.
     /// This takes into account recurrence items and the previous date
     /// of completion, if any.
     /// <note>
-    /// This method evaluates the recurrence pattern for this TODO
+    /// This method evaluates the recurrence pattern for this item
     /// as necessary to ensure all relevant information is taken
     /// into account to give the most accurate result possible.
     /// </note>
     /// </summary>
-    /// <returns>True if the todo item has been completed</returns>
+    /// <returns>True if the item has been completed</returns>
     public virtual bool IsCompleted(CalDateTime currDt)
     {
         if (Status == TodoStatus.Completed)
@@ -144,7 +144,7 @@ public virtual bool IsCompleted(CalDateTime currDt)
             }
 
             // Evaluate to the previous occurrence.
-            var periods = _mEvaluator.EvaluateToPreviousOccurrence(Completed, currDt, options: default);
+            var periods = _mEvaluator.EvaluateToPreviousOccurrence(Completed, currDt, options: null);
 
             return periods.All(p => !p.StartTime.GreaterThan(Completed) || !currDt.GreaterThanOrEqual(p.StartTime));
         }
@@ -152,7 +152,7 @@ public virtual bool IsCompleted(CalDateTime currDt)
     }
 
     /// <summary>
-    /// Returns 'True' if the todo item is Active as of <paramref name="currDt"/>.
+    /// Returns 'True' if the item is Active as of <paramref name="currDt"/>.
     /// An item is Active if it requires action of some sort.
     /// </summary>
     /// <param name="currDt">The date and time to test.</param>
@@ -162,9 +162,9 @@ public virtual bool IsActive(CalDateTime currDt)
            && (!IsCompleted(currDt) && !IsCancelled);
 
     /// <summary>
-    /// Returns True if the todo item was cancelled.
+    /// Returns True if the item was cancelled.
     /// </summary>
-    /// <returns>True if the todo was cancelled, False otherwise.</returns>
+    /// <returns>True if the item was cancelled, False otherwise.</returns>
     public virtual bool IsCancelled => string.Equals(Status, TodoStatus.Cancelled, TodoStatus.Comparison);
 
     public override IEvaluator Evaluator => _mEvaluator;
diff --git a/Ical.Net/CalendarComponents/UniqueComponent.cs b/Ical.Net/CalendarComponents/UniqueComponent.cs
index e4e68fea5..701374ede 100644
--- a/Ical.Net/CalendarComponents/UniqueComponent.cs
+++ b/Ical.Net/CalendarComponents/UniqueComponent.cs
@@ -3,11 +3,11 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Runtime.Serialization;
 using Ical.Net.DataTypes;
-using Ical.Net.Utility;
 
 namespace Ical.Net.CalendarComponents;
 
@@ -61,13 +61,13 @@ public virtual IList<string> Comments
         set => Properties.Set("COMMENT", value);
     }
 
-    public virtual CalDateTime DtStamp
+    public virtual CalDateTime? DtStamp
     {
         get => Properties.Get<CalDateTime>("DTSTAMP");
         set => Properties.Set("DTSTAMP", value);
     }
 
-    public virtual Organizer Organizer
+    public virtual Organizer? Organizer
     {
         get => Properties.Get<Organizer>("ORGANIZER");
         set => Properties.Set("ORGANIZER", value);
@@ -79,7 +79,7 @@ public virtual IList<RequestStatus> RequestStatuses
         set => Properties.Set("REQUEST-STATUS", value);
     }
 
-    public virtual Uri Url
+    public virtual Uri? Url
     {
         get => Properties.Get<Uri>("URL");
         set => Properties.Set("URL", value);
@@ -92,26 +92,25 @@ protected override void OnDeserialized(StreamingContext context)
         EnsureProperties();
     }
 
-    public int CompareTo(UniqueComponent other)
-        => string.Compare(Uid, other.Uid, StringComparison.OrdinalIgnoreCase);
+    public int CompareTo(UniqueComponent? other)
+        => string.Compare(Uid, other?.Uid, StringComparison.OrdinalIgnoreCase);
 
-    public override bool Equals(object obj)
+    public override bool Equals(object? obj)
     {
-        if (obj is RecurringComponent && obj != this)
+        if (obj is not RecurringComponent rec || rec == this)
+            return base.Equals(obj);
+
+        if (Uid != null)
         {
-            var r = (RecurringComponent)obj;
-            if (Uid != null)
-            {
-                return Uid.Equals(r.Uid);
-            }
-            return Uid == r.Uid;
+            return Uid.Equals(rec.Uid);
         }
-        return base.Equals(obj);
+
+        return Uid == rec.Uid;
     }
 
     public override int GetHashCode() => Uid?.GetHashCode() ?? base.GetHashCode();
 
-    public virtual string Uid
+    public virtual string? Uid
     {
         get => Properties.Get<string>("UID");
         set => Properties.Set("UID", value);
diff --git a/Ical.Net/CalendarComponents/VTimeZone.cs b/Ical.Net/CalendarComponents/VTimeZone.cs
index d563cdd12..26e39164a 100644
--- a/Ical.Net/CalendarComponents/VTimeZone.cs
+++ b/Ical.Net/CalendarComponents/VTimeZone.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using Ical.Net.DataTypes;
 using Ical.Net.Proxies;
 using Ical.Net.Utility;
@@ -83,24 +84,33 @@ public static VTimeZone FromDateTimeZone(string tzId, DateTime earliestDateTimeT
         }
         else
         {
-            // first, get the latest standard and daylight intervals, find the oldest recurring date in both, set the RRULES for it, and create a VTimeZoneInfos out of them.
-            //standard
+            // first, get the latest standard and daylight intervals, find the oldest recurring date in both,
+            // set the RRULES for it, and create a VTimeZoneInfos out of them.
+
+            // Standard
             var standardIntervals = intervals.Where(x => x.Savings.ToTimeSpan() == new TimeSpan(0)).ToList();
             var latestStandardInterval = standardIntervals.OrderByDescending(x => x.Start).FirstOrDefault();
+
+            if (latestStandardInterval == null)
+            {
+                return vTimeZone;
+            }
+
             matchingStandardIntervals = GetMatchingIntervals(standardIntervals, latestStandardInterval, true);
             var latestStandardTimeZoneInfo = CreateTimeZoneInfo(matchingStandardIntervals, intervals);
             vTimeZone.AddChild(latestStandardTimeZoneInfo);
 
             // check to see if there is no active, future daylight savings (ie, America/Phoenix)
-            if (latestStandardInterval != null && (latestStandardInterval.HasEnd ? latestStandardInterval.End : Instant.MaxValue) != Instant.MaxValue)
+            if ((latestStandardInterval.HasEnd ? latestStandardInterval.End : Instant.MaxValue) != Instant.MaxValue)
             {
                 //daylight
                 var daylightIntervals = intervals.Where(x => x.Savings.ToTimeSpan() != new TimeSpan(0)).ToList();
 
-                if (daylightIntervals.Any())
+                if (daylightIntervals.Count != 0)
                 {
-                    var latestDaylightInterval = daylightIntervals.OrderByDescending(x => x.Start).FirstOrDefault();
-                    matchingDaylightIntervals = GetMatchingIntervals(daylightIntervals, latestDaylightInterval, true);
+                    var latestDaylightInterval = daylightIntervals.OrderByDescending(x => x.Start).First();
+                    matchingDaylightIntervals =
+                        GetMatchingIntervals(daylightIntervals, latestDaylightInterval, true);
                     var latestDaylightTimeZoneInfo = CreateTimeZoneInfo(matchingDaylightIntervals, intervals);
                     vTimeZone.AddChild(latestDaylightTimeZoneInfo);
                 }
@@ -136,9 +146,9 @@ public static VTimeZone FromDateTimeZone(string tzId, DateTime earliestDateTimeT
     private static VTimeZoneInfo CreateTimeZoneInfo(List<ZoneInterval> matchedIntervals, List<ZoneInterval> intervals, bool isRRule = true,
         bool isOnlyInterval = false)
     {
-        if (matchedIntervals == null || !matchedIntervals.Any())
+        if (matchedIntervals == null || matchedIntervals.Count == 0)
         {
-            throw new ArgumentException("No intervals found in matchedIntervals");
+            throw new InvalidOperationException("No intervals found in matchedIntervals");
         }
 
         var oldestInterval = matchedIntervals.OrderBy(x => x.Start).FirstOrDefault();
@@ -157,7 +167,7 @@ private static VTimeZoneInfo CreateTimeZoneInfo(List<ZoneInterval> matchedInterv
         }
         else if (isOnlyInterval)
         {
-            delta = new TimeSpan();
+            delta = TimeSpan.Zero;
         }
 
         var utcOffset = oldestInterval.StandardOffset.ToTimeSpan();
@@ -242,11 +252,7 @@ private static void PopulateTimeZoneInfoRecurrenceDates(VTimeZoneInfo tzi, List<
         foreach (var interval in intervals)
         {
             var time = interval.IsoLocalStart.ToDateTimeUnspecified();
-            var date = new CalDateTime(time, true).Add(delta.ToDurationExact()) as CalDateTime;
-            if (date == null)
-            {
-                continue;
-            }
+            var date = new CalDateTime(time, true).Add(delta.ToDurationExact());
 
             tzi.RecurrenceDates.Add(date);
         }
@@ -293,8 +299,8 @@ public VTimeZone(string tzId) : this()
         Location = _nodaZone.Id;
     }
 
-    private DateTimeZone _nodaZone;
-    private string _tzId;
+    private DateTimeZone _nodaZone = DateTimeZone.Utc; // must initialize
+    private string? _tzId;
     public virtual string TzId
     {
         get
@@ -336,8 +342,8 @@ public virtual string TzId
         }
     }
 
-    private Uri _url;
-    public virtual Uri Url
+    private Uri? _url;
+    public virtual Uri? Url
     {
         get => _url ?? (_url = Properties.Get<Uri>("TZURL"));
         set
@@ -347,10 +353,10 @@ public virtual Uri Url
         }
     }
 
-    private string _location;
-    public string Location
+    private string? _location;
+    public string? Location
     {
-        get => _location ?? (_location = Properties.Get<string>("X-LIC-LOCATION"));
+        get => _location ??= Properties.Get<string>("X-LIC-LOCATION");
         set
         {
             _location = value;
@@ -365,20 +371,20 @@ protected bool Equals(VTimeZone other)
             && string.Equals(TzId, other.TzId, StringComparison.OrdinalIgnoreCase)
             && Equals(Url, other.Url);
 
-    public override bool Equals(object obj)
+    public override bool Equals(object? obj)
     {
         if (ReferenceEquals(null, obj)) return false;
         if (ReferenceEquals(this, obj)) return true;
-        if (obj.GetType() != this.GetType()) return false;
-        return Equals((VTimeZone)obj);
+        if (obj.GetType() != GetType()) return false;
+        return Equals((VTimeZone) obj);
     }
 
     public override int GetHashCode()
     {
         unchecked
         {
-            var hashCode = Name.GetHashCode();
-            hashCode = (hashCode * 397) ^ (TzId?.GetHashCode() ?? 0);
+            var hashCode = Name?.GetHashCode() ?? 0;
+            hashCode = (hashCode * 397) ^ (TzId.GetHashCode());
             hashCode = (hashCode * 397) ^ (Url?.GetHashCode() ?? 0);
             return hashCode;
         }
diff --git a/Ical.Net/Evaluation/TodoEvaluator.cs b/Ical.Net/Evaluation/TodoEvaluator.cs
index ba97d0cb8..6678c267d 100644
--- a/Ical.Net/Evaluation/TodoEvaluator.cs
+++ b/Ical.Net/Evaluation/TodoEvaluator.cs
@@ -9,7 +9,6 @@
 using System.Linq;
 using Ical.Net.CalendarComponents;
 using Ical.Net.DataTypes;
-using Ical.Net.Utility;
 
 namespace Ical.Net.Evaluation;
 
@@ -19,31 +18,30 @@ public class TodoEvaluator : RecurringEvaluator
 
     public TodoEvaluator(Todo todo) : base(todo) { }
 
-    internal IEnumerable<Period> EvaluateToPreviousOccurrence(CalDateTime completedDate, CalDateTime currDt, EvaluationOptions options)
+    internal IEnumerable<Period> EvaluateToPreviousOccurrence(CalDateTime completedDate, CalDateTime currDt, EvaluationOptions? options)
     {
         var beginningDate = completedDate.Copy();
 
-        if (Todo.RecurrenceRules != null)
+        foreach (var rrule in Todo.RecurrenceRules)
         {
-            foreach (var rrule in Todo.RecurrenceRules)
-            {
-                DetermineStartingRecurrence(rrule, ref beginningDate);
-            }
+            DetermineStartingRecurrence(rrule, ref beginningDate);
         }
 
         DetermineStartingRecurrence(Todo.RecurrenceDates.GetAllPeriods(), ref beginningDate);
         DetermineStartingRecurrence(Todo.RecurrenceDates.GetAllDates(), ref beginningDate);
 
-        if (Todo.ExceptionRules != null)
+        foreach (var exrule in Todo.ExceptionRules)
         {
-            foreach (var exrule in Todo.ExceptionRules)
-            {
-                DetermineStartingRecurrence(exrule, ref beginningDate);
-            }
+            DetermineStartingRecurrence(exrule, ref beginningDate);
         }
 
         DetermineStartingRecurrence(Todo.ExceptionDates.GetAllDates(), ref beginningDate);
 
+        if (Todo.Start == null)
+        {
+            throw new InvalidOperationException("Todo.Start must not be null.");
+        }
+
         return Evaluate(Todo.Start, beginningDate, currDt.AddSeconds(1), options);
     }
 
@@ -67,6 +65,8 @@ private static void DetermineStartingRecurrence(IEnumerable<CalDateTime> rdate,
 
     private void DetermineStartingRecurrence(RecurrencePattern recur, ref CalDateTime referenceDateTime)
     {
+        if (Todo.Start is null) return;
+
         if (recur.Count.HasValue)
         {
             referenceDateTime = Todo.Start.Copy();
diff --git a/Ical.Net/VTimeZoneInfo.cs b/Ical.Net/VTimeZoneInfo.cs
index a9034d37a..6973eba3b 100644
--- a/Ical.Net/VTimeZoneInfo.cs
+++ b/Ical.Net/VTimeZoneInfo.cs
@@ -47,8 +47,7 @@ protected override void OnDeserializing(StreamingContext context)
 
     public override bool Equals(object? obj)
     {
-        var tzi = obj as VTimeZoneInfo;
-        if (tzi != null)
+        if (obj is VTimeZoneInfo tzi)
         {
             return Equals(TimeZoneName, tzi.TimeZoneName) &&
                    Equals(OffsetFrom, tzi.OffsetFrom) &&
@@ -100,44 +99,32 @@ public virtual string? TimeZoneName
             TimeZoneNames.Add(value ?? string.Empty);
         }
     }
-
-    public virtual UtcOffset TZOffsetFrom
-    {
-        get => OffsetFrom;
-        set => OffsetFrom = value;
-    }
-
-    public virtual UtcOffset OffsetFrom
+    
+    public virtual UtcOffset? OffsetFrom
     {
         get => Properties.Get<UtcOffset>("TZOFFSETFROM");
         set => Properties.Set("TZOFFSETFROM", value);
     }
 
-    public virtual UtcOffset OffsetTo
+    public virtual UtcOffset? OffsetTo
     {
         get => Properties.Get<UtcOffset>("TZOFFSETTO");
         set => Properties.Set("TZOFFSETTO", value);
     }
-
-    public virtual UtcOffset TZOffsetTo
-    {
-        get => OffsetTo;
-        set => OffsetTo = value;
-    }
-
+    
     public virtual IList<string> TimeZoneNames
     {
         get => Properties.GetMany<string>("TZNAME");
         set => Properties.Set("TZNAME", value);
     }
 
-    public virtual CalDateTime DtStart
+    public virtual CalDateTime? DtStart
     {
         get => Start;
         set => Start = value;
     }
 
-    public virtual CalDateTime Start
+    public virtual CalDateTime? Start
     {
         get => Properties.Get<CalDateTime>("DTSTART");
         set => Properties.Set("DTSTART", value);
@@ -171,7 +158,7 @@ public virtual IList<RecurrencePattern> RecurrenceRules
         set => Properties.Set("RRULE", value);
     }
 
-    public virtual CalDateTime RecurrenceId
+    public virtual CalDateTime? RecurrenceId
     {
         get => Properties.Get<CalDateTime>("RECURRENCE-ID");
         set => Properties.Set("RECURRENCE-ID", value);