diff --git a/Ical.Net.Tests/RecurrenceTests.cs b/Ical.Net.Tests/RecurrenceTests.cs
index 62a341960..10fc184cf 100644
--- a/Ical.Net.Tests/RecurrenceTests.cs
+++ b/Ical.Net.Tests/RecurrenceTests.cs
@@ -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), default);
+        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 ad9bf7a95..68f7eeb53 100644
--- a/Ical.Net/CalendarComponents/Alarm.cs
+++ b/Ical.Net/CalendarComponents/Alarm.cs
@@ -141,7 +141,7 @@ public virtual IList<AlarmOccurrence> GetOccurrences(IRecurringComponent rc, Cal
     /// since the provided <paramref name="start"/> date/time.  If <paramref name="start"/>
     /// is null, all triggered alarms will be returned.
     /// </summary>
-    /// <param name="start">The earliest date/time to poll trigered alarms for.</param>
+    /// <param name="start">The earliest date/time to poll triggered alarms for.</param>
     /// <param name="end"></param>
     /// <param name="options"></param>
     /// <returns>A list of <see cref="AlarmOccurrence"/> objects, each containing a triggered alarm.</returns>
@@ -171,12 +171,20 @@ private void AddRepeatedItems(List<AlarmOccurrence> occurrences)
         for (var i = 0; i < len; i++)
         {
             var ao = occurrences[i];
+            if (ao?.DateTime == null || ao.Component == null)
+            {
+                continue;
+            }
+
             var alarmTime = ao.DateTime.Copy();
 
             for (var j = 0; j < Repeat; j++)
             {
-                alarmTime = alarmTime.Add(Duration);
-                occurrences.Add(new AlarmOccurrence(this, alarmTime.Copy(), ao.Component));
+                alarmTime = alarmTime?.Add(Duration);
+                if (alarmTime != null)
+                {
+                    occurrences.Add(new AlarmOccurrence(this, alarmTime.Copy(), ao.Component));
+                }
             }
         }
     }
diff --git a/Ical.Net/DataTypes/AlarmOccurrence.cs b/Ical.Net/DataTypes/AlarmOccurrence.cs
index ea1d5640c..f11a78e24 100644
--- a/Ical.Net/DataTypes/AlarmOccurrence.cs
+++ b/Ical.Net/DataTypes/AlarmOccurrence.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using Ical.Net.CalendarComponents;
 
@@ -18,16 +19,16 @@ namespace Ical.Net.DataTypes;
 /// </remarks>
 public class AlarmOccurrence : IComparable<AlarmOccurrence>
 {
-    public Period Period { get; set; }
+    public Period? Period { get; set; }
 
-    public IRecurringComponent Component { get; set; }
+    public IRecurringComponent? Component { get; set; }
 
-    public Alarm Alarm { get; set; }
+    public Alarm? Alarm { get; set; }
 
-    public CalDateTime DateTime
+    public CalDateTime? DateTime
     {
-        get => Period.StartTime;
-        set => Period = new Period(value);
+        get => Period?.StartTime;
+        set => Period = value != null ? new Period(value) : null;
     }
 
     public AlarmOccurrence(AlarmOccurrence ao)
@@ -44,14 +45,19 @@ public AlarmOccurrence(Alarm a, CalDateTime dt, IRecurringComponent rc)
         Component = rc;
     }
 
-    public int CompareTo(AlarmOccurrence other) => Period.CompareTo(other.Period);
+    public int CompareTo(AlarmOccurrence? other)
+    {
+        if (other == null) return 1;
+        if (Period == null) return other.Period == null ? 0 : -1;
+        return Period.CompareTo(other.Period);
+    }
 
     protected bool Equals(AlarmOccurrence other)
         => Equals(Period, other.Period)
            && Equals(Component, other.Component)
            && Equals(Alarm, other.Alarm);
 
-    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/DataTypes/Attachment.cs b/Ical.Net/DataTypes/Attachment.cs
index 7e277488f..22c567349 100644
--- a/Ical.Net/DataTypes/Attachment.cs
+++ b/Ical.Net/DataTypes/Attachment.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.Linq;
 using System.Text;
@@ -18,8 +19,8 @@ namespace Ical.Net.DataTypes;
 /// </summary>
 public class Attachment : EncodableDataType
 {
-    public virtual Uri Uri { get; set; }
-    public virtual byte[] Data { get; private set; } // private set for CopyFrom
+    public virtual Uri? Uri { get; set; }
+    public virtual byte[]? Data { get; private set; } // private set for CopyFrom
 
     private Encoding _valueEncoding = System.Text.Encoding.UTF8;
     public virtual Encoding ValueEncoding
@@ -43,7 +44,7 @@ public virtual string FormatType
 
     public Attachment() { }
 
-    public Attachment(byte[] value) : this()
+    public Attachment(byte[]? value) : this()
     {
         if (value != null)
         {
@@ -93,15 +94,21 @@ public override void CopyFrom(ICopyable obj)
         FormatType = att.FormatType;
     }
 
-    protected bool Equals(Attachment other)
+    protected bool Equals(Attachment? other)
     {
-        var firstPart = Equals(Uri, other.Uri) && ValueEncoding.Equals(other.ValueEncoding);
-        return Data == null
-            ? firstPart
-            : firstPart && Data.SequenceEqual(other.Data);
+        var firstPart = Equals(Uri, other?.Uri) && ValueEncoding.Equals(other?.ValueEncoding);
+        if (Data == null && other?.Data == null)
+        {
+            return firstPart;
+        }
+        if (Data == null || other?.Data == null)
+        {
+            return false;
+        }
+        return firstPart && Data.SequenceEqual(other.Data);
     }
 
-    public override bool Equals(object obj)
+    public override bool Equals(object? obj)
     {
         if (ReferenceEquals(null, obj)) return false;
         if (ReferenceEquals(this, obj)) return true;
@@ -118,4 +125,4 @@ public override int GetHashCode()
             return hashCode;
         }
     }
-}
\ No newline at end of file
+}
diff --git a/Ical.Net/DataTypes/Attendee.cs b/Ical.Net/DataTypes/Attendee.cs
index 37395ca4d..f4a1b4a9f 100644
--- a/Ical.Net/DataTypes/Attendee.cs
+++ b/Ical.Net/DataTypes/Attendee.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -12,9 +13,9 @@ namespace Ical.Net.DataTypes;
 
 public class Attendee : EncodableDataType
 {
-    private Uri _sentBy;
+    private Uri? _sentBy;
     /// <summary> SENT-BY, to indicate who is acting on behalf of the ATTENDEE </summary>
-    public virtual Uri SentBy
+    public virtual Uri? SentBy
     {
         get
         {
@@ -38,7 +39,7 @@ public virtual Uri SentBy
         }
     }
 
-    private string _commonName;
+    private string? _commonName;
     /// <summary> CN: to show the common or displayable name associated with the calendar address </summary>
     public virtual string CommonName
     {
@@ -61,9 +62,9 @@ public virtual string CommonName
         }
     }
 
-    private Uri _directoryEntry;
+    private Uri? _directoryEntry;
     /// <summary> DIR, to indicate the URI that points to the directory information corresponding to the attendee </summary>
-    public virtual Uri DirectoryEntry
+    public virtual Uri? DirectoryEntry
     {
         get
         {
@@ -87,7 +88,7 @@ public virtual Uri DirectoryEntry
         }
     }
 
-    private string _type;
+    private string? _type;
     /// <summary> CUTYPE: the type of calendar user </summary>
     public virtual string Type
     {
@@ -111,7 +112,7 @@ public virtual string Type
         }
     }
 
-    private List<string> _members;
+    private List<string>? _members;
     /// <summary> MEMBER: the groups the user belongs to </summary>
     public virtual IList<string> Members
     {
@@ -123,7 +124,7 @@ public virtual IList<string> Members
         }
     }
 
-    private string _role;
+    private string? _role;
     /// <summary> ROLE: the intended role the attendee will have </summary>
     public virtual string Role
     {
@@ -146,7 +147,7 @@ public virtual string Role
         }
     }
 
-    private string _participationStatus;
+    private string? _participationStatus;
     public virtual string ParticipationStatus
     {
         get
@@ -179,9 +180,8 @@ public virtual bool Rsvp
                 return _rsvp.Value;
             }
 
-            bool val;
             var rsvp = Parameters.Get("RSVP");
-            if (rsvp != null && bool.TryParse(rsvp, out val))
+            if (rsvp != null && bool.TryParse(rsvp, out var val))
             {
                 _rsvp = val;
                 return _rsvp.Value;
@@ -196,40 +196,32 @@ public virtual bool Rsvp
         }
     }
 
-    private List<string> _delegatedTo;
+    private List<string>? _delegatedTo;
     /// <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")));
         set
         {
-            if (value == null)
-            {
-                return;
-            }
             _delegatedTo = new List<string>(value);
             Parameters.Set("DELEGATED-TO", value);
         }
     }
 
-    private List<string> _delegatedFrom;
+    private List<string>? _delegatedFrom;
     /// <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")));
         set
         {
-            if (value == null)
-            {
-                return;
-            }
             _delegatedFrom = new List<string>(value);
             Parameters.Set("DELEGATED-FROM", value);
         }
     }
 
     /// <summary> Uri associated with the attendee, typically an email address </summary>
-    public virtual Uri Value { get; set; }
+    public virtual Uri? Value { get; set; }
 
     public Attendee() { }
 
@@ -278,8 +270,7 @@ protected bool Equals(Attendee other) => Equals(SentBy, other.SentBy)
                                              && Members.SequenceEqual(other.Members)
                                              && DelegatedTo.SequenceEqual(other.DelegatedTo)
                                              && DelegatedFrom.SequenceEqual(other.DelegatedFrom);
-
-    public override bool Equals(object obj)
+    public override bool Equals(object? obj)
     {
         if (ReferenceEquals(null, obj)) return false;
         if (ReferenceEquals(this, obj)) return true;
@@ -305,4 +296,4 @@ public override int GetHashCode()
             return hashCode;
         }
     }
-}
\ No newline at end of file
+}
diff --git a/Ical.Net/DataTypes/CalendarDataType.cs b/Ical.Net/DataTypes/CalendarDataType.cs
index 48381c8b4..f41fc4c25 100644
--- a/Ical.Net/DataTypes/CalendarDataType.cs
+++ b/Ical.Net/DataTypes/CalendarDataType.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.Runtime.Serialization;
 using Ical.Net.Proxies;
@@ -14,10 +15,11 @@ namespace Ical.Net.DataTypes;
 /// </summary>
 public abstract class CalendarDataType : ICalendarDataType
 {
-    private IParameterCollection _parameters;
-    private ParameterCollectionProxy _proxy;
+    // Well be set with Initialize()
+    private IParameterCollection _parameters = null!;
+    private ParameterCollectionProxy _proxy = null!;
 
-    private ICalendarObject _associatedObject;
+    private ICalendarObject? _associatedObject;
 
     protected CalendarDataType()
     {
@@ -49,10 +51,10 @@ protected virtual void OnDeserializing(StreamingContext context)
 
     protected virtual void OnDeserialized(StreamingContext context) { }
 
-    public virtual Type GetValueType()
+    public virtual Type? GetValueType()
     {
         // See RFC 5545 Section 3.2.20.
-        if (_proxy != null && _proxy.ContainsKey("VALUE"))
+        if (_proxy.ContainsKey("VALUE"))
         {
             switch (_proxy.Get("VALUE"))
             {
@@ -92,12 +94,12 @@ public virtual Type GetValueType()
         return null;
     }
 
-    public virtual void SetValueType(string type)
+    public virtual void SetValueType(string? type)
     {
-        _proxy?.Set("VALUE", type?.ToUpper());
+        _proxy.Set("VALUE", type?.ToUpper());
     }
 
-    public virtual ICalendarObject AssociatedObject
+    public virtual ICalendarObject? AssociatedObject
     {
         get => _associatedObject;
         set
@@ -124,9 +126,9 @@ public virtual ICalendarObject AssociatedObject
         }
     }
 
-    public virtual Calendar Calendar => _associatedObject?.Calendar;
+    public virtual Calendar? Calendar => _associatedObject?.Calendar;
 
-    public virtual string Language
+    public virtual string? Language
     {
         get => Parameters.Get("LANGUAGE");
         set => Parameters.Set("LANGUAGE", value);
@@ -149,7 +151,7 @@ public virtual void CopyFrom(ICopyable obj)
     /// Creates a deep copy of the <see cref="T"/> object.
     /// </summary>
     /// <returns>The copy of the <see cref="T"/> object.</returns>
-    public virtual T Copy<T>()
+    public virtual T? Copy<T>()
     {
         var type = GetType();
         var obj = Activator.CreateInstance(type, true) as ICopyable;
diff --git a/Ical.Net/DataTypes/Duration.cs b/Ical.Net/DataTypes/Duration.cs
index d0313c4c3..4c751ac64 100644
--- a/Ical.Net/DataTypes/Duration.cs
+++ b/Ical.Net/DataTypes/Duration.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.IO;
 using Ical.Net.Serialization.DataTypes;
@@ -118,7 +119,8 @@ public static Duration FromSeconds(int seconds) =>
     /// Parses the specified value according to RFC 5545.
     /// </summary>
     /// <exception cref="System.FormatException">Thrown if the value is not a valid duration.</exception>
-    public static Duration Parse(string value) => (Duration)new DurationSerializer().Deserialize(new StringReader(value));
+    public static Duration Parse(string value) =>
+        (Duration) new DurationSerializer().Deserialize(new StringReader(value))!; // throws if null
 
     /// <summary>
     /// Creates an instance that represents the given time span as exact value, that is, time-only.
@@ -143,32 +145,12 @@ internal static Duration FromTimeSpan(TimeSpan t)
     /// <summary>
     /// Gets a value representing the time parts of the given instance.
     /// </summary>
-    internal TimeSpan TimeAsTimeSpan
-    {
-        get
-        {
-            return new TimeSpan(
-                0,
-                Hours ?? 0,
-                Minutes ?? 0,
-                Seconds ?? 0);
-        }
-    }
+    internal TimeSpan TimeAsTimeSpan => new(0, Hours ?? 0, Minutes ?? 0, Seconds ?? 0);
 
     /// <summary>
     /// Gets a value representing the date parts (days and weeks) of the given instance.
     /// </summary>
-    internal TimeSpan DateAsTimeSpan
-    {
-        get
-        {
-            return new TimeSpan(
-                (Weeks ?? 0) * 7 + (Days ?? 0),
-                0,
-                0,
-                0);
-        }
-    }
+    internal TimeSpan DateAsTimeSpan => new((Weeks ?? 0) * 7 + (Days ?? 0), 0, 0, 0);
 
     /// <summary>
     /// Convert the instance to a <see cref="TimeSpan"/>, ignoring potential
@@ -181,11 +163,7 @@ internal TimeSpan DateAsTimeSpan
     /// nominal durations, use <see cref="ToTimeSpan"/>.
     /// </remarks>
     public TimeSpan ToTimeSpanUnspecified()
-        => new TimeSpan(
-            (Weeks ?? 0) * 7 + (Days ?? 0),
-            Hours ?? 0,
-            Minutes ?? 0,
-            Seconds ?? 0);
+        => new TimeSpan((Weeks ?? 0) * 7 + (Days ?? 0), Hours ?? 0, Minutes ?? 0, Seconds ?? 0);
 
     /// <summary>
     /// Convert the instance to a <see cref="TimeSpan"/>, treating the days as nominal duration and
@@ -226,7 +204,7 @@ internal bool IsEmpty
         new Duration(-d.Weeks, -d.Days, -d.Hours, -d.Minutes, -d.Seconds);
 
     /// <inheritdoc/>
-    public override string ToString()
+    public override string? ToString()
         => new DurationSerializer().SerializeToString(this);
 
     private static int? GetSign(int? v) =>
diff --git a/Ical.Net/DataTypes/ICalendarDataType.cs b/Ical.Net/DataTypes/ICalendarDataType.cs
index ac3ceeda5..eaf4b65ba 100644
--- a/Ical.Net/DataTypes/ICalendarDataType.cs
+++ b/Ical.Net/DataTypes/ICalendarDataType.cs
@@ -3,16 +3,16 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 
 namespace Ical.Net.DataTypes;
 
 public interface ICalendarDataType : ICalendarParameterCollectionContainer, ICopyable
 {
-    Type GetValueType();
+    Type? GetValueType();
     void SetValueType(string type);
-    ICalendarObject AssociatedObject { get; set; }
-    Calendar Calendar { get; }
-
-    string Language { get; set; }
+    ICalendarObject? AssociatedObject { get; set; }
+    Calendar? Calendar { get; }
+    string? Language { get; set; }
 }
diff --git a/Ical.Net/DataTypes/ICalendarParameterCollectionContainer.cs b/Ical.Net/DataTypes/ICalendarParameterCollectionContainer.cs
index 36ca6a57b..18874174c 100644
--- a/Ical.Net/DataTypes/ICalendarParameterCollectionContainer.cs
+++ b/Ical.Net/DataTypes/ICalendarParameterCollectionContainer.cs
@@ -3,9 +3,10 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 namespace Ical.Net.DataTypes;
 
 public interface ICalendarParameterCollectionContainer
 {
     IParameterCollection Parameters { get; }
-}
\ No newline at end of file
+}
diff --git a/Ical.Net/DataTypes/IEncodableDataType.cs b/Ical.Net/DataTypes/IEncodableDataType.cs
index 82cd0283f..83590f870 100644
--- a/Ical.Net/DataTypes/IEncodableDataType.cs
+++ b/Ical.Net/DataTypes/IEncodableDataType.cs
@@ -3,9 +3,10 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 namespace Ical.Net.DataTypes;
 
 public interface IEncodableDataType
 {
-    string Encoding { get; set; }
-}
\ No newline at end of file
+    string? Encoding { get; set; }
+}
diff --git a/Ical.Net/DataTypes/Organizer.cs b/Ical.Net/DataTypes/Organizer.cs
index 1401eccd7..4273eb654 100644
--- a/Ical.Net/DataTypes/Organizer.cs
+++ b/Ical.Net/DataTypes/Organizer.cs
@@ -21,7 +21,7 @@ public virtual Uri? SentBy
     {
         get
         {
-            string sentBy = Parameters.Get("SENT-BY");
+            var sentBy = Parameters.Get("SENT-BY");
             if (!string.IsNullOrWhiteSpace(sentBy))
             {
                 return new Uri(sentBy);
diff --git a/Ical.Net/DataTypes/PeriodList.cs b/Ical.Net/DataTypes/PeriodList.cs
index f43729691..b90bacc11 100644
--- a/Ical.Net/DataTypes/PeriodList.cs
+++ b/Ical.Net/DataTypes/PeriodList.cs
@@ -69,7 +69,7 @@ public override void CopyFrom(ICopyable obj)
 
         foreach (var p in list)
         {
-            Add(p.Copy<Period>());
+            Add(p.Copy<Period>()!);
         }
     }
 
@@ -125,7 +125,7 @@ public void Insert(int index, Period item)
     }
 
     /// <inheritdoc/>
-    public void RemoveAt(int index) => Periods.RemoveAt(index);
+    public void RemoveAt(int index) => Periods?.RemoveAt(index);
 
     /// <summary>
     /// Adds a <see cref="Period"/> to the list if it does not already exist.<br/>
@@ -159,7 +159,7 @@ public void Add(CalDateTime dt)
     public bool Contains(Period item) => Periods.Contains(item);
 
     /// <inheritdoc/>
-    public void CopyTo(Period[] array, int arrayIndex) => Periods.CopyTo(array, arrayIndex);
+    public void CopyTo(Period[] array, int arrayIndex) => Periods?.CopyTo(array, arrayIndex);
 
     /// <inheritdoc/>
     public IEnumerator<Period> GetEnumerator() => Periods.GetEnumerator();
diff --git a/Ical.Net/Evaluation/TodoEvaluator.cs b/Ical.Net/Evaluation/TodoEvaluator.cs
index 5dafb061a..ba97d0cb8 100644
--- a/Ical.Net/Evaluation/TodoEvaluator.cs
+++ b/Ical.Net/Evaluation/TodoEvaluator.cs
@@ -79,7 +79,7 @@ private void DetermineStartingRecurrence(RecurrencePattern recur, ref CalDateTim
 
     public override IEnumerable<Period> Evaluate(CalDateTime referenceDate, CalDateTime? periodStart, CalDateTime? periodEnd, EvaluationOptions? options)
     {
-        // TODO items can only recur if a start date is specified
+        // Items can only recur if a start date is specified
         if (Todo.Start == null)
             return [];