Nullable annotations for AvaloniaObject/AvaloniaProperty etc. (#7078)

* Add nullable annotations...

...to `AvaloniaObject`/`AvaloniaProperty`-related classes and a few other uncontroversial files.

* Check for null before first use.

* Don't need this check as base class does it.
# Conflicts:
#	src/Avalonia.Styling/Styling/Activators/PropertyEqualsActivator.cs
#	src/Avalonia.Styling/Styling/PropertyEqualsSelector.cs

src/Avalonia.Animation/Animatable.cs

@@ -232,7 +232,7 @@ namespace Avalonia.Animation
             }
         }
 
-        private object GetAnimationBaseValue(AvaloniaProperty property)
+        private object? GetAnimationBaseValue(AvaloniaProperty property)
         {
             var value = this.GetBaseValue(property, BindingPriority.LocalValue);
 

src/Avalonia.Base/AttachedProperty.cs

@@ -1,5 +1,7 @@
 using System;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -21,7 +23,7 @@ namespace Avalonia
             Type ownerType,
             StyledPropertyMetadata<TValue> metadata,
             bool inherits = false,
-            Func<TValue, bool> validate = null)
+            Func<TValue, bool>? validate = null)
             : base(name, ownerType, metadata, inherits, validate)
         {
         }

src/Avalonia.Base/AvaloniaInternalException.cs

@@ -1,5 +1,7 @@
 using System;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>

src/Avalonia.Base/AvaloniaObject.cs

@@ -103,7 +103,7 @@ namespace Avalonia
         /// Gets or sets the value of a <see cref="AvaloniaProperty"/>.
         /// </summary>
         /// <param name="property">The property.</param>
-        public object this[AvaloniaProperty property]
+        public object? this[AvaloniaProperty property]
         {
             get { return GetValue(property); }
             set { SetValue(property, value); }
@@ -211,7 +211,7 @@ namespace Avalonia
         /// See https://github.com/AvaloniaUI/Avalonia/pull/2747 for the discussion that prompted
         /// this.
         /// </remarks>
-        public sealed override bool Equals(object obj) => base.Equals(obj);
+        public sealed override bool Equals(object? obj) => base.Equals(obj);
 
         /// <summary>
         /// Gets the hash code for the object.
@@ -233,7 +233,7 @@ namespace Avalonia
         /// </summary>
         /// <param name="property">The property.</param>
         /// <returns>The value.</returns>
-        public object GetValue(AvaloniaProperty property)
+        public object? GetValue(AvaloniaProperty property)
         {
             property = property ?? throw new ArgumentNullException(nameof(property));
 
@@ -567,7 +567,7 @@ namespace Avalonia
         /// <param name="oldParent">The old inheritance parent.</param>
         internal void InheritanceParentChanged<T>(
             StyledPropertyBase<T> property,
-            IAvaloniaObject oldParent)
+            IAvaloniaObject? oldParent)
         {
             var oldValue = oldParent switch
             {

src/Avalonia.Base/AvaloniaObjectExtensions.cs

@@ -6,6 +6,8 @@ using System.Reactive.Subjects;
 using Avalonia.Data;
 using Avalonia.Reactive;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -21,7 +23,7 @@ namespace Avalonia
         /// <returns>An <see cref="IBinding"/>.</returns>
         public static IBinding ToBinding<T>(this IObservable<T> source)
         {
-            return new BindingAdaptor(source.Select(x => (object)x));
+            return new BindingAdaptor(source.Select(x => (object?)x));
         }
 
         /// <summary>
@@ -36,12 +38,11 @@ namespace Avalonia
         /// <remarks>
         /// The subscription to <paramref name="o"/> is created using a weak reference.
         /// </remarks>
-        public static IObservable<object> GetObservable(this IAvaloniaObject o, AvaloniaProperty property)
+        public static IObservable<object?> GetObservable(this IAvaloniaObject o, AvaloniaProperty property)
         {
-            Contract.Requires<ArgumentNullException>(o != null);
-            Contract.Requires<ArgumentNullException>(property != null);
-
-            return new AvaloniaPropertyObservable<object>(o, property);
+            return new AvaloniaPropertyObservable<object?>(
+                o ?? throw new ArgumentNullException(nameof(o)), 
+                property ?? throw new ArgumentNullException(nameof(property)));
         }
 
         /// <summary>
@@ -59,10 +60,9 @@ namespace Avalonia
         /// </remarks>
         public static IObservable<T> GetObservable<T>(this IAvaloniaObject o, AvaloniaProperty<T> property)
         {
-            Contract.Requires<ArgumentNullException>(o != null);
-            Contract.Requires<ArgumentNullException>(property != null);
-
-            return new AvaloniaPropertyObservable<T>(o, property);
+            return new AvaloniaPropertyObservable<T>(
+                o ?? throw new ArgumentNullException(nameof(o)),
+                property ?? throw new ArgumentNullException(nameof(property)));
         }
 
         /// <summary>
@@ -77,14 +77,13 @@ namespace Avalonia
         /// <remarks>
         /// The subscription to <paramref name="o"/> is created using a weak reference.
         /// </remarks>
-        public static IObservable<BindingValue<object>> GetBindingObservable(
+        public static IObservable<BindingValue<object?>> GetBindingObservable(
             this IAvaloniaObject o,
             AvaloniaProperty property)
         {
-            Contract.Requires<ArgumentNullException>(o != null);
-            Contract.Requires<ArgumentNullException>(property != null);
-
-            return new AvaloniaPropertyBindingObservable<object>(o, property);
+            return new AvaloniaPropertyBindingObservable<object?>(
+                o ?? throw new ArgumentNullException(nameof(o)),
+                property ?? throw new ArgumentNullException(nameof(property)));
         }
 
         /// <summary>
@@ -104,10 +103,10 @@ namespace Avalonia
             this IAvaloniaObject o,
             AvaloniaProperty<T> property)
         {
-            Contract.Requires<ArgumentNullException>(o != null);
-            Contract.Requires<ArgumentNullException>(property != null);
+            return new AvaloniaPropertyBindingObservable<T>(
+                o ?? throw new ArgumentNullException(nameof(o)),
+                property ?? throw new ArgumentNullException(nameof(property)));
 
-            return new AvaloniaPropertyBindingObservable<T>(o, property);
         }
 
         /// <summary>
@@ -125,10 +124,9 @@ namespace Avalonia
             this IAvaloniaObject o,
             AvaloniaProperty property)
         {
-            Contract.Requires<ArgumentNullException>(o != null);
-            Contract.Requires<ArgumentNullException>(property != null);
-
-            return new AvaloniaPropertyChangedObservable(o, property);
+            return new AvaloniaPropertyChangedObservable(
+                o ?? throw new ArgumentNullException(nameof(o)),
+                property ?? throw new ArgumentNullException(nameof(property)));
         }
 
         /// <summary>
@@ -143,13 +141,13 @@ namespace Avalonia
         /// An <see cref="ISubject{Object}"/> which can be used for two-way binding to/from the 
         /// property.
         /// </returns>
-        public static ISubject<object> GetSubject(
+        public static ISubject<object?> GetSubject(
             this IAvaloniaObject o,
             AvaloniaProperty property,
             BindingPriority priority = BindingPriority.LocalValue)
         {
-            return Subject.Create<object>(
-                Observer.Create<object>(x => o.SetValue(property, x, priority)),
+            return Subject.Create<object?>(
+                Observer.Create<object?>(x => o.SetValue(property, x, priority)),
                 o.GetObservable(property));
         }
 
@@ -188,13 +186,13 @@ namespace Avalonia
         /// An <see cref="ISubject{Object}"/> which can be used for two-way binding to/from the 
         /// property.
         /// </returns>
-        public static ISubject<BindingValue<object>> GetBindingSubject(
+        public static ISubject<BindingValue<object?>> GetBindingSubject(
             this IAvaloniaObject o,
             AvaloniaProperty property,
             BindingPriority priority = BindingPriority.LocalValue)
         {
-            return Subject.Create<BindingValue<object>>(
-                Observer.Create<BindingValue<object>>(x =>
+            return Subject.Create<BindingValue<object?>>(
+                Observer.Create<BindingValue<object?>>(x =>
                 {
                     if (x.HasValue)
                     {
@@ -246,7 +244,7 @@ namespace Avalonia
         public static IDisposable Bind(
             this IAvaloniaObject target,
             AvaloniaProperty property,
-            IObservable<BindingValue<object>> source,
+            IObservable<BindingValue<object?>> source,
             BindingPriority priority = BindingPriority.LocalValue)
         {
             target = target ?? throw new ArgumentNullException(nameof(target));
@@ -298,7 +296,7 @@ namespace Avalonia
         public static IDisposable Bind(
             this IAvaloniaObject target,
             AvaloniaProperty property,
-            IObservable<object> source,
+            IObservable<object?> source,
             BindingPriority priority = BindingPriority.LocalValue)
         {
             target = target ?? throw new ArgumentNullException(nameof(target));
@@ -354,7 +352,7 @@ namespace Avalonia
             this IAvaloniaObject target,
             AvaloniaProperty property,
             IBinding binding,
-            object anchor = null)
+            object? anchor = null)
         {
             target = target ?? throw new ArgumentNullException(nameof(target));
             property = property ?? throw new ArgumentNullException(nameof(property));
@@ -420,7 +418,7 @@ namespace Avalonia
         /// <param name="target">The object.</param>
         /// <param name="property">The property.</param>
         /// <returns>The value.</returns>
-        public static object GetValue(this IAvaloniaObject target, AvaloniaProperty property)
+        public static object? GetValue(this IAvaloniaObject target, AvaloniaProperty property)
         {
             target = target ?? throw new ArgumentNullException(nameof(target));
             property = property ?? throw new ArgumentNullException(nameof(property));
@@ -462,7 +460,7 @@ namespace Avalonia
         /// 
         /// For direct properties returns <see cref="GetValue(IAvaloniaObject, AvaloniaProperty)"/>.
         /// </remarks>
-        public static object GetBaseValue(
+        public static object? GetBaseValue(
             this IAvaloniaObject target,
             AvaloniaProperty property,
             BindingPriority maxPriority)
@@ -517,10 +515,10 @@ namespace Avalonia
         /// <returns>
         /// An <see cref="IDisposable"/> if setting the property can be undone, otherwise null.
         /// </returns>
-        public static IDisposable SetValue(
+        public static IDisposable? SetValue(
             this IAvaloniaObject target,
             AvaloniaProperty property,
-            object value,
+            object? value,
             BindingPriority priority = BindingPriority.LocalValue)
         {
             target = target ?? throw new ArgumentNullException(nameof(target));
@@ -540,7 +538,7 @@ namespace Avalonia
         /// <returns>
         /// An <see cref="IDisposable"/> if setting the property can be undone, otherwise null.
         /// </returns>
-        public static IDisposable SetValue<T>(
+        public static IDisposable? SetValue<T>(
             this IAvaloniaObject target,
             AvaloniaProperty<T> property,
             T value,
@@ -633,9 +631,9 @@ namespace Avalonia
 
         private class BindingAdaptor : IBinding
         {
-            private IObservable<object> _source;
+            private IObservable<object?> _source;
 
-            public BindingAdaptor(IObservable<object> source)
+            public BindingAdaptor(IObservable<object?> source)
             {
                 this._source = source;
             }
@@ -643,7 +641,7 @@ namespace Avalonia
             public InstancedBinding Initiate(
                 IAvaloniaObject target,
                 AvaloniaProperty targetProperty,
-                object anchor = null,
+                object? anchor = null,
                 bool enableDataValidation = false)
             {
                 return InstancedBinding.OneWay(_source);

src/Avalonia.Base/AvaloniaProperty.cs

@@ -4,6 +4,8 @@ using Avalonia.Data;
 using Avalonia.Data.Core;
 using Avalonia.Utilities;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -36,12 +38,9 @@ namespace Avalonia
             Type valueType,
             Type ownerType,
             AvaloniaPropertyMetadata metadata,
-            Action<IAvaloniaObject, bool> notifying = null)
+            Action<IAvaloniaObject, bool>? notifying = null)
         {
-            Contract.Requires<ArgumentNullException>(name != null);
-            Contract.Requires<ArgumentNullException>(valueType != null);
-            Contract.Requires<ArgumentNullException>(ownerType != null);
-            Contract.Requires<ArgumentNullException>(metadata != null);
+            _ = name ?? throw new ArgumentNullException(nameof(name));
 
             if (name.Contains("."))
             {
@@ -51,12 +50,12 @@ namespace Avalonia
             _metadata = new Dictionary<Type, AvaloniaPropertyMetadata>();
 
             Name = name;
-            PropertyType = valueType;
-            OwnerType = ownerType;
+            PropertyType = valueType ?? throw new ArgumentNullException(nameof(valueType));
+            OwnerType = ownerType ?? throw new ArgumentNullException(nameof(ownerType));
             Notifying = notifying;
             Id = s_nextId++;
 
-            _metadata.Add(ownerType, metadata);
+            _metadata.Add(ownerType, metadata ?? throw new ArgumentNullException(nameof(metadata)));
             _defaultMetadata = metadata;
         }
 
@@ -69,16 +68,13 @@ namespace Avalonia
         protected AvaloniaProperty(
             AvaloniaProperty source,
             Type ownerType,
-            AvaloniaPropertyMetadata metadata)
+            AvaloniaPropertyMetadata? metadata)
         {
-            Contract.Requires<ArgumentNullException>(source != null);
-            Contract.Requires<ArgumentNullException>(ownerType != null);
-
             _metadata = new Dictionary<Type, AvaloniaPropertyMetadata>();
 
-            Name = source.Name;
+            Name = source?.Name ?? throw new ArgumentNullException(nameof(source));
             PropertyType = source.PropertyType;
-            OwnerType = ownerType;
+            OwnerType = ownerType ?? throw new ArgumentNullException(nameof(ownerType));
             Notifying = source.Notifying;
             Id = source.Id;
             _defaultMetadata = source._defaultMetadata;
@@ -149,7 +145,7 @@ namespace Avalonia
         /// will be true before the property change notifications are sent and false afterwards. This
         /// callback is intended to support Control.IsDataContextChanging.
         /// </remarks>
-        public Action<IAvaloniaObject, bool> Notifying { get; }
+        public Action<IAvaloniaObject, bool>? Notifying { get; }
 
         /// <summary>
         /// Gets the integer ID that represents this property.
@@ -192,13 +188,13 @@ namespace Avalonia
         /// <param name="a">The first property.</param>
         /// <param name="b">The second property.</param>
         /// <returns>True if the properties are equal, otherwise false.</returns>
-        public static bool operator ==(AvaloniaProperty a, AvaloniaProperty b)
+        public static bool operator ==(AvaloniaProperty? a, AvaloniaProperty? b)
         {
             if (object.ReferenceEquals(a, b))
             {
                 return true;
             }
-            else if (((object)a == null) || ((object)b == null))
+            else if (a is null || b is null)
             {
                 return false;
             }
@@ -214,7 +210,7 @@ namespace Avalonia
         /// <param name="a">The first property.</param>
         /// <param name="b">The second property.</param>
         /// <returns>True if the properties are equal, otherwise false.</returns>
-        public static bool operator !=(AvaloniaProperty a, AvaloniaProperty b)
+        public static bool operator !=(AvaloniaProperty? a, AvaloniaProperty? b)
         {
             return !(a == b);
         }
@@ -238,15 +234,15 @@ namespace Avalonia
         /// <returns>A <see cref="StyledProperty{TValue}"/></returns>
         public static StyledProperty<TValue> Register<TOwner, TValue>(
             string name,
-            TValue defaultValue = default(TValue),
+            TValue defaultValue = default!,
             bool inherits = false,
             BindingMode defaultBindingMode = BindingMode.OneWay,
-            Func<TValue, bool> validate = null,
-            Func<IAvaloniaObject, TValue, TValue> coerce = null,
-            Action<IAvaloniaObject, bool> notifying = null)
+            Func<TValue, bool>? validate = null,
+            Func<IAvaloniaObject, TValue, TValue>? coerce = null,
+            Action<IAvaloniaObject, bool>? notifying = null)
                 where TOwner : IAvaloniaObject
         {
-            Contract.Requires<ArgumentNullException>(name != null);
+            _ = name ?? throw new ArgumentNullException(nameof(name));
 
             var metadata = new StyledPropertyMetadata<TValue>(
                 defaultValue,
@@ -279,14 +275,14 @@ namespace Avalonia
         /// <returns>A <see cref="AvaloniaProperty{TValue}"/></returns>
         public static AttachedProperty<TValue> RegisterAttached<TOwner, THost, TValue>(
             string name,
-            TValue defaultValue = default(TValue),
+            TValue defaultValue = default!,
             bool inherits = false,
             BindingMode defaultBindingMode = BindingMode.OneWay,
-            Func<TValue, bool> validate = null,
-            Func<IAvaloniaObject, TValue, TValue> coerce = null)
+            Func<TValue, bool>? validate = null,
+            Func<IAvaloniaObject, TValue, TValue>? coerce = null)
                 where THost : IAvaloniaObject
         {
-            Contract.Requires<ArgumentNullException>(name != null);
+            _ = name ?? throw new ArgumentNullException(nameof(name));
 
             var metadata = new StyledPropertyMetadata<TValue>(
                 defaultValue,
@@ -316,14 +312,14 @@ namespace Avalonia
         public static AttachedProperty<TValue> RegisterAttached<THost, TValue>(
             string name,
             Type ownerType,
-            TValue defaultValue = default(TValue),
+            TValue defaultValue = default!,
             bool inherits = false,
             BindingMode defaultBindingMode = BindingMode.OneWay,
-            Func<TValue, bool> validate = null,
-            Func<IAvaloniaObject, TValue, TValue> coerce = null)
+            Func<TValue, bool>? validate = null,
+            Func<IAvaloniaObject, TValue, TValue>? coerce = null)
                 where THost : IAvaloniaObject
         {
-            Contract.Requires<ArgumentNullException>(name != null);
+            _ = name ?? throw new ArgumentNullException(nameof(name));
 
             var metadata = new StyledPropertyMetadata<TValue>(
                 defaultValue,
@@ -354,14 +350,14 @@ namespace Avalonia
         public static DirectProperty<TOwner, TValue> RegisterDirect<TOwner, TValue>(
             string name,
             Func<TOwner, TValue> getter,
-            Action<TOwner, TValue> setter = null,
-            TValue unsetValue = default(TValue),
+            Action<TOwner, TValue>? setter = null,
+            TValue unsetValue = default!,
             BindingMode defaultBindingMode = BindingMode.OneWay,
             bool enableDataValidation = false)
                 where TOwner : IAvaloniaObject
         {
-            Contract.Requires<ArgumentNullException>(name != null);
-            Contract.Requires<ArgumentNullException>(getter != null);
+            _ = name ?? throw new ArgumentNullException(nameof(name));
+            _ = getter ?? throw new ArgumentNullException(nameof(getter));
 
             var metadata = new DirectPropertyMetadata<TValue>(
                 unsetValue: unsetValue,
@@ -394,16 +390,16 @@ namespace Avalonia
         }
 
         /// <inheritdoc/>
-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
         {
             var p = obj as AvaloniaProperty;
-            return p != null && Equals(p);
+            return p is not null && Equals(p);
         }
 
         /// <inheritdoc/>
-        public bool Equals(AvaloniaProperty other)
+        public bool Equals(AvaloniaProperty? other)
         {
-            return other != null && Id == other.Id;
+            return Id == other?.Id;
         }
 
         /// <inheritdoc/>
@@ -431,7 +427,6 @@ namespace Avalonia
         /// <returns>
         /// The property metadata.
         /// </returns>
-        ///
         public AvaloniaPropertyMetadata GetMetadata(Type type)
         {
             if (!_hasMetadataOverrides)
@@ -447,9 +442,9 @@ namespace Avalonia
         /// </summary>
         /// <param name="value">The value.</param>
         /// <returns>True if the value is valid, otherwise false.</returns>
-        public bool IsValidValue(object value)
+        public bool IsValidValue(object? value)
         {
-            return TypeUtilities.TryConvertImplicit(PropertyType, value, out value);
+            return TypeUtilities.TryConvertImplicit(PropertyType, value, out _);
         }
 
         /// <summary>
@@ -480,14 +475,14 @@ namespace Avalonia
         /// Routes an untyped GetValue call to a typed call.
         /// </summary>
         /// <param name="o">The object instance.</param>
-        internal abstract object RouteGetValue(IAvaloniaObject o);
+        internal abstract object? RouteGetValue(IAvaloniaObject o);
 
         /// <summary>
         /// Routes an untyped GetBaseValue call to a typed call.
         /// </summary>
         /// <param name="o">The object instance.</param>
         /// <param name="maxPriority">The maximum priority for the value.</param>
-        internal abstract object RouteGetBaseValue(IAvaloniaObject o, BindingPriority maxPriority);
+        internal abstract object? RouteGetBaseValue(IAvaloniaObject o, BindingPriority maxPriority);
 
         /// <summary>
         /// Routes an untyped SetValue call to a typed call.
@@ -498,9 +493,9 @@ 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,
+            object? value,
             BindingPriority priority);
 
         /// <summary>
@@ -511,10 +506,10 @@ namespace Avalonia
         /// <param name="priority">The priority.</param>
         internal abstract IDisposable RouteBind(
             IAvaloniaObject o,
-            IObservable<BindingValue<object>> source,
+            IObservable<BindingValue<object?>> source,
             BindingPriority priority);
 
-        internal abstract void RouteInheritanceParentChanged(AvaloniaObject o, IAvaloniaObject oldParent);
+        internal abstract void RouteInheritanceParentChanged(AvaloniaObject o, IAvaloniaObject? oldParent);
 
         /// <summary>
         /// Overrides the metadata for the property on the specified type.
@@ -523,8 +518,8 @@ namespace Avalonia
         /// <param name="metadata">The metadata.</param>
         protected void OverrideMetadata(Type type, AvaloniaPropertyMetadata metadata)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
-            Contract.Requires<ArgumentNullException>(metadata != null);
+            _ = type ?? throw new ArgumentNullException(nameof(type));
+            _ = metadata ?? throw new ArgumentNullException(nameof(metadata));
 
             if (_metadata.ContainsKey(type))
             {
@@ -575,8 +570,8 @@ namespace Avalonia
 
         bool IPropertyInfo.CanGet => true;
         bool IPropertyInfo.CanSet => true;
-        object IPropertyInfo.Get(object target) => ((AvaloniaObject)target).GetValue(this);
-        void IPropertyInfo.Set(object target, object value) => ((AvaloniaObject)target).SetValue(this, value);
+        object? IPropertyInfo.Get(object target) => ((AvaloniaObject)target).GetValue(this);
+        void IPropertyInfo.Set(object target, object? value) => ((AvaloniaObject)target).SetValue(this, value);
     }
 
     /// <summary>

src/Avalonia.Base/AvaloniaPropertyMetadata.cs

@@ -1,5 +1,7 @@
 using Avalonia.Data;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>

src/Avalonia.Base/AvaloniaPropertyRegistry.cs

@@ -2,6 +2,8 @@ using System;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -44,7 +46,7 @@ namespace Avalonia
         /// <returns>A collection of <see cref="AvaloniaProperty"/> definitions.</returns>
         public IReadOnlyList<AvaloniaProperty> GetRegistered(Type type)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
+            _ = type ?? throw new ArgumentNullException(nameof(type));
 
             if (_registeredCache.TryGetValue(type, out var result))
             {
@@ -78,7 +80,7 @@ namespace Avalonia
         /// <returns>A collection of <see cref="AvaloniaProperty"/> definitions.</returns>
         public IReadOnlyList<AvaloniaProperty> GetRegisteredAttached(Type type)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
+            _ = type ?? throw new ArgumentNullException(nameof(type));
 
             if (_attachedCache.TryGetValue(type, out var result))
             {
@@ -109,7 +111,7 @@ namespace Avalonia
         /// <returns>A collection of <see cref="AvaloniaProperty"/> definitions.</returns>
         public IReadOnlyList<AvaloniaProperty> GetRegisteredDirect(Type type)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
+            _ = type ?? throw new ArgumentNullException(nameof(type));
 
             if (_directCache.TryGetValue(type, out var result))
             {
@@ -140,7 +142,7 @@ namespace Avalonia
         /// <returns>A collection of <see cref="AvaloniaProperty"/> definitions.</returns>
         public IReadOnlyList<AvaloniaProperty> GetRegisteredInherited(Type type)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
+            _ = type ?? throw new ArgumentNullException(nameof(type));
 
             if (_inheritedCache.TryGetValue(type, out var result))
             {
@@ -191,18 +193,18 @@ namespace Avalonia
         /// <returns>A collection of <see cref="AvaloniaProperty"/> definitions.</returns>
         public IReadOnlyList<AvaloniaProperty> GetRegistered(IAvaloniaObject o)
         {
-            Contract.Requires<ArgumentNullException>(o != null);
+            _ = o ?? throw new ArgumentNullException(nameof(o));
 
             return GetRegistered(o.GetType());
         }
 
         /// <summary>
-        /// Finds a direct property as registered on an object.
+        /// Gets a direct property as registered on an object.
         /// </summary>
         /// <param name="o">The object.</param>
         /// <param name="property">The direct property.</param>
         /// <returns>
-        /// The registered property or null if no matching property found.
+        /// The registered.
         /// </returns>
         public DirectPropertyBase<T> GetRegisteredDirect<T>(
             IAvaloniaObject o,
@@ -223,10 +225,10 @@ namespace Avalonia
         /// <exception cref="InvalidOperationException">
         /// The property name contains a '.'.
         /// </exception>
-        public AvaloniaProperty FindRegistered(Type type, string name)
+        public AvaloniaProperty? FindRegistered(Type type, string name)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
-            Contract.Requires<ArgumentNullException>(name != null);
+            _ = type ?? throw new ArgumentNullException(nameof(type));
+            _ = name ?? throw new ArgumentNullException(nameof(name));
 
             if (name.Contains("."))
             {
@@ -260,10 +262,10 @@ namespace Avalonia
         /// <exception cref="InvalidOperationException">
         /// The property name contains a '.'.
         /// </exception>
-        public AvaloniaProperty FindRegistered(IAvaloniaObject o, string name)
+        public AvaloniaProperty? FindRegistered(IAvaloniaObject o, string name)
         {
-            Contract.Requires<ArgumentNullException>(o != null);
-            Contract.Requires<ArgumentNullException>(name != null);
+            _ = o ?? throw new ArgumentNullException(nameof(o));
+            _ = name ?? throw new ArgumentNullException(nameof(name));
 
             return FindRegistered(o.GetType(), name);
         }
@@ -276,7 +278,7 @@ namespace Avalonia
         /// <returns>
         /// The registered property or null if no matching property found.
         /// </returns>
-        public DirectPropertyBase<T> FindRegisteredDirect<T>(
+        public DirectPropertyBase<T>? FindRegisteredDirect<T>(
             IAvaloniaObject o,
             DirectPropertyBase<T> property)
         {
@@ -306,7 +308,7 @@ namespace Avalonia
         /// </summary>
         /// <param name="id">The property Id.</param>
         /// <returns>The registered property or null if no matching property found.</returns>
-        internal AvaloniaProperty FindRegistered(int id)
+        internal AvaloniaProperty? FindRegistered(int id)
         {
             return id < _properties.Count ? _properties[id] : null;
         }
@@ -319,8 +321,8 @@ namespace Avalonia
         /// <returns>True if the property is registered, otherwise false.</returns>
         public bool IsRegistered(Type type, AvaloniaProperty property)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
-            Contract.Requires<ArgumentNullException>(property != null);
+            _ = type ?? throw new ArgumentNullException(nameof(type));
+            _ = property ?? throw new ArgumentNullException(nameof(property));
 
             static bool ContainsProperty(IReadOnlyList<AvaloniaProperty> properties, AvaloniaProperty property)
             {
@@ -349,8 +351,8 @@ namespace Avalonia
         /// <returns>True if the property is registered, otherwise false.</returns>
         public bool IsRegistered(object o, AvaloniaProperty property)
         {
-            Contract.Requires<ArgumentNullException>(o != null);
-            Contract.Requires<ArgumentNullException>(property != null);
+            _ = o ?? throw new ArgumentNullException(nameof(o));
+            _ = property ?? throw new ArgumentNullException(nameof(property));
 
             return IsRegistered(o.GetType(), property);
         }
@@ -367,8 +369,8 @@ namespace Avalonia
         /// </remarks>
         public void Register(Type type, AvaloniaProperty property)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
-            Contract.Requires<ArgumentNullException>(property != null);
+            _ = type ?? throw new ArgumentNullException(nameof(type));
+            _ = property ?? throw new ArgumentNullException(nameof(property));
 
             if (!_registered.TryGetValue(type, out var inner))
             {
@@ -418,8 +420,8 @@ namespace Avalonia
         /// </remarks>
         public void RegisterAttached(Type type, AvaloniaProperty property)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
-            Contract.Requires<ArgumentNullException>(property != null);
+            _ = type ?? throw new ArgumentNullException(nameof(type));
+            _ = property ?? throw new ArgumentNullException(nameof(property));
 
             if (!property.IsAttached)
             {

src/Avalonia.Base/AvaloniaProperty`1.cs

@@ -3,6 +3,8 @@ using System.Reactive.Subjects;
 using Avalonia.Data;
 using Avalonia.Utilities;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -24,7 +26,7 @@ namespace Avalonia
             string name,
             Type ownerType,
             AvaloniaPropertyMetadata metadata,
-            Action<IAvaloniaObject, bool> notifying = null)
+            Action<IAvaloniaObject, bool>? notifying = null)
             : base(name, typeof(TValue), ownerType, metadata, notifying)
         {
             _changed = new Subject<AvaloniaPropertyChangedEventArgs<TValue>>();
@@ -40,7 +42,7 @@ namespace Avalonia
         protected AvaloniaProperty(
             AvaloniaProperty source,
             Type ownerType,
-            AvaloniaPropertyMetadata metadata)
+            AvaloniaPropertyMetadata? metadata)
             : this(source as AvaloniaProperty<TValue> ?? throw new InvalidOperationException(), ownerType, metadata)
         {
         }
@@ -54,7 +56,7 @@ namespace Avalonia
         protected AvaloniaProperty(
             AvaloniaProperty<TValue> source,
             Type ownerType,
-            AvaloniaPropertyMetadata metadata)
+            AvaloniaPropertyMetadata? metadata)
             : base(source, ownerType, metadata)
         {
             _changed = source._changed;
@@ -82,7 +84,7 @@ namespace Avalonia
 
         protected override IObservable<AvaloniaPropertyChangedEventArgs> GetChanged() => Changed;
 
-        protected BindingValue<object> TryConvert(object value)
+        protected BindingValue<object> TryConvert(object? value)
         {
             if (value == UnsetValue)
             {

src/Avalonia.Base/Data/BindingValue.cs

@@ -416,7 +416,7 @@ namespace Avalonia.Data
         /// <remarks>
         /// Note that this method uses reflection and as such may be slow.
         /// </remarks>
-        public static BindingValue<T> Convert<T>(this BindingValue<object> value)
+        public static BindingValue<T> Convert<T>(this BindingValue<object?> value)
         {
             return value.Type switch
             {

src/Avalonia.Base/Data/Core/IPropertyInfo.cs

@@ -1,12 +1,14 @@
 using System;
 
+#nullable enable
+
 namespace Avalonia.Data.Core
 {
     public interface IPropertyInfo
     {
         string Name { get; }
-        object Get(object target);
-        void Set(object target, object value);
+        object? Get(object target);
+        void Set(object target, object? value);
         bool CanSet { get; }
         bool CanGet { get; }
         Type PropertyType { get; }

src/Avalonia.Base/DirectProperty.cs

@@ -1,6 +1,9 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 using Avalonia.Data;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -26,13 +29,11 @@ namespace Avalonia
         public DirectProperty(
             string name,
             Func<TOwner, TValue> getter,
-            Action<TOwner, TValue> setter,
+            Action<TOwner, TValue>? setter,
             DirectPropertyMetadata<TValue> metadata)
             : base(name, typeof(TOwner), metadata)
         {
-            Contract.Requires<ArgumentNullException>(getter != null);
-
-            Getter = getter;
+            Getter = getter ?? throw new ArgumentNullException(nameof(getter));
             Setter = setter;
         }
 
@@ -46,13 +47,11 @@ namespace Avalonia
         private DirectProperty(
             DirectPropertyBase<TValue> source,
             Func<TOwner, TValue> getter,
-            Action<TOwner, TValue> setter,
+            Action<TOwner, TValue>? setter,
             DirectPropertyMetadata<TValue> metadata)
             : base(source, typeof(TOwner), metadata)
         {
-            Contract.Requires<ArgumentNullException>(getter != null);
-
-            Getter = getter;
+            Getter = getter ?? throw new ArgumentNullException(nameof(getter));
             Setter = setter;
         }
 
@@ -73,7 +72,7 @@ namespace Avalonia
         /// <summary>
         /// Gets the setter function.
         /// </summary>
-        public Action<TOwner, TValue> Setter { get; }
+        public Action<TOwner, TValue>? Setter { get; }
 
         /// <summary>
         /// Registers the direct property on another type.
@@ -91,8 +90,8 @@ namespace Avalonia
         /// <returns>The property.</returns>
         public DirectProperty<TNewOwner, TValue> AddOwner<TNewOwner>(
             Func<TNewOwner, TValue> getter,
-            Action<TNewOwner, TValue> setter = null,
-            TValue unsetValue = default(TValue),
+            Action<TNewOwner, TValue>? setter = null,
+            TValue unsetValue = default!,
             BindingMode defaultBindingMode = BindingMode.Default,
             bool enableDataValidation = false)
                 where TNewOwner : AvaloniaObject
@@ -131,7 +130,7 @@ namespace Avalonia
         public DirectProperty<TNewOwner, TValue> AddOwnerWithDataValidation<TNewOwner>(
             Func<TNewOwner, TValue> getter,
             Action<TNewOwner,TValue> setter,
-            TValue unsetValue = default(TValue),
+            TValue unsetValue = default!,
             BindingMode defaultBindingMode = BindingMode.Default,
             bool enableDataValidation = false)
                 where TNewOwner : AvaloniaObject
@@ -174,20 +173,20 @@ namespace Avalonia
         }
 
         /// <inheritdoc/>
-        object IDirectPropertyAccessor.GetValue(IAvaloniaObject instance)
+        object? IDirectPropertyAccessor.GetValue(IAvaloniaObject instance)
         {
             return Getter((TOwner)instance);
         }
 
         /// <inheritdoc/>
-        void IDirectPropertyAccessor.SetValue(IAvaloniaObject instance, object value)
+        void IDirectPropertyAccessor.SetValue(IAvaloniaObject instance, object? value)
         {
             if (Setter == null)
             {
                 throw new ArgumentException($"The property {Name} is readonly.");
             }
 
-            Setter((TOwner)instance, (TValue)value);
+            Setter((TOwner)instance, (TValue)value!);
         }
     }
 }

src/Avalonia.Base/DirectPropertyBase.cs

@@ -148,7 +148,7 @@ namespace Avalonia
         /// <inheritdoc/>
         internal override IDisposable? RouteSetValue(
             IAvaloniaObject o,
-            object value,
+            object? value,
             BindingPriority priority)
         {
             var v = TryConvert(value);
@@ -172,14 +172,14 @@ namespace Avalonia
         /// <inheritdoc/>
         internal override IDisposable RouteBind(
             IAvaloniaObject o,
-            IObservable<BindingValue<object>> source,
+            IObservable<BindingValue<object?>> source,
             BindingPriority priority)
         {
             var adapter = TypedBindingAdapter<TValue>.Create(o, this, source);
             return o.Bind<TValue>(this, adapter);
         }
 
-        internal override void RouteInheritanceParentChanged(AvaloniaObject o, IAvaloniaObject oldParent)
+        internal override void RouteInheritanceParentChanged(AvaloniaObject o, IAvaloniaObject? oldParent)
         {
             throw new NotSupportedException("Direct properties do not support inheritance.");
         }

src/Avalonia.Base/DirectPropertyMetadata`1.cs

@@ -1,5 +1,7 @@
 using Avalonia.Data;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -18,7 +20,7 @@ namespace Avalonia
         /// Whether the property is interested in data validation.
         /// </param>
         public DirectPropertyMetadata(
-            TValue unsetValue = default(TValue),
+            TValue unsetValue = default!,
             BindingMode defaultBindingMode = BindingMode.Default,
             bool? enableDataValidation = null)
                 : base(defaultBindingMode)
@@ -44,7 +46,7 @@ namespace Avalonia
         public bool? EnableDataValidation { get; private set; }
 
         /// <inheritdoc/>
-        object IDirectPropertyMetadata.UnsetValue => UnsetValue;
+        object? IDirectPropertyMetadata.UnsetValue => UnsetValue;
 
         /// <inheritdoc/>
         public override void Merge(AvaloniaPropertyMetadata baseMetadata, AvaloniaProperty property)

src/Avalonia.Base/EnumExtensions.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Runtime.CompilerServices;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>

src/Avalonia.Base/IDescription.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -11,6 +13,6 @@ namespace Avalonia
         /// <value>
         /// The description of the object.
         /// </value>
-        string Description { get; }
+        string? Description { get; }
     }
 }

src/Avalonia.Base/IDirectPropertyAccessor.cs

@@ -1,5 +1,7 @@
 using System;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -23,13 +25,13 @@ namespace Avalonia
         /// </summary>
         /// <param name="instance">The instance.</param>
         /// <returns>The property value.</returns>
-        object GetValue(IAvaloniaObject instance);
+        object? GetValue(IAvaloniaObject instance);
 
         /// <summary>
         /// Sets the value of the property on the instance.
         /// </summary>
         /// <param name="instance">The instance.</param>
         /// <param name="value">The value.</param>
-        void SetValue(IAvaloniaObject instance, object value);
+        void SetValue(IAvaloniaObject instance, object? value);
     }
 }

src/Avalonia.Base/IDirectPropertyMetadata.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -8,11 +10,11 @@ namespace Avalonia
         /// <summary>
         /// Gets the to use when the property is set to <see cref="AvaloniaProperty.UnsetValue"/>.
         /// </summary>
-        object UnsetValue { get; }
+        object? UnsetValue { get; }
 
         /// <summary>
         /// Gets a value indicating whether the property is interested in data validation.
         /// </summary>
         bool? EnableDataValidation { get; }
     }
-}
+}

src/Avalonia.Base/IStyledPropertyAccessor.cs

@@ -1,5 +1,7 @@
 using System;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -14,6 +16,6 @@ namespace Avalonia
         /// <returns>
         /// The default value.
         /// </returns>
-        object GetDefaultValue(Type type);
+        object? GetDefaultValue(Type type);
     }
 }

src/Avalonia.Base/IStyledPropertyMetadata.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -8,6 +10,6 @@ namespace Avalonia
         /// <summary>
         /// Gets the default value for the property.
         /// </summary>
-        object DefaultValue { get; }
+        object? DefaultValue { get; }
     }
 }

src/Avalonia.Base/Reactive/AvaloniaPropertyBindingObservable.cs

@@ -28,7 +28,7 @@ namespace Avalonia.Reactive
         {
             if (_target.TryGetTarget(out var target))
             {
-                _value = (T)target.GetValue(_property);
+                _value = (T)target.GetValue(_property)!;
                 target.PropertyChanged += PropertyChanged;
             }
         }
@@ -66,7 +66,7 @@ namespace Avalonia.Reactive
 
                     if (!Equals(newValue, _value))
                     {
-                        _value = (T)newValue;
+                        _value = (T)newValue!;
                         PublishNext(_value);
                     }
                 }

src/Avalonia.Base/Reactive/TypedBindingAdapter.cs

@@ -7,24 +7,24 @@ using Avalonia.Logging;
 namespace Avalonia.Reactive
 {
     internal class TypedBindingAdapter<T> : SingleSubscriberObservableBase<BindingValue<T>>,
-        IObserver<BindingValue<object>>
+        IObserver<BindingValue<object?>>
     {
         private readonly IAvaloniaObject _target;
         private readonly AvaloniaProperty<T> _property;
-        private readonly IObservable<BindingValue<object>> _source;
+        private readonly IObservable<BindingValue<object?>> _source;
         private IDisposable? _subscription;
 
         public TypedBindingAdapter(
             IAvaloniaObject target,
             AvaloniaProperty<T> property,
-            IObservable<BindingValue<object>> source)
+            IObservable<BindingValue<object?>> source)
         {
             _target = target;
             _property = property;
             _source = source;
         }
 
-        public void OnNext(BindingValue<object> value)
+        public void OnNext(BindingValue<object?> value)
         {
             try
             {
@@ -49,7 +49,7 @@ namespace Avalonia.Reactive
         public static IObservable<BindingValue<T>> Create(
             IAvaloniaObject target,
             AvaloniaProperty<T> property,
-            IObservable<BindingValue<object>> source)
+            IObservable<BindingValue<object?>> source)
         {
             return source is IObservable<BindingValue<T>> result ?
                 result :

src/Avalonia.Base/StyledProperty.cs

@@ -1,5 +1,7 @@
 using System;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -21,8 +23,8 @@ namespace Avalonia
             Type ownerType,
             StyledPropertyMetadata<TValue> metadata,
             bool inherits = false,
-            Func<TValue, bool> validate = null,
-            Action<IAvaloniaObject, bool> notifying = null)
+            Func<TValue, bool>? validate = null,
+            Action<IAvaloniaObject, bool>? notifying = null)
             : base(name, ownerType, metadata, inherits, validate, notifying)
         {
         }

src/Avalonia.Base/StyledPropertyBase.cs

@@ -4,6 +4,8 @@ using Avalonia.Data;
 using Avalonia.Reactive;
 using Avalonia.Utilities;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -27,18 +29,10 @@ namespace Avalonia
             Type ownerType,            
             StyledPropertyMetadata<TValue> metadata,
             bool inherits = false,
-            Func<TValue, bool> validate = null,
-            Action<IAvaloniaObject, bool> notifying = null)
+            Func<TValue, bool>? validate = null,
+            Action<IAvaloniaObject, bool>? notifying = null)
                 : base(name, ownerType, metadata, notifying)
         {
-            Contract.Requires<ArgumentNullException>(name != null);
-            Contract.Requires<ArgumentNullException>(ownerType != null);
-
-            if (name.Contains("."))
-            {
-                throw new ArgumentException("'name' may not contain periods.");
-            }
-
             _inherits = inherits;
             ValidateValue = validate;
             HasCoercion |= metadata.CoerceValue != null;
@@ -72,7 +66,7 @@ namespace Avalonia
         /// <summary>
         /// Gets the value validation callback for the property.
         /// </summary>
-        public Func<TValue, bool> ValidateValue { get; }
+        public Func<TValue, bool>? ValidateValue { get; }
 
         /// <summary>
         /// Gets a value indicating whether this property has any value coercion callbacks defined
@@ -99,8 +93,6 @@ namespace Avalonia
         /// <returns>The default value.</returns>
         public TValue GetDefaultValue(Type type)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
-
             return GetMetadata(type).DefaultValue;
         }
 
@@ -113,6 +105,7 @@ namespace Avalonia
         /// </returns>
         public new StyledPropertyMetadata<TValue> GetMetadata(Type type)
         {
+            _ = type ?? throw new ArgumentNullException(nameof(type));
             return (StyledPropertyMetadata<TValue>)base.GetMetadata(type);
         }
 
@@ -183,7 +176,7 @@ namespace Avalonia
         }
 
         /// <inheritdoc/>
-        object IStyledPropertyAccessor.GetDefaultValue(Type type) => GetDefaultBoxedValue(type);
+        object? IStyledPropertyAccessor.GetDefaultValue(Type type) => GetDefaultBoxedValue(type);
 
         /// <inheritdoc/>
         internal override void RouteClearValue(IAvaloniaObject o)
@@ -192,22 +185,22 @@ namespace Avalonia
         }
 
         /// <inheritdoc/>
-        internal override object RouteGetValue(IAvaloniaObject o)
+        internal override object? RouteGetValue(IAvaloniaObject o)
         {
             return o.GetValue<TValue>(this);
         }
 
         /// <inheritdoc/>
-        internal override object RouteGetBaseValue(IAvaloniaObject o, BindingPriority maxPriority)
+        internal override object? RouteGetBaseValue(IAvaloniaObject o, BindingPriority maxPriority)
         {
             var value = o.GetBaseValue<TValue>(this, maxPriority);
             return value.HasValue ? value.Value : AvaloniaProperty.UnsetValue;
         }
 
         /// <inheritdoc/>
-        internal override IDisposable RouteSetValue(
+        internal override IDisposable? RouteSetValue(
             IAvaloniaObject o,
-            object value,
+            object? value,
             BindingPriority priority)
         {
             var v = TryConvert(value);
@@ -222,7 +215,7 @@ namespace Avalonia
             }
             else if (v.HasError)
             {
-                throw v.Error;
+                throw v.Error!;
             }
 
             return null;
@@ -231,7 +224,7 @@ namespace Avalonia
         /// <inheritdoc/>
         internal override IDisposable RouteBind(
             IAvaloniaObject o,
-            IObservable<BindingValue<object>> source,
+            IObservable<BindingValue<object?>> source,
             BindingPriority priority)
         {
             var adapter = TypedBindingAdapter<TValue>.Create(o, this, source);
@@ -241,15 +234,14 @@ namespace Avalonia
         /// <inheritdoc/>
         internal override void RouteInheritanceParentChanged(
             AvaloniaObject o,
-            IAvaloniaObject oldParent)
+            IAvaloniaObject? oldParent)
         {
             o.InheritanceParentChanged(this, oldParent);
         }
 
-        private object GetDefaultBoxedValue(Type type)
+        private object? GetDefaultBoxedValue(Type type)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
-
+            _ = type ?? throw new ArgumentNullException(nameof(type));
             return GetMetadata(type).DefaultValue;
         }
 

src/Avalonia.Base/StyledPropertyMetadata`1.cs

@@ -1,6 +1,8 @@
 using System;
 using Avalonia.Data;
 
+#nullable enable
+
 namespace Avalonia
 {
     /// <summary>
@@ -19,7 +21,7 @@ namespace Avalonia
         public StyledPropertyMetadata(
             Optional<TValue> defaultValue = default,
             BindingMode defaultBindingMode = BindingMode.Default,
-            Func<IAvaloniaObject, TValue, TValue> coerce = null)
+            Func<IAvaloniaObject, TValue, TValue>? coerce = null)
                 : base(defaultBindingMode)
         {
             _defaultValue = defaultValue;
@@ -29,14 +31,14 @@ namespace Avalonia
         /// <summary>
         /// Gets the default value for the property.
         /// </summary>
-        public TValue DefaultValue => _defaultValue.GetValueOrDefault();
+        public TValue DefaultValue => _defaultValue.GetValueOrDefault()!;
 
         /// <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;
+        object? IStyledPropertyMetadata.DefaultValue => DefaultValue;
 
         /// <inheritdoc/>
         public override void Merge(AvaloniaPropertyMetadata baseMetadata, AvaloniaProperty property)

src/Avalonia.Input/Avalonia.Input.csproj

@@ -2,7 +2,6 @@
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
     <Nullable>Enable</Nullable>
-    <WarningsAsErrors>CS8600;CS8602;CS8603</WarningsAsErrors>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="..\Avalonia.Base\Metadata\NullableAttributes.cs" Link="NullableAttributes.cs" />

src/Avalonia.Styling/Styling/Activators/PropertyEqualsActivator.cs

@@ -7,7 +7,7 @@ namespace Avalonia.Styling.Activators
     /// <summary>
     /// An <see cref="IStyleActivator"/> which listens to a property value on a control.
     /// </summary>
-    internal class PropertyEqualsActivator : StyleActivatorBase, IObserver<object>
+    internal class PropertyEqualsActivator : StyleActivatorBase, IObserver<object?>
     {
         private readonly IStyleable _control;
         private readonly AvaloniaProperty _property;
@@ -31,8 +31,8 @@ namespace Avalonia.Styling.Activators
 
         protected override void Deinitialize() => _subscription?.Dispose();
 
-        void IObserver<object>.OnCompleted() { }
-        void IObserver<object>.OnError(Exception error) { }
-        void IObserver<object>.OnNext(object value) => PublishNext(Equals(value, _value));
+        void IObserver<object?>.OnCompleted() { }
+        void IObserver<object?>.OnError(Exception error) { }
+        void IObserver<object?>.OnNext(object value) => PublishNext(Equals(value, _value));
     }
 }

src/Markup/Avalonia.Markup/Data/TemplateBinding.cs

@@ -10,10 +10,10 @@ namespace Avalonia.Data
     /// <summary>
     /// A XAML binding to a property on a control's templated parent.
     /// </summary>
-    public class TemplateBinding : SingleSubscriberObservableBase<object>,
+    public class TemplateBinding : SingleSubscriberObservableBase<object?>,
         IBinding,
         IDescription,
-        ISubject<object>,
+        ISubject<object?>,
         ISetterValue
     {
         private bool _isSetterValue;
@@ -88,10 +88,10 @@ namespace Avalonia.Data
         /// <inheritdoc/>
         public string Description => "TemplateBinding: " + Property;
 
-        void IObserver<object>.OnCompleted() => throw new NotImplementedException();
-        void IObserver<object>.OnError(Exception error) => throw new NotImplementedException();
+        void IObserver<object?>.OnCompleted() => throw new NotImplementedException();
+        void IObserver<object?>.OnError(Exception error) => throw new NotImplementedException();
 
-        void IObserver<object>.OnNext(object value)
+        void IObserver<object?>.OnNext(object? value)
         {
             if (_target.TemplatedParent != null && Property != null)
             {