diff --git a/src/Avalonia.Styling/Avalonia.Styling.csproj b/src/Avalonia.Styling/Avalonia.Styling.csproj
index 20b3183b00..3548749846 100644
--- a/src/Avalonia.Styling/Avalonia.Styling.csproj
+++ b/src/Avalonia.Styling/Avalonia.Styling.csproj
@@ -9,4 +9,5 @@
     <ProjectReference Include="..\Avalonia.Base\Avalonia.Base.csproj" />
   </ItemGroup>
   <Import Project="..\..\build\ApiDiff.props" />
+  <Import Project="..\..\build\NullableEnable.props" />
 </Project>
diff --git a/src/Avalonia.Styling/Controls/ChildNameScope.cs b/src/Avalonia.Styling/Controls/ChildNameScope.cs
index e6707e71db..58114a57fd 100644
--- a/src/Avalonia.Styling/Controls/ChildNameScope.cs
+++ b/src/Avalonia.Styling/Controls/ChildNameScope.cs
@@ -15,20 +15,20 @@ namespace Avalonia.Controls
         
         public void Register(string name, object element) => _inner.Register(name, element);
 
-        public SynchronousCompletionAsyncResult<object> FindAsync(string name)
+        public SynchronousCompletionAsyncResult<object?> FindAsync(string name)
         {
             var found = Find(name);
             if (found != null)
-                return new SynchronousCompletionAsyncResult<object>(found);
+                return new SynchronousCompletionAsyncResult<object?>(found);
             // Not found and both current and parent scope are in completed state
             if(IsCompleted)
-                return new SynchronousCompletionAsyncResult<object>(null);
+                return new SynchronousCompletionAsyncResult<object?>(null);
             return DoFindAsync(name);
         }
 
-        public SynchronousCompletionAsyncResult<object> DoFindAsync(string name)
+        public SynchronousCompletionAsyncResult<object?> DoFindAsync(string name)
         {
-            var src = new SynchronousCompletionAsyncResultSource<object>();
+            var src = new SynchronousCompletionAsyncResultSource<object?>();
 
             void ParentSearch()
             {
@@ -56,7 +56,7 @@ namespace Avalonia.Controls
             return src.AsyncResult;
         }
 
-        public object Find(string name)
+        public object? Find(string name)
         {
             var found = _inner.Find(name);
             if (found != null)
diff --git a/src/Avalonia.Styling/Controls/INameScope.cs b/src/Avalonia.Styling/Controls/INameScope.cs
index 2d5295fe45..1ca7db2f37 100644
--- a/src/Avalonia.Styling/Controls/INameScope.cs
+++ b/src/Avalonia.Styling/Controls/INameScope.cs
@@ -22,14 +22,14 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="name">The name.</param>
         /// <returns>The element, or null if the name was not found.</returns>
-        SynchronousCompletionAsyncResult<object> FindAsync(string name);
+        SynchronousCompletionAsyncResult<object?> FindAsync(string name);
         
         /// <summary>
         /// Finds a named element in the name scope, returns immediately, doesn't traverse the name scope stack
         /// </summary>
         /// <param name="name">The name.</param>
         /// <returns>The element, or null if the name was not found.</returns>
-        object Find(string name);
+        object? Find(string name);
 
         /// <summary>
         /// Marks the name scope as completed, no further registrations will be allowed
diff --git a/src/Avalonia.Styling/Controls/NameScope.cs b/src/Avalonia.Styling/Controls/NameScope.cs
index 62a04eac8b..77f98f85c4 100644
--- a/src/Avalonia.Styling/Controls/NameScope.cs
+++ b/src/Avalonia.Styling/Controls/NameScope.cs
@@ -22,8 +22,8 @@ namespace Avalonia.Controls
         
         private readonly Dictionary<string, object> _inner = new Dictionary<string, object>();
 
-        private readonly Dictionary<string, SynchronousCompletionAsyncResultSource<object>> _pendingSearches =
-            new Dictionary<string, SynchronousCompletionAsyncResultSource<object>>();
+        private readonly Dictionary<string, SynchronousCompletionAsyncResultSource<object?>> _pendingSearches =
+            new Dictionary<string, SynchronousCompletionAsyncResultSource<object?>>();
         
         /// <summary>
         /// Gets the value of the attached <see cref="NameScopeProperty"/> on a styled element.
@@ -32,7 +32,7 @@ namespace Avalonia.Controls
         /// <returns>The value of the NameScope attached property.</returns>
         public static INameScope GetNameScope(StyledElement styled)
         {
-            Contract.Requires<ArgumentNullException>(styled != null);
+            _ = styled ?? throw new ArgumentNullException(nameof(styled));
 
             return styled.GetValue(NameScopeProperty);
         }
@@ -44,7 +44,7 @@ namespace Avalonia.Controls
         /// <param name="value">The value to set.</param>
         public static void SetNameScope(StyledElement styled, INameScope value)
         {
-            Contract.Requires<ArgumentNullException>(styled != null);
+            _ = styled ?? throw new ArgumentNullException(nameof(styled));
 
             styled.SetValue(NameScopeProperty, value);
         }
@@ -54,12 +54,11 @@ namespace Avalonia.Controls
         {
             if (IsCompleted)
                 throw new InvalidOperationException("NameScope is completed, no further registrations are allowed");
-            Contract.Requires<ArgumentNullException>(name != null);
-            Contract.Requires<ArgumentNullException>(element != null);
 
-            object existing;
+            _ = name ?? throw new ArgumentNullException(nameof(name));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
 
-            if (_inner.TryGetValue(name, out existing))
+            if (_inner.TryGetValue(name, out var existing))
             {
                 if (existing != element)
                 {
@@ -77,27 +76,26 @@ namespace Avalonia.Controls
             }
         }
 
-        public SynchronousCompletionAsyncResult<object> FindAsync(string name)
+        public SynchronousCompletionAsyncResult<object?> FindAsync(string name)
         {
             var found = Find(name);
             if (found != null)
-                return new SynchronousCompletionAsyncResult<object>(found);
+                return new SynchronousCompletionAsyncResult<object?>(found);
             if (IsCompleted)
-                return new SynchronousCompletionAsyncResult<object>((object)null);
+                return new SynchronousCompletionAsyncResult<object?>(null);
             if (!_pendingSearches.TryGetValue(name, out var tcs))
                 // We are intentionally running continuations synchronously here
-                _pendingSearches[name] = tcs = new SynchronousCompletionAsyncResultSource<object>();
+                _pendingSearches[name] = tcs = new SynchronousCompletionAsyncResultSource<object?>();
 
             return tcs.AsyncResult;
         }
 
         /// <inheritdoc />
-        public object Find(string name)
+        public object? Find(string name)
         {
-            Contract.Requires<ArgumentNullException>(name != null);
+            _ = name ?? throw new ArgumentNullException(nameof(name));
 
-            object result;
-            _inner.TryGetValue(name, out result);
+            _inner.TryGetValue(name, out var result);
             return result;
         }
 
diff --git a/src/Avalonia.Styling/Controls/NameScopeExtensions.cs b/src/Avalonia.Styling/Controls/NameScopeExtensions.cs
index 75630711b8..3895b6ceb9 100644
--- a/src/Avalonia.Styling/Controls/NameScopeExtensions.cs
+++ b/src/Avalonia.Styling/Controls/NameScopeExtensions.cs
@@ -17,11 +17,11 @@ namespace Avalonia.Controls
         /// <param name="nameScope">The name scope.</param>
         /// <param name="name">The name.</param>
         /// <returns>The named element or null if not found.</returns>
-        public static T Find<T>(this INameScope nameScope, string name)
+        public static T? Find<T>(this INameScope nameScope, string name)
             where T : class
         {
-            Contract.Requires<ArgumentNullException>(nameScope != null);
-            Contract.Requires<ArgumentNullException>(name != null);
+            _ = nameScope ?? throw new ArgumentNullException(nameof(nameScope));
+            _ = name ?? throw new ArgumentNullException(nameof(name));
 
             var result = nameScope.Find(name);
 
@@ -31,7 +31,7 @@ namespace Avalonia.Controls
                     $"Expected control '{name}' to be '{typeof(T)} but it was '{result.GetType()}'.");
             }
 
-            return (T)result;
+            return (T?)result;
         }
 
         /// <summary>
@@ -41,11 +41,11 @@ namespace Avalonia.Controls
         /// <param name="anchor">The control to take the name scope from.</param>
         /// <param name="name">The name.</param>
         /// <returns>The named element or null if not found.</returns>
-        public static T Find<T>(this ILogical anchor, string name)
+        public static T? Find<T>(this ILogical anchor, string name)
             where T : class
         {
-            Contract.Requires<ArgumentNullException>(anchor != null);
-            Contract.Requires<ArgumentNullException>(name != null);
+            _ = anchor ?? throw new ArgumentNullException(nameof(anchor));
+            _ = name ?? throw new ArgumentNullException(nameof(name));
             var styledAnchor = anchor as StyledElement;
             if (styledAnchor == null)
                 return null;
@@ -64,8 +64,8 @@ namespace Avalonia.Controls
         public static T Get<T>(this INameScope nameScope, string name)
             where T : class
         {
-            Contract.Requires<ArgumentNullException>(nameScope != null);
-            Contract.Requires<ArgumentNullException>(name != null);
+            _ = nameScope ?? throw new ArgumentNullException(nameof(nameScope));
+            _ = name ?? throw new ArgumentNullException(nameof(name));
 
             var result = nameScope.Find(name);
 
@@ -94,9 +94,9 @@ namespace Avalonia.Controls
         public static T Get<T>(this ILogical anchor, string name)
             where T : class
         {
-            Contract.Requires<ArgumentNullException>(anchor != null);
-            Contract.Requires<ArgumentNullException>(name != null);
-               
+            _ = anchor ?? throw new ArgumentNullException(nameof(anchor));
+            _ = name ?? throw new ArgumentNullException(nameof(name));
+
             var nameScope = (anchor as INameScope) ?? NameScope.GetNameScope((StyledElement)anchor);
             if (nameScope == null)
                 throw new InvalidOperationException(
@@ -105,9 +105,9 @@ namespace Avalonia.Controls
             return nameScope.Get<T>(name);
         }
         
-        public static INameScope FindNameScope(this ILogical control)
+        public static INameScope? FindNameScope(this ILogical control)
         {
-            Contract.Requires<ArgumentNullException>(control != null);
+            _ = control ?? throw new ArgumentNullException(nameof(control));
 
             var scope = control.GetSelfAndLogicalAncestors()
                 .OfType<StyledElement>()
diff --git a/src/Avalonia.Styling/Controls/NameScopeLocator.cs b/src/Avalonia.Styling/Controls/NameScopeLocator.cs
index 51f4c5c4eb..f0ce7f8a5b 100644
--- a/src/Avalonia.Styling/Controls/NameScopeLocator.cs
+++ b/src/Avalonia.Styling/Controls/NameScopeLocator.cs
@@ -11,9 +11,9 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="scope">The scope relative from which the object should be resolved.</param>
         /// <param name="name">The name of the object to find.</param>
-        public static IObservable<object> Track(INameScope scope, string name)
+        public static IObservable<object?> Track(INameScope scope, string name)
         {
-            return new NeverEndingSynchronousCompletionAsyncResultObservable<object>(scope.FindAsync(name));
+            return new NeverEndingSynchronousCompletionAsyncResultObservable<object?>(scope.FindAsync(name));
         }
         
         // This class is implemented in such weird way because for some reason
@@ -22,7 +22,7 @@ namespace Avalonia.Controls
 
         private class NeverEndingSynchronousCompletionAsyncResultObservable<T> : IObservable<T>
         {
-            private T _value;
+            private T? _value;
             private SynchronousCompletionAsyncResult<T>? _asyncResult;
 
             public NeverEndingSynchronousCompletionAsyncResultObservable(SynchronousCompletionAsyncResult<T> task)
@@ -47,7 +47,7 @@ namespace Avalonia.Controls
                         observer.OnNext(_asyncResult.Value.GetResult());
                     });
                 else
-                    observer.OnNext(_value);
+                    observer.OnNext(_value!);
                 
                 return Disposable.Empty;
             }
diff --git a/src/Avalonia.Styling/Controls/PseudoClassesExtensions.cs b/src/Avalonia.Styling/Controls/PseudoClassesExtensions.cs
index 7b28d1a911..4cddc8c8fb 100644
--- a/src/Avalonia.Styling/Controls/PseudoClassesExtensions.cs
+++ b/src/Avalonia.Styling/Controls/PseudoClassesExtensions.cs
@@ -12,7 +12,7 @@ namespace Avalonia.Controls
         /// <param name="value">True to add the pseudoclass or false to remove.</param>
         public static void Set(this IPseudoClasses classes, string name, bool value)
         {
-            Contract.Requires<ArgumentNullException>(classes != null);
+            _ = classes ?? throw new ArgumentNullException(nameof(classes));
 
             if (value)
             {
diff --git a/src/Avalonia.Styling/Controls/ResourceDictionary.cs b/src/Avalonia.Styling/Controls/ResourceDictionary.cs
index e797f8cf8b..3af14daa83 100644
--- a/src/Avalonia.Styling/Controls/ResourceDictionary.cs
+++ b/src/Avalonia.Styling/Controls/ResourceDictionary.cs
@@ -177,7 +177,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void OnCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             Owner?.NotifyHostedResourcesChanged(ResourcesChangedEventArgs.Empty);
         }
diff --git a/src/Avalonia.Styling/Controls/ResourceNodeExtensions.cs b/src/Avalonia.Styling/Controls/ResourceNodeExtensions.cs
index be1069a4da..513b3f2424 100644
--- a/src/Avalonia.Styling/Controls/ResourceNodeExtensions.cs
+++ b/src/Avalonia.Styling/Controls/ResourceNodeExtensions.cs
@@ -109,7 +109,7 @@ namespace Avalonia.Controls
                 observer.OnNext(Convert(_target.FindResource(_key)));
             }
 
-            private void ResourcesChanged(object sender, ResourcesChangedEventArgs e)
+            private void ResourcesChanged(object? sender, ResourcesChangedEventArgs e)
             {
                 PublishNext(Convert(_target.FindResource(_key)));
             }
@@ -159,7 +159,7 @@ namespace Avalonia.Controls
                 }
             }
 
-            private void OwnerChanged(object sender, EventArgs e)
+            private void OwnerChanged(object? sender, EventArgs e)
             {
                 if (_owner is object)
                 {
@@ -176,7 +176,7 @@ namespace Avalonia.Controls
                 PublishNext();
             }
 
-            private void ResourcesChanged(object sender, ResourcesChangedEventArgs e)
+            private void ResourcesChanged(object? sender, ResourcesChangedEventArgs e)
             {
                 PublishNext();
             }
diff --git a/src/Avalonia.Styling/INamed.cs b/src/Avalonia.Styling/INamed.cs
index 1f8d269b07..df83f3b6ef 100644
--- a/src/Avalonia.Styling/INamed.cs
+++ b/src/Avalonia.Styling/INamed.cs
@@ -8,6 +8,6 @@ namespace Avalonia
         /// <summary>
         /// Gets the element name.
         /// </summary>
-        string Name { get; }
+        string? Name { get; }
     }
 }
diff --git a/src/Avalonia.Styling/LogicalTree/ControlLocator.cs b/src/Avalonia.Styling/LogicalTree/ControlLocator.cs
index 6d0302ace4..f60ed5aab8 100644
--- a/src/Avalonia.Styling/LogicalTree/ControlLocator.cs
+++ b/src/Avalonia.Styling/LogicalTree/ControlLocator.cs
@@ -9,19 +9,19 @@ namespace Avalonia.LogicalTree
     /// </summary>
     public static class ControlLocator
     {
-        public static IObservable<ILogical> Track(ILogical relativeTo, int ancestorLevel, Type ancestorType = null)
+        public static IObservable<ILogical?> Track(ILogical relativeTo, int ancestorLevel, Type? ancestorType = null)
         {
             return new ControlTracker(relativeTo, ancestorLevel, ancestorType);
         }
 
-        private class ControlTracker : LightweightObservableBase<ILogical>
+        private class ControlTracker : LightweightObservableBase<ILogical?>
         {
             private readonly ILogical _relativeTo;
             private readonly int _ancestorLevel;
-            private readonly Type _ancestorType;
-            private ILogical _value;
+            private readonly Type? _ancestorType;
+            private ILogical? _value;
 
-            public ControlTracker(ILogical relativeTo, int ancestorLevel, Type ancestorType)
+            public ControlTracker(ILogical relativeTo, int ancestorLevel, Type? ancestorType)
             {
                 _relativeTo = relativeTo;
                 _ancestorLevel = ancestorLevel;
@@ -43,18 +43,18 @@ namespace Avalonia.LogicalTree
                 _value = null;
             }
 
-            protected override void Subscribed(IObserver<ILogical> observer, bool first)
+            protected override void Subscribed(IObserver<ILogical?> observer, bool first)
             {
                 observer.OnNext(_value);
             }
 
-            private void Attached(object sender, LogicalTreeAttachmentEventArgs e)
+            private void Attached(object? sender, LogicalTreeAttachmentEventArgs e)
             {
                 Update();
                 PublishNext(_value);
             }
 
-            private void Detached(object sender, LogicalTreeAttachmentEventArgs e)
+            private void Detached(object? sender, LogicalTreeAttachmentEventArgs e)
             {
                 _value = null;
                 PublishNext(null);
diff --git a/src/Avalonia.Styling/LogicalTree/ILogical.cs b/src/Avalonia.Styling/LogicalTree/ILogical.cs
index 9c4223618f..caff3d8150 100644
--- a/src/Avalonia.Styling/LogicalTree/ILogical.cs
+++ b/src/Avalonia.Styling/LogicalTree/ILogical.cs
@@ -12,12 +12,12 @@ namespace Avalonia.LogicalTree
         /// <summary>
         /// Raised when the control is attached to a rooted logical tree.
         /// </summary>
-        event EventHandler<LogicalTreeAttachmentEventArgs> AttachedToLogicalTree;
+        event EventHandler<LogicalTreeAttachmentEventArgs>? AttachedToLogicalTree;
 
         /// <summary>
         /// Raised when the control is detached from a rooted logical tree.
         /// </summary>
-        event EventHandler<LogicalTreeAttachmentEventArgs> DetachedFromLogicalTree;
+        event EventHandler<LogicalTreeAttachmentEventArgs>? DetachedFromLogicalTree;
 
         /// <summary>
         /// Gets a value indicating whether the element is attached to a rooted logical tree.
@@ -27,7 +27,7 @@ namespace Avalonia.LogicalTree
         /// <summary>
         /// Gets the logical parent.
         /// </summary>
-        ILogical LogicalParent { get; }
+        ILogical? LogicalParent { get; }
 
         /// <summary>
         /// Gets the logical children.
diff --git a/src/Avalonia.Styling/LogicalTree/LogicalExtensions.cs b/src/Avalonia.Styling/LogicalTree/LogicalExtensions.cs
index 458ab0fce2..74720c0a77 100644
--- a/src/Avalonia.Styling/LogicalTree/LogicalExtensions.cs
+++ b/src/Avalonia.Styling/LogicalTree/LogicalExtensions.cs
@@ -15,14 +15,14 @@ namespace Avalonia.LogicalTree
         /// <returns>The logical's ancestors.</returns>
         public static IEnumerable<ILogical> GetLogicalAncestors(this ILogical logical)
         {
-            Contract.Requires<ArgumentNullException>(logical != null);
+            _ = logical ?? throw new ArgumentNullException(nameof(logical));
 
-            logical = logical.LogicalParent;
+            ILogical? l = logical.LogicalParent;
 
-            while (logical != null)
+            while (l != null)
             {
-                yield return logical;
-                logical = logical.LogicalParent;
+                yield return l;
+                l = l.LogicalParent;
             }
         }
 
@@ -48,14 +48,14 @@ namespace Avalonia.LogicalTree
         /// <param name="logical">The logical.</param>
         /// <param name="includeSelf">If given logical should be included in search.</param>
         /// <returns>First ancestor of given type.</returns>
-        public static T FindLogicalAncestorOfType<T>(this ILogical logical, bool includeSelf = false) where T : class
+        public static T? FindLogicalAncestorOfType<T>(this ILogical logical, bool includeSelf = false) where T : class
         {
             if (logical is null)
             {
                 return null;
             }
 
-            ILogical parent = includeSelf ? logical : logical.LogicalParent;
+            var parent = includeSelf ? logical : logical.LogicalParent;
 
             while (parent != null)
             {
@@ -120,7 +120,7 @@ namespace Avalonia.LogicalTree
         /// <param name="logical">The logical.</param>
         /// <param name="includeSelf">If given logical should be included in search.</param>
         /// <returns>First descendant of given type.</returns>
-        public static T FindLogicalDescendantOfType<T>(this ILogical logical, bool includeSelf = false) where T : class
+        public static T? FindLogicalDescendantOfType<T>(this ILogical logical, bool includeSelf = false) where T : class
         {
             if (logical is null)
             {
@@ -140,7 +140,7 @@ namespace Avalonia.LogicalTree
         /// </summary>
         /// <param name="logical">The logical.</param>
         /// <returns>The parent, or null if the logical is unparented.</returns>
-        public static ILogical GetLogicalParent(this ILogical logical)
+        public static ILogical? GetLogicalParent(this ILogical logical)
         {
             return logical.LogicalParent;
         }
@@ -153,7 +153,7 @@ namespace Avalonia.LogicalTree
         /// <returns>
         /// The parent, or null if the logical is unparented or its parent is not of type <typeparamref name="T"/>.
         /// </returns>
-        public static T GetLogicalParent<T>(this ILogical logical) where T : class
+        public static T? GetLogicalParent<T>(this ILogical logical) where T : class
         {
             return logical.LogicalParent as T;
         }
@@ -165,7 +165,7 @@ namespace Avalonia.LogicalTree
         /// <returns>The logical siblings.</returns>
         public static IEnumerable<ILogical> GetLogicalSiblings(this ILogical logical)
         {
-            ILogical parent = logical.LogicalParent;
+            var parent = logical.LogicalParent;
 
             if (parent != null)
             {
@@ -187,7 +187,7 @@ namespace Avalonia.LogicalTree
         /// </returns>
         public static bool IsLogicalAncestorOf(this ILogical logical, ILogical target)
         {
-            ILogical current = target?.LogicalParent;
+            var current = target?.LogicalParent;
 
             while (current != null)
             {
@@ -202,7 +202,7 @@ namespace Avalonia.LogicalTree
             return false;
         }
 
-        private static T FindDescendantOfTypeCore<T>(ILogical logical) where T : class
+        private static T? FindDescendantOfTypeCore<T>(ILogical logical) where T : class
         {
             var logicalChildren = logical.LogicalChildren;
             var logicalChildrenCount = logicalChildren.Count;
diff --git a/src/Avalonia.Styling/LogicalTree/LogicalTreeAttachmentEventArgs.cs b/src/Avalonia.Styling/LogicalTree/LogicalTreeAttachmentEventArgs.cs
index 891724599c..826fc5b2a4 100644
--- a/src/Avalonia.Styling/LogicalTree/LogicalTreeAttachmentEventArgs.cs
+++ b/src/Avalonia.Styling/LogicalTree/LogicalTreeAttachmentEventArgs.cs
@@ -19,11 +19,8 @@ namespace Avalonia.LogicalTree
             ILogical source,
             ILogical parent)
         {
-            Contract.Requires<ArgumentNullException>(root != null);
-            Contract.Requires<ArgumentNullException>(source != null);
-
-            Root = root;
-            Source = source;
+            Root = root ?? throw new ArgumentNullException(nameof(root));
+            Source = source ?? throw new ArgumentNullException(nameof(source));
             Parent = parent;
         }
 
diff --git a/src/Avalonia.Styling/StyledElement.cs b/src/Avalonia.Styling/StyledElement.cs
index 2292f5c518..5f498623e1 100644
--- a/src/Avalonia.Styling/StyledElement.cs
+++ b/src/Avalonia.Styling/StyledElement.cs
@@ -427,7 +427,7 @@ namespace Avalonia
 
                 if (_logicalRoot != null)
                 {
-                    var e = new LogicalTreeAttachmentEventArgs(_logicalRoot, this, old);
+                    var e = new LogicalTreeAttachmentEventArgs(_logicalRoot, this, old!);
                     OnDetachedFromLogicalTreeCore(e);
                 }
 
@@ -435,7 +435,7 @@ namespace Avalonia
 
                 if (newRoot is object)
                 {
-                    var e = new LogicalTreeAttachmentEventArgs(newRoot, this, parent);
+                    var e = new LogicalTreeAttachmentEventArgs(newRoot, this, parent!);
                     OnAttachedToLogicalTreeCore(e);
                 }
                 else if (parent is null)
@@ -495,21 +495,21 @@ namespace Avalonia
             DetachStylesFromThisAndDescendents(allStyles);
         }
 
-        protected virtual void LogicalChildrenCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        protected virtual void LogicalChildrenCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
-                    SetLogicalParent(e.NewItems);
+                    SetLogicalParent(e.NewItems!);
                     break;
 
                 case NotifyCollectionChangedAction.Remove:
-                    ClearLogicalParent(e.OldItems);
+                    ClearLogicalParent(e.OldItems!);
                     break;
 
                 case NotifyCollectionChangedAction.Replace:
-                    ClearLogicalParent(e.OldItems);
-                    SetLogicalParent(e.NewItems);
+                    ClearLogicalParent(e.OldItems!);
+                    SetLogicalParent(e.NewItems!);
                     break;
 
                 case NotifyCollectionChangedAction.Reset:
@@ -729,7 +729,7 @@ namespace Avalonia
 
             for (var i = 0; i < count; i++)
             {
-                var logical = (ILogical) children[i];
+                var logical = (ILogical) children[i]!;
                 
                 if (logical.LogicalParent is null)
                 {
@@ -744,7 +744,7 @@ namespace Avalonia
 
             for (var i = 0; i < count; i++)
             {
-                var logical = (ILogical) children[i];
+                var logical = (ILogical) children[i]!;
                 
                 if (logical.LogicalParent == this)
                 {
diff --git a/src/Avalonia.Styling/Styling/Activators/NthChildActivator.cs b/src/Avalonia.Styling/Styling/Activators/NthChildActivator.cs
index 803809a8ce..6f54cd5904 100644
--- a/src/Avalonia.Styling/Styling/Activators/NthChildActivator.cs
+++ b/src/Avalonia.Styling/Styling/Activators/NthChildActivator.cs
@@ -37,7 +37,7 @@ namespace Avalonia.Styling.Activators
             _provider.ChildIndexChanged -= ChildIndexChanged;
         }
 
-        private void ChildIndexChanged(object sender, ChildIndexChangedEventArgs e)
+        private void ChildIndexChanged(object? sender, ChildIndexChangedEventArgs e)
         {
             // Run matching again if:
             // 1. Selector is reversed, so other item insertion/deletion might affect total count without changing subscribed item index.
diff --git a/src/Avalonia.Styling/Styling/Activators/StyleClassActivator.cs b/src/Avalonia.Styling/Styling/Activators/StyleClassActivator.cs
index 7906a29cb5..98d3f16a0a 100644
--- a/src/Avalonia.Styling/Styling/Activators/StyleClassActivator.cs
+++ b/src/Avalonia.Styling/Styling/Activators/StyleClassActivator.cs
@@ -66,7 +66,7 @@ namespace Avalonia.Styling.Activators
             _classes.CollectionChanged -= ClassesChangedHandler;
         }
 
-        private void ClassesChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void ClassesChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             if (e.Action != NotifyCollectionChangedAction.Move)
             {
diff --git a/src/Avalonia.Styling/Styling/ChildSelector.cs b/src/Avalonia.Styling/Styling/ChildSelector.cs
index 85e7aaabde..5c92182b80 100644
--- a/src/Avalonia.Styling/Styling/ChildSelector.cs
+++ b/src/Avalonia.Styling/Styling/ChildSelector.cs
@@ -6,7 +6,7 @@ namespace Avalonia.Styling
     internal class ChildSelector : Selector
     {
         private readonly Selector _parent;
-        private string _selectorString;
+        private string? _selectorString;
 
         public ChildSelector(Selector parent)
         {
@@ -25,7 +25,7 @@ namespace Avalonia.Styling
         public override bool IsCombinator => true;
 
         /// <inheritdoc/>
-        public override Type TargetType => null;
+        public override Type? TargetType => null;
 
         public override string ToString()
         {
@@ -64,6 +64,6 @@ namespace Avalonia.Styling
             }
         }
 
-        protected override Selector MovePrevious() => null;
+        protected override Selector? MovePrevious() => null;
     }
 }
diff --git a/src/Avalonia.Styling/Styling/OrSelector.cs b/src/Avalonia.Styling/Styling/OrSelector.cs
index 8251915504..3d6db9b01e 100644
--- a/src/Avalonia.Styling/Styling/OrSelector.cs
+++ b/src/Avalonia.Styling/Styling/OrSelector.cs
@@ -120,7 +120,7 @@ namespace Avalonia.Styling
                 }
                 else
                 {
-                    while (!result.IsAssignableFrom(selector.TargetType))
+                    while (result is not null && !result.IsAssignableFrom(selector.TargetType))
                     {
                         result = result.BaseType;
                     }
diff --git a/src/Avalonia.Styling/Styling/PropertyEqualsSelector.cs b/src/Avalonia.Styling/Styling/PropertyEqualsSelector.cs
index ebdfff222a..1cd1a650ef 100644
--- a/src/Avalonia.Styling/Styling/PropertyEqualsSelector.cs
+++ b/src/Avalonia.Styling/Styling/PropertyEqualsSelector.cs
@@ -109,7 +109,7 @@ namespace Avalonia.Styling
             var converter = TypeDescriptor.GetConverter(propertyType);
             if (converter?.CanConvertFrom(valueType) == true)
             {
-                return Equals(propertyValue, converter.ConvertFrom(null, CultureInfo.InvariantCulture, value));
+                return Equals(propertyValue, converter.ConvertFrom(null, CultureInfo.InvariantCulture, value!));
             }
 
             return false;
diff --git a/src/Avalonia.Styling/Styling/Selectors.cs b/src/Avalonia.Styling/Styling/Selectors.cs
index 64d0a0e96b..7c66469cf1 100644
--- a/src/Avalonia.Styling/Styling/Selectors.cs
+++ b/src/Avalonia.Styling/Styling/Selectors.cs
@@ -25,10 +25,14 @@ namespace Avalonia.Styling
         /// <param name="previous">The previous selector.</param>
         /// <param name="name">The name of the style class.</param>
         /// <returns>The selector.</returns>
-        public static Selector Class(this Selector previous, string name)
+        public static Selector Class(this Selector? previous, string name)
         {
-            Contract.Requires<ArgumentNullException>(name != null);
-            Contract.Requires<ArgumentException>(!string.IsNullOrWhiteSpace(name));
+            _ = name ?? throw new ArgumentNullException(nameof(name));
+
+            if (string.IsNullOrWhiteSpace(name))
+            {
+                throw new ArgumentException("Name may not be empty", nameof(name));
+            }
 
             var tac = previous as TypeNameAndClassSelector;
 
@@ -48,7 +52,7 @@ namespace Avalonia.Styling
         /// </summary>
         /// <param name="previous">The previous selector.</param>
         /// <returns>The selector.</returns>
-        public static Selector Descendant(this Selector previous)
+        public static Selector Descendant(this Selector? previous)
         {
             return new DescendantSelector(previous);
         }
@@ -59,9 +63,9 @@ namespace Avalonia.Styling
         /// <param name="previous">The previous selector.</param>
         /// <param name="type">The type.</param>
         /// <returns>The selector.</returns>
-        public static Selector Is(this Selector previous, Type type)
+        public static Selector Is(this Selector? previous, Type type)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
+            _ = type ?? throw new ArgumentNullException(nameof(type));
 
             return TypeNameAndClassSelector.Is(previous, type);
         }
@@ -72,7 +76,7 @@ namespace Avalonia.Styling
         /// <typeparam name="T">The type.</typeparam>
         /// <param name="previous">The previous selector.</param>
         /// <returns>The selector.</returns>
-        public static Selector Is<T>(this Selector previous) where T : IStyleable
+        public static Selector Is<T>(this Selector? previous) where T : IStyleable
         {
             return previous.Is(typeof(T));
         }
@@ -83,10 +87,14 @@ namespace Avalonia.Styling
         /// <param name="previous">The previous selector.</param>
         /// <param name="name">The name.</param>
         /// <returns>The selector.</returns>
-        public static Selector Name(this Selector previous, string name)
+        public static Selector Name(this Selector? previous, string name)
         {
-            Contract.Requires<ArgumentNullException>(name != null);
-            Contract.Requires<ArgumentException>(!string.IsNullOrWhiteSpace(name));
+            _ = name ?? throw new ArgumentNullException(nameof(name));
+
+            if (string.IsNullOrWhiteSpace(name))
+            {
+                throw new ArgumentException("Name may not be empty", nameof(name));
+            }
 
             var tac = previous as TypeNameAndClassSelector;
 
@@ -107,7 +115,7 @@ namespace Avalonia.Styling
         /// <param name="previous">The previous selector.</param>
         /// <param name="argument">The selector to be not-ed.</param>
         /// <returns>The selector.</returns>
-        public static Selector Not(this Selector previous, Func<Selector, Selector> argument)
+        public static Selector Not(this Selector? previous, Func<Selector?, Selector> argument)
         {
             return new NotSelector(previous, argument(null));
         }
@@ -118,7 +126,7 @@ namespace Avalonia.Styling
         /// <param name="previous">The previous selector.</param>
         /// <param name="argument">The selector to be not-ed.</param>
         /// <returns>The selector.</returns>
-        public static Selector Not(this Selector previous, Selector argument)
+        public static Selector Not(this Selector? previous, Selector argument)
         {
             return new NotSelector(previous, argument);
         }
@@ -126,7 +134,7 @@ namespace Avalonia.Styling
         /// <inheritdoc cref="NthChildSelector"/>
         /// <inheritdoc cref="NthChildSelector(Selector?, int, int)"/>
         /// <returns>The selector.</returns>
-        public static Selector NthChild(this Selector previous, int step, int offset)
+        public static Selector NthChild(this Selector? previous, int step, int offset)
         {
             return new NthChildSelector(previous, step, offset);
         }
@@ -134,7 +142,7 @@ namespace Avalonia.Styling
         /// <inheritdoc cref="NthLastChildSelector"/>
         /// <inheritdoc cref="NthLastChildSelector(Selector?, int, int)"/>
         /// <returns>The selector.</returns>
-        public static Selector NthLastChild(this Selector previous, int step, int offset)
+        public static Selector NthLastChild(this Selector? previous, int step, int offset)
         {
             return new NthLastChildSelector(previous, step, offset);
         }
@@ -145,9 +153,9 @@ namespace Avalonia.Styling
         /// <param name="previous">The previous selector.</param>
         /// <param name="type">The type.</param>
         /// <returns>The selector.</returns>
-        public static Selector OfType(this Selector previous, Type type)
+        public static Selector OfType(this Selector? previous, Type type)
         {
-            Contract.Requires<ArgumentNullException>(type != null);
+            _ = type ?? throw new ArgumentNullException(nameof(type));
 
             return TypeNameAndClassSelector.OfType(previous, type);
         }
@@ -158,7 +166,7 @@ namespace Avalonia.Styling
         /// <typeparam name="T">The type.</typeparam>
         /// <param name="previous">The previous selector.</param>
         /// <returns>The selector.</returns>
-        public static Selector OfType<T>(this Selector previous) where T : IStyleable
+        public static Selector OfType<T>(this Selector? previous) where T : IStyleable
         {
             return previous.OfType(typeof(T));
         }
@@ -191,9 +199,9 @@ namespace Avalonia.Styling
         /// <param name="property">The property.</param>
         /// <param name="value">The property value.</param>
         /// <returns>The selector.</returns>
-        public static Selector PropertyEquals<T>(this Selector previous, AvaloniaProperty<T> property, object value)
+        public static Selector PropertyEquals<T>(this Selector? previous, AvaloniaProperty<T> property, object? value)
         {
-            Contract.Requires<ArgumentNullException>(property != null);
+            _ = property ?? throw new ArgumentNullException(nameof(property));
 
             return new PropertyEqualsSelector(previous, property, value);
         }
@@ -205,9 +213,9 @@ namespace Avalonia.Styling
         /// <param name="property">The property.</param>
         /// <param name="value">The property value.</param>
         /// <returns>The selector.</returns>
-        public static Selector PropertyEquals(this Selector previous, AvaloniaProperty property, object value)
+        public static Selector PropertyEquals(this Selector? previous, AvaloniaProperty property, object? value)
         {
-            Contract.Requires<ArgumentNullException>(property != null);
+            _ = property ?? throw new ArgumentNullException(nameof(property));
 
             return new PropertyEqualsSelector(previous, property, value);
         }
diff --git a/src/Avalonia.Styling/Styling/Styles.cs b/src/Avalonia.Styling/Styling/Styles.cs
index c752bdfeb8..81502f1570 100644
--- a/src/Avalonia.Styling/Styling/Styles.cs
+++ b/src/Avalonia.Styling/Styling/Styles.cs
@@ -262,7 +262,7 @@ namespace Avalonia.Styling
             }
         }
 
-        private void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void OnCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             static IReadOnlyList<T> ToReadOnlyList<T>(IList list)
             {
@@ -282,7 +282,7 @@ namespace Avalonia.Styling
             {
                 for (var i = 0; i < items.Count; ++i)
                 {
-                    var style = (IStyle)items[i];
+                    var style = (IStyle)items[i]!;
 
                     if (Owner is object && style is IResourceProvider resourceProvider)
                     {
@@ -299,7 +299,7 @@ namespace Avalonia.Styling
             {
                 for (var i = 0; i < items.Count; ++i)
                 {
-                    var style = (IStyle)items[i];
+                    var style = (IStyle)items[i]!;
 
                     if (Owner is object && style is IResourceProvider resourceProvider)
                     {
@@ -315,14 +315,14 @@ namespace Avalonia.Styling
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
-                    Add(e.NewItems);
+                    Add(e.NewItems!);
                     break;
                 case NotifyCollectionChangedAction.Remove:
-                    Remove(e.OldItems);
+                    Remove(e.OldItems!);
                     break;
                 case NotifyCollectionChangedAction.Replace:
-                    Remove(e.OldItems);
-                    Add(e.NewItems);
+                    Remove(e.OldItems!);
+                    Add(e.NewItems!);
                     break;
                 case NotifyCollectionChangedAction.Reset:
                     throw new InvalidOperationException("Reset should not be called on Styles.");
diff --git a/src/Avalonia.Styling/Styling/TemplateSelector.cs b/src/Avalonia.Styling/Styling/TemplateSelector.cs
index 5ea8defeda..e8051efa6d 100644
--- a/src/Avalonia.Styling/Styling/TemplateSelector.cs
+++ b/src/Avalonia.Styling/Styling/TemplateSelector.cs
@@ -5,7 +5,7 @@ namespace Avalonia.Styling
     internal class TemplateSelector : Selector
     {
         private readonly Selector _parent;
-        private string _selectorString;
+        private string? _selectorString;
 
         public TemplateSelector(Selector parent)
         {
@@ -24,7 +24,7 @@ namespace Avalonia.Styling
         public override bool IsCombinator => true;
 
         /// <inheritdoc/>
-        public override Type TargetType => null;
+        public override Type? TargetType => null;
 
         public override string ToString()
         {
@@ -48,6 +48,6 @@ namespace Avalonia.Styling
             return _parent.Match(templatedParent, subscribe);
         }
 
-        protected override Selector MovePrevious() => null;
+        protected override Selector? MovePrevious() => null;
     }
 }
diff --git a/src/Avalonia.Visuals/Visual.cs b/src/Avalonia.Visuals/Visual.cs
index 78c6d9c057..324b253a0f 100644
--- a/src/Avalonia.Visuals/Visual.cs
+++ b/src/Avalonia.Visuals/Visual.cs
@@ -377,7 +377,7 @@ namespace Avalonia
             }
         }
 
-        protected override void LogicalChildrenCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        protected override void LogicalChildrenCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             base.LogicalChildrenCollectionChanged(sender, e);
             VisualRoot?.Renderer?.RecalculateChildren(this);
diff --git a/src/Markup/Avalonia.Markup/Markup/Parsers/SelectorParser.cs b/src/Markup/Avalonia.Markup/Markup/Parsers/SelectorParser.cs
index 1b7ee9025e..5b6522064a 100644
--- a/src/Markup/Avalonia.Markup/Markup/Parsers/SelectorParser.cs
+++ b/src/Markup/Avalonia.Markup/Markup/Parsers/SelectorParser.cs
@@ -138,16 +138,16 @@ namespace Avalonia.Markup.Parsers
 
                         break;
                     case SelectorGrammar.ChildSyntax child:
-                        result = result.Child();
+                        result = result!.Child();
                         break;
                     case SelectorGrammar.DescendantSyntax descendant:
                         result = result.Descendant();
                         break;
                     case SelectorGrammar.TemplateSyntax template:
-                        result = result.Template();
+                        result = result!.Template();
                         break;
                     case SelectorGrammar.NotSyntax not:
-                        result = result.Not(x => Create(not.Argument));
+                        result = result.Not(x => Create(not.Argument)!);
                         break;
                     case SelectorGrammar.NthChildSyntax nth:
                         result = result.NthChild(nth.Step, nth.Offset);