diff --git a/src/Avalonia.Base/AvaloniaObject.cs b/src/Avalonia.Base/AvaloniaObject.cs
index fdd688cf9d..6a9cff6b71 100644
--- a/src/Avalonia.Base/AvaloniaObject.cs
+++ b/src/Avalonia.Base/AvaloniaObject.cs
@@ -7,6 +7,8 @@ using Avalonia.Logging;
 using Avalonia.PropertyStore;
 using Avalonia.Threading;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -17,12 +19,12 @@ namespace Avalonia
     /// </remarks>
     public class AvaloniaObject : IAvaloniaObject, IAvaloniaObjectDebug, INotifyPropertyChanged, IValueSink
     {
-        private IAvaloniaObject _inheritanceParent;
-        private List<IDisposable> _directBindings;
-        private PropertyChangedEventHandler _inpcChanged;
-        private EventHandler<AvaloniaPropertyChangedEventArgs> _propertyChanged;
-        private List<IAvaloniaObject> _inheritanceChildren;
-        private ValueStore _values;
+        private IAvaloniaObject? _inheritanceParent;
+        private List<IDisposable>? _directBindings;
+        private PropertyChangedEventHandler? _inpcChanged;
+        private EventHandler<AvaloniaPropertyChangedEventArgs>? _propertyChanged;
+        private List<IAvaloniaObject>? _inheritanceChildren;
+        private ValueStore? _values;
         private bool _batchUpdate;
 
         /// <summary>
@@ -36,7 +38,7 @@ namespace Avalonia
         /// <summary>
         /// Raised when a <see cref="AvaloniaProperty"/> value changes on this object.
         /// </summary>
-        public event EventHandler<AvaloniaPropertyChangedEventArgs> PropertyChanged
+        public event EventHandler<AvaloniaPropertyChangedEventArgs>? PropertyChanged
         {
             add { _propertyChanged += value; }
             remove { _propertyChanged -= value; }
@@ -58,7 +60,7 @@ namespace Avalonia
         /// <value>
         /// The inheritance parent.
         /// </value>
-        protected IAvaloniaObject InheritanceParent
+        protected IAvaloniaObject? InheritanceParent
         {
             get
             {
@@ -289,7 +291,8 @@ namespace Avalonia
         /// <returns>True if the property is animating, otherwise false.</returns>
         public bool IsAnimating(AvaloniaProperty property)
         {
-            Contract.Requires<ArgumentNullException>(property != null);
+            property = property ?? throw new ArgumentNullException(nameof(property));
+
             VerifyAccess();
 
             return _values?.IsAnimating(property) ?? false;
@@ -306,7 +309,8 @@ namespace Avalonia
         /// </remarks>
         public bool IsSet(AvaloniaProperty property)
         {
-            Contract.Requires<ArgumentNullException>(property != null);
+            property = property ?? throw new ArgumentNullException(nameof(property));
+
             VerifyAccess();
 
             return _values?.IsSet(property) ?? false;
@@ -320,7 +324,7 @@ namespace Avalonia
         /// <param name="priority">The priority of the value.</param>
         public void SetValue(
             AvaloniaProperty property,
-            object value,
+            object? value,
             BindingPriority priority = BindingPriority.LocalValue)
         {
             property = property ?? throw new ArgumentNullException(nameof(property));
@@ -338,7 +342,7 @@ namespace Avalonia
         /// <returns>
         /// An <see cref="IDisposable"/> if setting the property can be undone, otherwise null.
         /// </returns>
-        public IDisposable SetValue<T>(
+        public IDisposable? SetValue<T>(
             StyledPropertyBase<T> property,
             T value,
             BindingPriority priority = BindingPriority.LocalValue)
@@ -497,7 +501,7 @@ namespace Avalonia
         }
 
         /// <inheritdoc/>
-        Delegate[] IAvaloniaObjectDebug.GetPropertyChangedSubscribers()
+        Delegate[]? IAvaloniaObjectDebug.GetPropertyChangedSubscribers()
         {
             return _propertyChanged?.GetInvocationList();
         }
@@ -723,7 +727,8 @@ namespace Avalonia
             {
                 var values = o._values;
 
-                if (values?.TryGetValue(property, maxPriority, out value) == true)
+                if (values != null
+                    && values.TryGetValue(property, maxPriority, out value) == true)
                 {
                     return value;
                 }
@@ -873,7 +878,7 @@ namespace Avalonia
                 }
                 else
                 {
-                    LogBindingError(property, value.Error);
+                    LogBindingError(property, value.Error!);
                 }
             }
         }
@@ -907,14 +912,14 @@ namespace Avalonia
             {
                 _owner = owner;
                 _property = property;
-                _owner._directBindings.Add(this);
+                _owner._directBindings!.Add(this);
                 _subscription = source.Subscribe(this);
             }
 
             public void Dispose()
             {
                 _subscription.Dispose();
-                _owner._directBindings.Remove(this);
+                _owner._directBindings!.Remove(this);
             }
 
             public void OnCompleted() => Dispose();
diff --git a/src/Avalonia.Base/Diagnostics/IAvaloniaObjectDebug.cs b/src/Avalonia.Base/Diagnostics/IAvaloniaObjectDebug.cs
index 7f09425905..4b9f12ddf8 100644
--- a/src/Avalonia.Base/Diagnostics/IAvaloniaObjectDebug.cs
+++ b/src/Avalonia.Base/Diagnostics/IAvaloniaObjectDebug.cs
@@ -1,5 +1,7 @@
 using System;
 
+#nullable enable
+
 namespace Avalonia.Diagnostics
 {
     /// <summary>
@@ -14,6 +16,6 @@ namespace Avalonia.Diagnostics
         /// <returns>
         /// The subscribers or null if no subscribers.
         /// </returns>
-        Delegate[] GetPropertyChangedSubscribers();
+        Delegate[]? GetPropertyChangedSubscribers();
     }
 }
diff --git a/src/Avalonia.Base/IAvaloniaObject.cs b/src/Avalonia.Base/IAvaloniaObject.cs
index 0452f77d4c..2e992f8616 100644
--- a/src/Avalonia.Base/IAvaloniaObject.cs
+++ b/src/Avalonia.Base/IAvaloniaObject.cs
@@ -1,6 +1,8 @@
 using System;
 using Avalonia.Data;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -11,7 +13,7 @@ namespace Avalonia
         /// <summary>
         /// Raised when a <see cref="AvaloniaProperty"/> value changes on this object.
         /// </summary>
-        event EventHandler<AvaloniaPropertyChangedEventArgs> PropertyChanged;
+        event EventHandler<AvaloniaPropertyChangedEventArgs>? PropertyChanged;
 
         /// <summary>
         /// Clears an <see cref="AvaloniaProperty"/>'s local value.
@@ -75,7 +77,10 @@ namespace Avalonia
         /// <param name="property">The property.</param>
         /// <param name="value">The value.</param>
         /// <param name="priority">The priority of the value.</param>
-        IDisposable SetValue<T>(
+        /// <returns>
+        /// An <see cref="IDisposable"/> if setting the property can be undone, otherwise null.
+        /// </returns>
+        IDisposable? SetValue<T>(
             StyledPropertyBase<T> property,
             T value,
             BindingPriority priority = BindingPriority.LocalValue);
diff --git a/src/Avalonia.Styling/StyledElement.cs b/src/Avalonia.Styling/StyledElement.cs
index 5f26f65512..ae14b12dae 100644
--- a/src/Avalonia.Styling/StyledElement.cs
+++ b/src/Avalonia.Styling/StyledElement.cs
@@ -300,7 +300,7 @@ namespace Avalonia
         bool IStyleHost.IsStylesInitialized => _styles != null;
 
         /// <inheritdoc/>
-        IStyleHost? IStyleHost.StylingParent => (IStyleHost)InheritanceParent;
+        IStyleHost? IStyleHost.StylingParent => (IStyleHost?)InheritanceParent;
 
         /// <inheritdoc/>
         public virtual void BeginInit()
diff --git a/src/Avalonia.Styling/Styling/PropertySetterInstance.cs b/src/Avalonia.Styling/Styling/PropertySetterInstance.cs
index b52ae6b146..1c3055fed6 100644
--- a/src/Avalonia.Styling/Styling/PropertySetterInstance.cs
+++ b/src/Avalonia.Styling/Styling/PropertySetterInstance.cs
@@ -50,7 +50,7 @@ namespace Avalonia.Styling
                 }
                 else
                 {
-                    _subscription = _target.Bind(_directProperty, this);
+                    _subscription = _target.Bind(_directProperty!, this);
                 }
             }
             else
@@ -100,7 +100,7 @@ namespace Avalonia.Styling
                 }
                 else
                 {
-                    _target.ClearValue(_directProperty);
+                    _target.ClearValue(_directProperty!);
                 }
             }
 
diff --git a/src/Avalonia.Styling/Styling/PropertySetterLazyInstance.cs b/src/Avalonia.Styling/Styling/PropertySetterLazyInstance.cs
index 03d33faff9..92653d0064 100644
--- a/src/Avalonia.Styling/Styling/PropertySetterLazyInstance.cs
+++ b/src/Avalonia.Styling/Styling/PropertySetterLazyInstance.cs
@@ -53,7 +53,7 @@ namespace Avalonia.Styling
             }
             else
             {
-                _subscription = _target.Bind(_directProperty, this);
+                _subscription = _target.Bind(_directProperty!, this);
             }
         }
 
@@ -91,7 +91,7 @@ namespace Avalonia.Styling
                 }
                 else
                 {
-                    _target.ClearValue(_directProperty);
+                    _target.ClearValue(_directProperty!);
                 }
             }