From 6ce1d94a090b76d29aec46f2b0eab09e16b66404 Mon Sep 17 00:00:00 2001
From: Dariusz Komosinski <darek.komosinski@gmail.com>
Date: Sun, 28 Jun 2020 12:59:53 +0200
Subject: [PATCH 1/3] Remove a few warnings.

---
 src/Avalonia.Base/PropertyStore/IPriorityValueEntry.cs | 7 +------
 src/Avalonia.Styling/IStyledElement.cs                 | 5 -----
 src/Avalonia.Styling/StyledElement.cs                  | 1 -
 src/Avalonia.Styling/Styling/Styles.cs                 | 1 -
 4 files changed, 1 insertion(+), 13 deletions(-)

diff --git a/src/Avalonia.Base/PropertyStore/IPriorityValueEntry.cs b/src/Avalonia.Base/PropertyStore/IPriorityValueEntry.cs
index 6ed6c2ef52..4d82381323 100644
--- a/src/Avalonia.Base/PropertyStore/IPriorityValueEntry.cs
+++ b/src/Avalonia.Base/PropertyStore/IPriorityValueEntry.cs
@@ -1,7 +1,4 @@
-using System;
-using Avalonia.Data;
-
-#nullable enable
+#nullable enable
 
 namespace Avalonia.PropertyStore
 {
@@ -10,8 +7,6 @@ namespace Avalonia.PropertyStore
     /// </summary>
     internal interface IPriorityValueEntry : IValue
     {
-        BindingPriority Priority { get; }
-
         void Reparent(IValueSink sink);
     }
 
diff --git a/src/Avalonia.Styling/IStyledElement.cs b/src/Avalonia.Styling/IStyledElement.cs
index 610c743a3e..37e6ed6fbb 100644
--- a/src/Avalonia.Styling/IStyledElement.cs
+++ b/src/Avalonia.Styling/IStyledElement.cs
@@ -17,11 +17,6 @@ namespace Avalonia
         /// </summary>
         event EventHandler Initialized;
 
-        /// <summary>
-        /// Raised when resources on the element are changed.
-        /// </summary>
-        event EventHandler<ResourcesChangedEventArgs> ResourcesChanged;
-
         /// <summary>
         /// Gets a value that indicates whether the element has finished initialization.
         /// </summary>
diff --git a/src/Avalonia.Styling/StyledElement.cs b/src/Avalonia.Styling/StyledElement.cs
index 05e031c9ec..65885ddebe 100644
--- a/src/Avalonia.Styling/StyledElement.cs
+++ b/src/Avalonia.Styling/StyledElement.cs
@@ -67,7 +67,6 @@ namespace Avalonia
         private List<IStyleInstance>? _appliedStyles;
         private ITemplatedControl? _templatedParent;
         private bool _dataContextUpdating;
-        private bool _notifyingResourcesChanged;
 
         /// <summary>
         /// Initializes static members of the <see cref="StyledElement"/> class.
diff --git a/src/Avalonia.Styling/Styling/Styles.cs b/src/Avalonia.Styling/Styling/Styles.cs
index 7c79060930..c752bdfeb8 100644
--- a/src/Avalonia.Styling/Styling/Styles.cs
+++ b/src/Avalonia.Styling/Styling/Styles.cs
@@ -21,7 +21,6 @@ namespace Avalonia.Styling
         private IResourceHost? _owner;
         private IResourceDictionary? _resources;
         private Dictionary<Type, List<IStyle>?>? _cache;
-        private bool _notifyingResourcesChanged;
 
         public Styles()
         {

From 128fbee51fc77b2b931be1cded41fd723921dc40 Mon Sep 17 00:00:00 2001
From: Dariusz Komosinski <darek.komosinski@gmail.com>
Date: Sun, 28 Jun 2020 13:51:34 +0200
Subject: [PATCH 2/3] Remove nullable and cref warnings.

---
 src/Avalonia.Base/AvaloniaProperty.cs         |  4 +--
 src/Avalonia.Base/AvaloniaPropertyRegistry.cs |  4 +--
 .../Collections/Pooled/IReadOnlyPooledList.cs |  2 ++
 .../Collections/Pooled/PooledList.cs          |  3 +-
 src/Avalonia.Base/Data/BindingValue.cs        | 36 +++++++++----------
 src/Avalonia.Base/Data/Optional.cs            | 10 +++---
 src/Avalonia.Base/DirectPropertyBase.cs       |  2 +-
 src/Avalonia.Base/IStyledPropertyMetadata.cs  |  2 --
 .../PropertyStore/LocalValueEntry.cs          |  4 +--
 src/Avalonia.Base/StyledPropertyMetadata`1.cs |  3 +-
 src/Avalonia.Base/ValueStore.cs               |  2 +-
 11 files changed, 35 insertions(+), 37 deletions(-)

diff --git a/src/Avalonia.Base/AvaloniaProperty.cs b/src/Avalonia.Base/AvaloniaProperty.cs
index daa7191cc5..09480f2701 100644
--- a/src/Avalonia.Base/AvaloniaProperty.cs
+++ b/src/Avalonia.Base/AvaloniaProperty.cs
@@ -159,8 +159,6 @@ namespace Avalonia
         /// </summary>
         internal int Id { get; }
 
-        internal bool HasChangedSubscriptions => _changed?.HasObservers ?? false;
-
         /// <summary>
         /// Provides access to a property's binding via the <see cref="AvaloniaObject"/>
         /// indexer.
@@ -512,7 +510,7 @@ namespace Avalonia
         /// <returns>
         /// An <see cref="IDisposable"/> if setting the property can be undone, otherwise null.
         /// </returns>
-        internal abstract IDisposable? RouteSetValue(
+        internal abstract IDisposable RouteSetValue(
             IAvaloniaObject o,
             object value,
             BindingPriority priority);
diff --git a/src/Avalonia.Base/AvaloniaPropertyRegistry.cs b/src/Avalonia.Base/AvaloniaPropertyRegistry.cs
index 4a3b104f2a..4cde965400 100644
--- a/src/Avalonia.Base/AvaloniaPropertyRegistry.cs
+++ b/src/Avalonia.Base/AvaloniaPropertyRegistry.cs
@@ -362,7 +362,7 @@ namespace Avalonia
         /// <param name="property">The property.</param>
         /// <remarks>
         /// You won't usually want to call this method directly, instead use the
-        /// <see cref="AvaloniaProperty.Register{TOwner, TValue}(string, TValue, bool, Data.BindingMode, Func{TOwner, TValue, TValue}, Action{IAvaloniaObject, bool})"/>
+        /// <see cref="AvaloniaProperty.Register{TOwner, TValue}(string, TValue, bool, Data.BindingMode, Func{TValue, bool}, Func{IAvaloniaObject, TValue, TValue}, Action{IAvaloniaObject, bool})"/>
         /// method.
         /// </remarks>
         public void Register(Type type, AvaloniaProperty property)
@@ -413,7 +413,7 @@ namespace Avalonia
         /// <param name="property">The property.</param>
         /// <remarks>
         /// You won't usually want to call this method directly, instead use the
-        /// <see cref="AvaloniaProperty.RegisterAttached{THost, TValue}(string, Type, TValue, bool, Data.BindingMode, Func{THost, TValue, TValue})"/>
+        /// <see cref="AvaloniaProperty.RegisterAttached{THost, TValue}(string, Type, TValue, bool, Data.BindingMode, Func{TValue, bool}, Func{IAvaloniaObject, TValue, TValue})"/>
         /// method.
         /// </remarks>
         public void RegisterAttached(Type type, AvaloniaProperty property)
diff --git a/src/Avalonia.Base/Collections/Pooled/IReadOnlyPooledList.cs b/src/Avalonia.Base/Collections/Pooled/IReadOnlyPooledList.cs
index 9bc3609dc5..7a233a62ab 100644
--- a/src/Avalonia.Base/Collections/Pooled/IReadOnlyPooledList.cs
+++ b/src/Avalonia.Base/Collections/Pooled/IReadOnlyPooledList.cs
@@ -13,9 +13,11 @@ namespace Avalonia.Collections.Pooled
 
     public interface IReadOnlyPooledList<T> : IReadOnlyList<T>
     {
+#pragma warning disable CS0419
         /// <summary>
         /// Gets a <see cref="System.ReadOnlySpan{T}"/> for the items currently in the collection.
         /// </summary>
+#pragma warning restore CS0419
         ReadOnlySpan<T> Span { get; }
     }
 }
diff --git a/src/Avalonia.Base/Collections/Pooled/PooledList.cs b/src/Avalonia.Base/Collections/Pooled/PooledList.cs
index f0d6b292cc..e50e100d32 100644
--- a/src/Avalonia.Base/Collections/Pooled/PooledList.cs
+++ b/src/Avalonia.Base/Collections/Pooled/PooledList.cs
@@ -138,7 +138,6 @@ namespace Avalonia.Collections.Pooled
         /// initially empty, but will have room for the given number of elements
         /// before any reallocations are required.
         /// </summary>
-        /// <param name="sizeToCapacity">If true, Count of list equals capacity. Depending on ClearMode, rented items may or may not hold dirty values.</param>
         public PooledList(int capacity, ClearMode clearMode, ArrayPool<T> customPool, bool sizeToCapacity)
         {
             if (capacity < 0)
@@ -499,11 +498,13 @@ namespace Avalonia.Collections.Pooled
         public void AddRange(T[] array)
             => AddRange(array.AsSpan());
 
+#pragma warning disable CS0419
         /// <summary>
         /// Adds the elements of the given <see cref="ReadOnlySpan{T}"/> to the end of this list. If
         /// required, the capacity of the list is increased to twice the previous
         /// capacity or the new size, whichever is larger.
         /// </summary>
+#pragma warning restore CS0419
         public void AddRange(ReadOnlySpan<T> span)
         {
             var newSpan = InsertSpan(_size, span.Length, false);
diff --git a/src/Avalonia.Base/Data/BindingValue.cs b/src/Avalonia.Base/Data/BindingValue.cs
index 9aac1bacba..56f37ff89c 100644
--- a/src/Avalonia.Base/Data/BindingValue.cs
+++ b/src/Avalonia.Base/Data/BindingValue.cs
@@ -190,7 +190,7 @@ namespace Avalonia.Data
         /// Gets the value of the binding value if present, otherwise the default value.
         /// </summary>
         /// <returns>The value.</returns>
-        public T GetValueOrDefault() => HasValue ? _value : default;
+        public T GetValueOrDefault() => HasValue ? _value : default!;
 
         /// <summary>
         /// Gets the value of the binding value if present, otherwise a default value.
@@ -209,8 +209,8 @@ namespace Avalonia.Data
         public TResult GetValueOrDefault<TResult>()
         {
             return HasValue ?
-                _value is TResult result ? result : default
-                : default;
+                _value is TResult result ? result : default!
+                : default!;
         }
 
         /// <summary>
@@ -225,7 +225,7 @@ namespace Avalonia.Data
         public TResult GetValueOrDefault<TResult>(TResult defaultValue)
         {
             return HasValue ?
-                _value is TResult result ? result : default
+                _value is TResult result ? result : default!
                 : defaultValue;
         }
 
@@ -242,7 +242,7 @@ namespace Avalonia.Data
                 UnsetValueType _ => Unset,
                 DoNothingType _ => DoNothing,
                 BindingNotification n => n.ToBindingValue().Cast<T>(),
-                _ => (T)value
+                _ => (T)value!
             };
         }
 
@@ -259,18 +259,18 @@ namespace Avalonia.Data
 
         public static implicit operator BindingValue<T>(Optional<T> optional)
         {
-            return optional.HasValue ? optional.Value : Unset;
+            return optional.HasValue ? optional.Value! : Unset;
         }
 
         /// <summary>
         /// Returns a binding value with a type of <see cref="BindingValueType.UnsetValue"/>.
         /// </summary>
-        public static BindingValue<T> Unset => new BindingValue<T>(BindingValueType.UnsetValue, default, null);
+        public static BindingValue<T> Unset => new BindingValue<T>(BindingValueType.UnsetValue, default!, null);
 
         /// <summary>
         /// Returns a binding value with a type of <see cref="BindingValueType.DoNothing"/>.
         /// </summary>
-        public static BindingValue<T> DoNothing => new BindingValue<T>(BindingValueType.DoNothing, default, null);
+        public static BindingValue<T> DoNothing => new BindingValue<T>(BindingValueType.DoNothing, default!, null);
 
         /// <summary>
         /// Returns a binding value with a type of <see cref="BindingValueType.BindingError"/>.
@@ -278,9 +278,9 @@ namespace Avalonia.Data
         /// <param name="e">The binding error.</param>
         public static BindingValue<T> BindingError(Exception e)
         {
-            e = e ?? throw new ArgumentNullException("e");
+            e = e ?? throw new ArgumentNullException(nameof(e));
 
-            return new BindingValue<T>(BindingValueType.BindingError, default, e);
+            return new BindingValue<T>(BindingValueType.BindingError, default!, e);
         }
 
         /// <summary>
@@ -290,7 +290,7 @@ namespace Avalonia.Data
         /// <param name="fallbackValue">The fallback value.</param>
         public static BindingValue<T> BindingError(Exception e, T fallbackValue)
         {
-            e = e ?? throw new ArgumentNullException("e");
+            e = e ?? throw new ArgumentNullException(nameof(e));
 
             return new BindingValue<T>(BindingValueType.BindingErrorWithFallback, fallbackValue, e);
         }
@@ -303,13 +303,13 @@ namespace Avalonia.Data
         /// <param name="fallbackValue">The fallback value.</param>
         public static BindingValue<T> BindingError(Exception e, Optional<T> fallbackValue)
         {
-            e = e ?? throw new ArgumentNullException("e");
+            e = e ?? throw new ArgumentNullException(nameof(e));
 
             return new BindingValue<T>(
                 fallbackValue.HasValue ?
                     BindingValueType.BindingErrorWithFallback :
                     BindingValueType.BindingError,
-                fallbackValue.HasValue ? fallbackValue.Value : default,
+                fallbackValue.HasValue ? fallbackValue.Value : default!,
                 e);
         }
 
@@ -319,9 +319,9 @@ namespace Avalonia.Data
         /// <param name="e">The data validation error.</param>
         public static BindingValue<T> DataValidationError(Exception e)
         {
-            e = e ?? throw new ArgumentNullException("e");
+            e = e ?? throw new ArgumentNullException(nameof(e));
 
-            return new BindingValue<T>(BindingValueType.DataValidationError, default, e);
+            return new BindingValue<T>(BindingValueType.DataValidationError, default!, e);
         }
 
         /// <summary>
@@ -331,7 +331,7 @@ namespace Avalonia.Data
         /// <param name="fallbackValue">The fallback value.</param>
         public static BindingValue<T> DataValidationError(Exception e, T fallbackValue)
         {
-            e = e ?? throw new ArgumentNullException("e");
+            e = e ?? throw new ArgumentNullException(nameof(e));
 
             return new BindingValue<T>(BindingValueType.DataValidationErrorWithFallback, fallbackValue, e);
         }
@@ -344,13 +344,13 @@ namespace Avalonia.Data
         /// <param name="fallbackValue">The fallback value.</param>
         public static BindingValue<T> DataValidationError(Exception e, Optional<T> fallbackValue)
         {
-            e = e ?? throw new ArgumentNullException("e");
+            e = e ?? throw new ArgumentNullException(nameof(e));
 
             return new BindingValue<T>(
                 fallbackValue.HasValue ?
                     BindingValueType.DataValidationErrorWithFallback :
                     BindingValueType.DataValidationError,
-                fallbackValue.HasValue ? fallbackValue.Value : default,
+                fallbackValue.HasValue ? fallbackValue.Value : default!,
                 e);
         }
 
diff --git a/src/Avalonia.Base/Data/Optional.cs b/src/Avalonia.Base/Data/Optional.cs
index dd952c895c..cc064e0faa 100644
--- a/src/Avalonia.Base/Data/Optional.cs
+++ b/src/Avalonia.Base/Data/Optional.cs
@@ -60,7 +60,7 @@ namespace Avalonia.Data
         /// Casts the value (if any) to an <see cref="object"/>.
         /// </summary>
         /// <returns>The cast optional value.</returns>
-        public Optional<object> ToObject() => HasValue ? new Optional<object>(_value) : default;
+        public Optional<object> ToObject() => HasValue ? new Optional<object>(_value!) : default;
 
         /// <inheritdoc/>
         public override string ToString() => HasValue ? _value?.ToString() ?? "(null)" : "(empty)";
@@ -69,7 +69,7 @@ namespace Avalonia.Data
         /// Gets the value if present, otherwise the default value.
         /// </summary>
         /// <returns>The value.</returns>
-        public T GetValueOrDefault() => HasValue ? _value : default;
+        public T GetValueOrDefault() => HasValue ? _value : default!;
 
         /// <summary>
         /// Gets the value if present, otherwise a default value.
@@ -88,8 +88,8 @@ namespace Avalonia.Data
         public TResult GetValueOrDefault<TResult>()
         {
             return HasValue ?
-                _value is TResult result ? result : default
-                : default;
+                _value is TResult result ? result : default!
+                : default!;
         }
 
         /// <summary>
@@ -104,7 +104,7 @@ namespace Avalonia.Data
         public TResult GetValueOrDefault<TResult>(TResult defaultValue)
         {
             return HasValue ?
-                _value is TResult result ? result : default
+                _value is TResult result ? result : default!
                 : defaultValue;
         }
 
diff --git a/src/Avalonia.Base/DirectPropertyBase.cs b/src/Avalonia.Base/DirectPropertyBase.cs
index 0e65379abd..b59e86f8d5 100644
--- a/src/Avalonia.Base/DirectPropertyBase.cs
+++ b/src/Avalonia.Base/DirectPropertyBase.cs
@@ -122,7 +122,7 @@ namespace Avalonia
 
         internal override object RouteGetBaseValue(IAvaloniaObject o, BindingPriority maxPriority)
         {
-            return o.GetValue<TValue>(this);
+            return o.GetValue<TValue>(this)!;
         }
 
         /// <inheritdoc/>
diff --git a/src/Avalonia.Base/IStyledPropertyMetadata.cs b/src/Avalonia.Base/IStyledPropertyMetadata.cs
index f567cd930c..a68b65e5e0 100644
--- a/src/Avalonia.Base/IStyledPropertyMetadata.cs
+++ b/src/Avalonia.Base/IStyledPropertyMetadata.cs
@@ -1,5 +1,3 @@
-using System;
-
 namespace Avalonia
 {
     /// <summary>
diff --git a/src/Avalonia.Base/PropertyStore/LocalValueEntry.cs b/src/Avalonia.Base/PropertyStore/LocalValueEntry.cs
index 59c017bc09..5d3930469d 100644
--- a/src/Avalonia.Base/PropertyStore/LocalValueEntry.cs
+++ b/src/Avalonia.Base/PropertyStore/LocalValueEntry.cs
@@ -15,11 +15,11 @@ namespace Avalonia.PropertyStore
 
         public LocalValueEntry(T value) => _value = value;
         public BindingPriority Priority => BindingPriority.LocalValue;
-        Optional<object> IValue.GetValue() => new Optional<object>(_value);
+        Optional<object> IValue.GetValue() => new Optional<object>(_value!);
         
         public Optional<T> GetValue(BindingPriority maxPriority)
         {
-            return BindingPriority.LocalValue >= maxPriority ? _value : Optional<T>.Empty;
+            return BindingPriority.LocalValue >= maxPriority ? _value! : Optional<T>.Empty;
         }
 
         public void SetValue(T value) => _value = value;
diff --git a/src/Avalonia.Base/StyledPropertyMetadata`1.cs b/src/Avalonia.Base/StyledPropertyMetadata`1.cs
index 300548db0a..cf0a0c34ec 100644
--- a/src/Avalonia.Base/StyledPropertyMetadata`1.cs
+++ b/src/Avalonia.Base/StyledPropertyMetadata`1.cs
@@ -1,5 +1,4 @@
 using System;
-using System.Diagnostics;
 using Avalonia.Data;
 
 namespace Avalonia
@@ -35,7 +34,7 @@ namespace Avalonia
         /// <summary>
         /// Gets the value coercion callback, if any.
         /// </summary>
-        public Func<IAvaloniaObject, TValue, TValue>? CoerceValue { get; private set; }
+        public Func<IAvaloniaObject, TValue, TValue> CoerceValue { get; private set; }
 
         object IStyledPropertyMetadata.DefaultValue => DefaultValue;
 
diff --git a/src/Avalonia.Base/ValueStore.cs b/src/Avalonia.Base/ValueStore.cs
index 05e66f2e0a..0e87a1f242 100644
--- a/src/Avalonia.Base/ValueStore.cs
+++ b/src/Avalonia.Base/ValueStore.cs
@@ -162,7 +162,7 @@ namespace Avalonia
                         _sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
                             _owner,
                             property,
-                            old,
+                            old!,
                             default,
                             BindingPriority.Unset));
                     }

From de6553a0dc52917c84198f10f420ea903fe884ed Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Thu, 2 Jul 2020 11:15:56 +0200
Subject: [PATCH 3/3] Added nullable attributes and use them instead of `!`.

---
 src/Avalonia.Base/Data/BindingValue.cs        |  44 +++---
 src/Avalonia.Base/Data/Optional.cs            |  26 ++--
 src/Avalonia.Base/DirectPropertyBase.cs       |   4 +-
 .../Metadata/NullableAttributes.cs            | 140 ++++++++++++++++++
 .../PropertyStore/LocalValueEntry.cs          |  11 +-
 src/Avalonia.Base/ValueStore.cs               |   2 +-
 6 files changed, 188 insertions(+), 39 deletions(-)
 create mode 100644 src/Avalonia.Base/Metadata/NullableAttributes.cs

diff --git a/src/Avalonia.Base/Data/BindingValue.cs b/src/Avalonia.Base/Data/BindingValue.cs
index 56f37ff89c..6e3c9ae67b 100644
--- a/src/Avalonia.Base/Data/BindingValue.cs
+++ b/src/Avalonia.Base/Data/BindingValue.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 using Avalonia.Utilities;
 
 #nullable enable
@@ -81,14 +82,14 @@ namespace Avalonia.Data
     /// </remarks>
     public readonly struct BindingValue<T>
     {
-        private readonly T _value;
+        [AllowNull] private readonly T _value;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="BindingValue{T}"/> struct with a type of
         /// <see cref="BindingValueType.Value"/>
         /// </summary>
         /// <param name="value">The value.</param>
-        public BindingValue(T value)
+        public BindingValue([AllowNull] T value)
         {
             ValidateValue(value);
             _value = value;
@@ -96,7 +97,7 @@ namespace Avalonia.Data
             Error = null;
         }
 
-        private BindingValue(BindingValueType type, T value, Exception? error)
+        private BindingValue(BindingValueType type, [AllowNull] T value, Exception? error)
         {
             _value = value;
             Type = type;
@@ -154,7 +155,7 @@ namespace Avalonia.Data
                 BindingValueType.UnsetValue => AvaloniaProperty.UnsetValue,
                 BindingValueType.DoNothing => BindingOperations.DoNothing,
                 BindingValueType.Value => _value,
-                BindingValueType.BindingError => 
+                BindingValueType.BindingError =>
                     new BindingNotification(Error, BindingErrorType.Error),
                 BindingValueType.BindingErrorWithFallback =>
                     new BindingNotification(Error, BindingErrorType.Error, Value),
@@ -175,7 +176,7 @@ namespace Avalonia.Data
         /// The binding type is <see cref="BindingValueType.UnsetValue"/> or
         /// <see cref="BindingValueType.DoNothing"/>.
         /// </exception>
-        public BindingValue<T> WithValue(T value)
+        public BindingValue<T> WithValue([AllowNull] T value)
         {
             if (Type == BindingValueType.DoNothing)
             {
@@ -190,7 +191,8 @@ namespace Avalonia.Data
         /// Gets the value of the binding value if present, otherwise the default value.
         /// </summary>
         /// <returns>The value.</returns>
-        public T GetValueOrDefault() => HasValue ? _value : default!;
+        [return: MaybeNull]
+        public T GetValueOrDefault() => HasValue ? _value : default;
 
         /// <summary>
         /// Gets the value of the binding value if present, otherwise a default value.
@@ -206,11 +208,12 @@ namespace Avalonia.Data
         /// The value if present and of the correct type, `default(TResult)` if the value is
         /// not present or of an incorrect type.
         /// </returns>
+        [return: MaybeNull]
         public TResult GetValueOrDefault<TResult>()
         {
             return HasValue ?
-                _value is TResult result ? result : default!
-                : default!;
+                _value is TResult result ? result : default
+                : default;
         }
 
         /// <summary>
@@ -222,10 +225,11 @@ namespace Avalonia.Data
         /// present but not of the correct type or null, or <paramref name="defaultValue"/> if the
         /// value is not present.
         /// </returns>
-        public TResult GetValueOrDefault<TResult>(TResult defaultValue)
+        [return: MaybeNull]
+        public TResult GetValueOrDefault<TResult>([AllowNull] TResult defaultValue)
         {
             return HasValue ?
-                _value is TResult result ? result : default!
+                _value is TResult result ? result : default
                 : defaultValue;
         }
 
@@ -242,7 +246,7 @@ namespace Avalonia.Data
                 UnsetValueType _ => Unset,
                 DoNothingType _ => DoNothing,
                 BindingNotification n => n.ToBindingValue().Cast<T>(),
-                _ => (T)value!
+                _ => new BindingValue<T>((T)value)
             };
         }
 
@@ -250,7 +254,7 @@ namespace Avalonia.Data
         /// Creates a binding value from an instance of the underlying value type.
         /// </summary>
         /// <param name="value">The value.</param>
-        public static implicit operator BindingValue<T>(T value) => new BindingValue<T>(value);
+        public static implicit operator BindingValue<T>([AllowNull] T value) => new BindingValue<T>(value);
 
         /// <summary>
         /// Creates a binding value from an <see cref="Optional{T}"/>.
@@ -259,18 +263,18 @@ namespace Avalonia.Data
 
         public static implicit operator BindingValue<T>(Optional<T> optional)
         {
-            return optional.HasValue ? optional.Value! : Unset;
+            return optional.HasValue ? optional.Value : Unset;
         }
 
         /// <summary>
         /// Returns a binding value with a type of <see cref="BindingValueType.UnsetValue"/>.
         /// </summary>
-        public static BindingValue<T> Unset => new BindingValue<T>(BindingValueType.UnsetValue, default!, null);
+        public static BindingValue<T> Unset => new BindingValue<T>(BindingValueType.UnsetValue, default, null);
 
         /// <summary>
         /// Returns a binding value with a type of <see cref="BindingValueType.DoNothing"/>.
         /// </summary>
-        public static BindingValue<T> DoNothing => new BindingValue<T>(BindingValueType.DoNothing, default!, null);
+        public static BindingValue<T> DoNothing => new BindingValue<T>(BindingValueType.DoNothing, default, null);
 
         /// <summary>
         /// Returns a binding value with a type of <see cref="BindingValueType.BindingError"/>.
@@ -280,7 +284,7 @@ namespace Avalonia.Data
         {
             e = e ?? throw new ArgumentNullException(nameof(e));
 
-            return new BindingValue<T>(BindingValueType.BindingError, default!, e);
+            return new BindingValue<T>(BindingValueType.BindingError, default, e);
         }
 
         /// <summary>
@@ -309,7 +313,7 @@ namespace Avalonia.Data
                 fallbackValue.HasValue ?
                     BindingValueType.BindingErrorWithFallback :
                     BindingValueType.BindingError,
-                fallbackValue.HasValue ? fallbackValue.Value : default!,
+                fallbackValue.HasValue ? fallbackValue.Value : default,
                 e);
         }
 
@@ -321,7 +325,7 @@ namespace Avalonia.Data
         {
             e = e ?? throw new ArgumentNullException(nameof(e));
 
-            return new BindingValue<T>(BindingValueType.DataValidationError, default!, e);
+            return new BindingValue<T>(BindingValueType.DataValidationError, default, e);
         }
 
         /// <summary>
@@ -350,11 +354,11 @@ namespace Avalonia.Data
                 fallbackValue.HasValue ?
                     BindingValueType.DataValidationErrorWithFallback :
                     BindingValueType.DataValidationError,
-                fallbackValue.HasValue ? fallbackValue.Value : default!,
+                fallbackValue.HasValue ? fallbackValue.Value : default,
                 e);
         }
 
-        private static void ValidateValue(T value)
+        private static void ValidateValue([AllowNull] T value)
         {
             if (value is UnsetValueType)
             {
diff --git a/src/Avalonia.Base/Data/Optional.cs b/src/Avalonia.Base/Data/Optional.cs
index cc064e0faa..8e044d7896 100644
--- a/src/Avalonia.Base/Data/Optional.cs
+++ b/src/Avalonia.Base/Data/Optional.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 #nullable enable
 
@@ -22,13 +23,13 @@ namespace Avalonia.Data
     /// </remarks>
     public readonly struct Optional<T> : IEquatable<Optional<T>>
     {
-        private readonly T _value;
+        [AllowNull] private readonly T _value;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Optional{T}"/> struct with value.
         /// </summary>
         /// <param name="value">The value.</param>
-        public Optional(T value)
+        public Optional([AllowNull] T value)
         {
             _value = value;
             HasValue = true;
@@ -48,7 +49,7 @@ namespace Avalonia.Data
         public T Value => HasValue ? _value : throw new InvalidOperationException("Optional has no value.");
 
         /// <inheritdoc/>
-        public override bool Equals(object obj) => obj is Optional<T> o && this == o;
+        public override bool Equals(object? obj) => obj is Optional<T> o && this == o;
 
         /// <inheritdoc/>
         public bool Equals(Optional<T> other) => this == other;
@@ -60,7 +61,7 @@ namespace Avalonia.Data
         /// Casts the value (if any) to an <see cref="object"/>.
         /// </summary>
         /// <returns>The cast optional value.</returns>
-        public Optional<object> ToObject() => HasValue ? new Optional<object>(_value!) : default;
+        public Optional<object> ToObject() => HasValue ? new Optional<object>(_value) : default;
 
         /// <inheritdoc/>
         public override string ToString() => HasValue ? _value?.ToString() ?? "(null)" : "(empty)";
@@ -69,7 +70,8 @@ namespace Avalonia.Data
         /// Gets the value if present, otherwise the default value.
         /// </summary>
         /// <returns>The value.</returns>
-        public T GetValueOrDefault() => HasValue ? _value : default!;
+        [return: MaybeNull]
+        public T GetValueOrDefault() => HasValue ? _value : default;
 
         /// <summary>
         /// Gets the value if present, otherwise a default value.
@@ -85,11 +87,12 @@ namespace Avalonia.Data
         /// The value if present and of the correct type, `default(TResult)` if the value is
         /// not present or of an incorrect type.
         /// </returns>
+        [return: MaybeNull]
         public TResult GetValueOrDefault<TResult>()
         {
             return HasValue ?
-                _value is TResult result ? result : default!
-                : default!;
+                _value is TResult result ? result : default
+                : default;
         }
 
         /// <summary>
@@ -101,10 +104,11 @@ namespace Avalonia.Data
         /// present but not of the correct type or null, or <paramref name="defaultValue"/> if the
         /// value is not present.
         /// </returns>
-        public TResult GetValueOrDefault<TResult>(TResult defaultValue)
+        [return: MaybeNull]
+        public TResult GetValueOrDefault<TResult>([AllowNull] TResult defaultValue)
         {
             return HasValue ?
-                _value is TResult result ? result : default!
+                _value is TResult result ? result : default
                 : defaultValue;
         }
 
@@ -112,7 +116,7 @@ namespace Avalonia.Data
         /// Creates an <see cref="Optional{T}"/> from an instance of the underlying value type.
         /// </summary>
         /// <param name="value">The value.</param>
-        public static implicit operator Optional<T>(T value) => new Optional<T>(value);
+        public static implicit operator Optional<T>([AllowNull] T value) => new Optional<T>(value);
 
         /// <summary>
         /// Compares two <see cref="Optional{T}"/>s for inequality.
@@ -128,7 +132,7 @@ namespace Avalonia.Data
         /// <param name="x">The first value.</param>
         /// <param name="y">The second value.</param>
         /// <returns>True if the values are equal; otherwise false.</returns>
-        public static bool operator==(Optional<T> x, Optional<T> y)
+        public static bool operator ==(Optional<T> x, Optional<T> y)
         {
             if (!x.HasValue && !y.HasValue)
             {
diff --git a/src/Avalonia.Base/DirectPropertyBase.cs b/src/Avalonia.Base/DirectPropertyBase.cs
index b59e86f8d5..d42c030245 100644
--- a/src/Avalonia.Base/DirectPropertyBase.cs
+++ b/src/Avalonia.Base/DirectPropertyBase.cs
@@ -120,9 +120,9 @@ namespace Avalonia
             return o.GetValue<TValue>(this);
         }
 
-        internal override object RouteGetBaseValue(IAvaloniaObject o, BindingPriority maxPriority)
+        internal override object? RouteGetBaseValue(IAvaloniaObject o, BindingPriority maxPriority)
         {
-            return o.GetValue<TValue>(this)!;
+            return o.GetValue<TValue>(this);
         }
 
         /// <inheritdoc/>
diff --git a/src/Avalonia.Base/Metadata/NullableAttributes.cs b/src/Avalonia.Base/Metadata/NullableAttributes.cs
new file mode 100644
index 0000000000..91f5e81863
--- /dev/null
+++ b/src/Avalonia.Base/Metadata/NullableAttributes.cs
@@ -0,0 +1,140 @@
+#pragma warning disable MA0048 // File name must match type name
+#define INTERNAL_NULLABLE_ATTRIBUTES
+#if NETSTANDARD2_0 ||  NETCOREAPP2_0 ||  NETCOREAPP2_1 ||  NETCOREAPP2_2 || NET45 || NET451 || NET452 || NET6 || NET461 || NET462 || NET47 || NET471 || NET472 || NET48
+
+// https://github.com/dotnet/corefx/blob/48363ac826ccf66fbe31a5dcb1dc2aab9a7dd768/src/Common/src/CoreLib/System/Diagnostics/CodeAnalysis/NullableAttributes.cs
+
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>Specifies that null is allowed as an input even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class AllowNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that null is disallowed as an input even if the corresponding type allows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class DisallowNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that an output may be null even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class MaybeNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that an output will not be null even if the corresponding type allows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class NotNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter may be null even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class MaybeNullWhenAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified return value condition.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated parameter may be null.
+        /// </param>
+        public MaybeNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+    }
+
+    /// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter will not be null even if the corresponding type allows it.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class NotNullWhenAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified return value condition.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated parameter will not be null.
+        /// </param>
+        public NotNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+    }
+
+    /// <summary>Specifies that the output will be non-null if the named parameter is non-null.</summary>
+    [AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, AllowMultiple = true, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class NotNullIfNotNullAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the associated parameter name.</summary>
+        /// <param name="parameterName">
+        /// The associated parameter name.  The output will be non-null if the argument to the parameter specified is non-null.
+        /// </param>
+        public NotNullIfNotNullAttribute(string parameterName) => ParameterName = parameterName;
+
+        /// <summary>Gets the associated parameter name.</summary>
+        public string ParameterName { get; }
+    }
+
+    /// <summary>Applied to a method that will never return under any circumstance.</summary>
+    [AttributeUsage(AttributeTargets.Method, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class DoesNotReturnAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that the method will not return if the associated Boolean parameter is passed the specified value.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+#if INTERNAL_NULLABLE_ATTRIBUTES
+    internal
+#else
+    public
+#endif
+        sealed class DoesNotReturnIfAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified parameter value.</summary>
+        /// <param name="parameterValue">
+        /// The condition parameter value. Code after the method will be considered unreachable by diagnostics if the argument to
+        /// the associated parameter matches this value.
+        /// </param>
+        public DoesNotReturnIfAttribute(bool parameterValue) => ParameterValue = parameterValue;
+
+        /// <summary>Gets the condition parameter value.</summary>
+        public bool ParameterValue { get; }
+    }
+}
+#endif
diff --git a/src/Avalonia.Base/PropertyStore/LocalValueEntry.cs b/src/Avalonia.Base/PropertyStore/LocalValueEntry.cs
index 5d3930469d..859e9ba81c 100644
--- a/src/Avalonia.Base/PropertyStore/LocalValueEntry.cs
+++ b/src/Avalonia.Base/PropertyStore/LocalValueEntry.cs
@@ -1,4 +1,5 @@
-using Avalonia.Data;
+using System.Diagnostics.CodeAnalysis;
+using Avalonia.Data;
 
 #nullable enable
 
@@ -11,15 +12,15 @@ namespace Avalonia.PropertyStore
     /// <typeparam name="T">The property type.</typeparam>
     internal class LocalValueEntry<T> : IValue<T>
     {
-        private T _value;
+        [AllowNull] private T _value;
 
-        public LocalValueEntry(T value) => _value = value;
+        public LocalValueEntry([AllowNull] T value) => _value = value;
         public BindingPriority Priority => BindingPriority.LocalValue;
-        Optional<object> IValue.GetValue() => new Optional<object>(_value!);
+        Optional<object> IValue.GetValue() => new Optional<object>(_value);
         
         public Optional<T> GetValue(BindingPriority maxPriority)
         {
-            return BindingPriority.LocalValue >= maxPriority ? _value! : Optional<T>.Empty;
+            return BindingPriority.LocalValue >= maxPriority ? _value : Optional<T>.Empty;
         }
 
         public void SetValue(T value) => _value = value;
diff --git a/src/Avalonia.Base/ValueStore.cs b/src/Avalonia.Base/ValueStore.cs
index 0e87a1f242..6b89fcbdb9 100644
--- a/src/Avalonia.Base/ValueStore.cs
+++ b/src/Avalonia.Base/ValueStore.cs
@@ -162,7 +162,7 @@ namespace Avalonia
                         _sink.ValueChanged(new AvaloniaPropertyChangedEventArgs<T>(
                             _owner,
                             property,
-                            old!,
+                            new Optional<T>(old),
                             default,
                             BindingPriority.Unset));
                     }