diff --git a/Ical.Net/CalendarObject.cs b/Ical.Net/CalendarObject.cs
index 0dcfd888..b0190d9f 100644
--- a/Ical.Net/CalendarObject.cs
+++ b/Ical.Net/CalendarObject.cs
@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 using System.Runtime.Serialization;
 using Ical.Net.Collections;
@@ -14,8 +15,9 @@ namespace Ical.Net;
 /// </summary>
 public class CalendarObject : CalendarObjectBase, ICalendarObject
 {
-    private ICalendarObjectList<ICalendarObject> _children;
-    private ServiceProvider _serviceProvider;
+    // Are initialized in the constructor
+    private ICalendarObjectList<ICalendarObject> _children = null!;
+    private ServiceProvider _serviceProvider = null!;
 
     internal CalendarObject()
     {
@@ -50,11 +52,11 @@ private void Initialize()
 
     protected virtual void OnDeserialized(StreamingContext context) { }
 
-    private void Children_ItemAdded(object sender, ObjectEventArgs<ICalendarObject, int> e) => e.First.Parent = this;
+    private void Children_ItemAdded(object? sender, ObjectEventArgs<ICalendarObject, int> e) => e.First.Parent = this;
 
     protected bool Equals(CalendarObject other) => string.Equals(Name, other.Name, StringComparison.OrdinalIgnoreCase);
 
-    public override bool Equals(object obj)
+    public override bool Equals(object? obj)
     {
         if (ReferenceEquals(null, obj)) return false;
         if (ReferenceEquals(this, obj)) return true;
@@ -89,7 +91,7 @@ public override void CopyFrom(ICopyable c)
     /// <summary>
     /// Returns the parent iCalObject that owns this one.
     /// </summary>
-    public virtual ICalendarObject Parent { get; set; }
+    public virtual ICalendarObject? Parent { get; set; }
 
     /// <summary>
     /// A collection of iCalObjects that are children of the current object.
@@ -99,18 +101,18 @@ public override void CopyFrom(ICopyable c)
     /// <summary>
     /// 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.
     /// The setter must be implemented in a derived class.
     /// </summary>
-    public virtual Calendar Calendar
+    public virtual Calendar? Calendar
     {
         get
         {
             ICalendarObject obj = this;
-            while (obj is not Net.Calendar && obj.Parent != null)
+            while (obj is not Net.Calendar && obj?.Parent != null)
             {
                 obj = obj.Parent;
             }
@@ -140,9 +142,9 @@ public virtual Calendar Calendar
 
     public virtual void RemoveService(string name) => _serviceProvider.RemoveService(name);
 
-    public virtual string Group
+    public virtual string? Group
     {
         get => Name;
         set => Name = value;
     }
-}
\ No newline at end of file
+}
diff --git a/Ical.Net/CalendarObjectBase.cs b/Ical.Net/CalendarObjectBase.cs
index 8d3cbcda..34d60a0a 100644
--- a/Ical.Net/CalendarObjectBase.cs
+++ b/Ical.Net/CalendarObjectBase.cs
@@ -3,12 +3,12 @@
 // Licensed under the MIT license.
 //
 
+#nullable enable
 using System;
 
 namespace Ical.Net;
 
-// This class should be declared as abstract
-public class CalendarObjectBase : ICopyable, ILoadable
+public abstract class CalendarObjectBase : ICopyable, ILoadable
 {
     private bool _mIsLoaded = true;
 
@@ -17,20 +17,18 @@ public class CalendarObjectBase : ICopyable, ILoadable
     /// to the current object. This method must be overridden in a derived class.
     /// </summary>
     public virtual void CopyFrom(ICopyable obj)
-    {
-        throw new NotImplementedException("Must be implemented in a derived class.");
-    }
+        => throw new NotImplementedException("Must be implemented in a derived class.");
 
     /// <summary>
     /// 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) as ICopyable;
 
-        if (obj is not T objOfT) return default(T);
+        if (obj is not T objOfT) return default(T?);
 
         obj.CopyFrom(this);
         return objOfT;
@@ -38,11 +36,11 @@ public virtual T Copy<T>()
 
     public virtual bool IsLoaded => _mIsLoaded;
 
-    public event EventHandler Loaded;
+    public event EventHandler? Loaded;
 
     public virtual void OnLoaded()
     {
         _mIsLoaded = true;
         Loaded?.Invoke(this, EventArgs.Empty);
     }
-}
\ No newline at end of file
+}