add nullable and refactor.

src/Avalonia.Controls/RelativePanel.AttachedProperties.cs

@@ -1,14 +1,17 @@
 using Avalonia.Layout;
 
+#nullable enable
+
 namespace Avalonia.Controls
 {
     public partial class RelativePanel
     {
         private static void OnAlignPropertiesChanged(AvaloniaObject d, AvaloniaPropertyChangedEventArgs e)
         {
-            var elm = d as Layoutable;
-            if (elm.Parent is Layoutable)
-                ((Layoutable)elm.Parent).InvalidateArrange();
+            if (d is Layoutable layoutable && layoutable.Parent is Layoutable layoutableParent)
+            {
+                layoutableParent.InvalidateArrange();
+            }
         }
 
         static RelativePanel()

src/Avalonia.Controls/RelativePanel.cs

@@ -4,6 +4,8 @@ using System.Collections.Generic;
 using System.Linq;
 using Avalonia.Layout;
 
+#nullable enable
+
 namespace Avalonia.Controls
 {
     public partial class RelativePanel : Panel
@@ -80,7 +82,7 @@ namespace Avalonia.Controls
             return size;
         }
 
-        private Layoutable GetDependencyElement(AvaloniaProperty property, AvaloniaObject child)
+        private Layoutable? GetDependencyElement(AvaloniaProperty property, AvaloniaObject child)
         {
             var dependency = child.GetValue(property);
 
@@ -89,7 +91,7 @@ namespace Avalonia.Controls
                 if (Children.Contains((ILayoutable)layoutable))
                     return layoutable;
 
-                throw new ArgumentException(string.Format("RelativePanel error: Element does not exist in the current context", property.Name));
+                throw new ArgumentException($"RelativePanel error: Element does not exist in the current context: {property.Name}");
             }
 
             return null;
@@ -105,25 +107,25 @@ namespace Avalonia.Controls
 
             public HashSet<GraphNode> OutgoingNodes { get; }
 
-            public GraphNode AlignLeftWithNode { get; set; }
+            public GraphNode? AlignLeftWithNode { get; set; }
 
-            public GraphNode AlignTopWithNode { get; set; }
+            public GraphNode? AlignTopWithNode { get; set; }
 
-            public GraphNode AlignRightWithNode { get; set; }
+            public GraphNode? AlignRightWithNode { get; set; }
 
-            public GraphNode AlignBottomWithNode { get; set; }
+            public GraphNode? AlignBottomWithNode { get; set; }
 
-            public GraphNode LeftOfNode { get; set; }
+            public GraphNode? LeftOfNode { get; set; }
 
-            public GraphNode AboveNode { get; set; }
+            public GraphNode? AboveNode { get; set; }
 
-            public GraphNode RightOfNode { get; set; }
+            public GraphNode? RightOfNode { get; set; }
 
-            public GraphNode BelowNode { get; set; }
+            public GraphNode? BelowNode { get; set; }
 
-            public GraphNode AlignHorizontalCenterWith { get; set; }
+            public GraphNode? AlignHorizontalCenterWith { get; set; }
 
-            public GraphNode AlignVerticalCenterWith { get; set; }
+            public GraphNode? AlignVerticalCenterWith { get; set; }
 
             public GraphNode(Layoutable element)
             {
@@ -143,7 +145,7 @@ namespace Avalonia.Controls
                 _nodeDic = new Dictionary<AvaloniaObject, GraphNode>();
             }
 
-            public GraphNode AddLink(GraphNode from, Layoutable to)
+            public GraphNode? AddLink(GraphNode from, Layoutable? to)
             {
                 if (to == null)
                     return null;
@@ -183,12 +185,9 @@ namespace Avalonia.Controls
 
             public bool CheckCyclic() => CheckCyclic(_nodeDic.Values, null);
 
-            private bool CheckCyclic(IEnumerable<GraphNode> nodes, HashSet<Layoutable> set)
+            private bool CheckCyclic(IEnumerable<GraphNode> nodes, HashSet<Layoutable>? set)
             {
-                if (set == null)
-                {
-                    set = new HashSet<Layoutable>();
-                }
+                set ??= new HashSet<Layoutable>();
 
                 foreach (var node in nodes)
                 {
@@ -341,7 +340,7 @@ namespace Avalonia.Controls
                     }
                     else
                     {
-                        childPos = childPos.WithY((_arrangeSize.Height + node.AlignLeftWithNode.Position.Y - childSize.Height) / 2);
+                        childPos = childPos.WithY((_arrangeSize.Height + node.AlignTopWithNode.Position.Y - childSize.Height) / 2);
                     }
                 }
 

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/ResolveByNameExtension.cs

@@ -2,6 +2,8 @@
 using Avalonia.Controls;
 using Avalonia.Data.Core;
 
+#nullable  enable
+
 namespace Avalonia.Markup.Xaml.MarkupExtensions
 {
     public class ResolveByNameExtension
@@ -13,20 +15,19 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
 
         public string Name { get; }
 
-        public object ProvideValue(IServiceProvider serviceProvider)
+        public object? ProvideValue(IServiceProvider serviceProvider)
         {
-            var namescope = serviceProvider.GetService<INameScope>();
+            var nameScope = serviceProvider.GetService<INameScope>();
             
-            var value = namescope.FindAsync(Name);
+            var value = nameScope.FindAsync(Name);
 
             if(value.IsCompleted)
                 return value.GetResult();
 
             var provideValueTarget = serviceProvider.GetService<IProvideValueTarget>();
             var target = provideValueTarget.TargetObject;
-            var property = provideValueTarget.TargetProperty as IPropertyInfo;
 
-            if (property != null) 
+            if (provideValueTarget.TargetProperty is IPropertyInfo property) 
                 value.OnCompleted(() => property.Set(target, value.GetResult()));
 
             return null;

src/Markup/Avalonia.Markup.Xaml/XamlIl/CompilerExtensions/Transformers/AvaloniaXamlIlResolveByNameMarkupExtensionReplacer.cs

@@ -5,47 +5,40 @@ using XamlX.Ast;
 using XamlX.Transform;
 using XamlX.TypeSystem;
 
+#nullable  enable
 namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
 {
     class AvaloniaXamlIlResolveByNameMarkupExtensionReplacer : IXamlAstTransformer
     {
         public IXamlAstNode Transform(AstTransformationContext context, IXamlAstNode node)
         {
-            if (node is XamlAstXamlPropertyValueNode propertyValueNode)
+            if (!(node is XamlAstXamlPropertyValueNode propertyValueNode)) return node;
+            
+            IEnumerable<IXamlCustomAttribute> attributes = propertyValueNode.Property.GetClrProperty().CustomAttributes;
+
+            if (propertyValueNode.Property is XamlAstClrProperty referenceNode &&
+                referenceNode.Getter != null)
             {
+                attributes = attributes.Concat(referenceNode.Getter.CustomAttributes);
+            }
 
-                IEnumerable<IXamlCustomAttribute> attributes = propertyValueNode.Property.GetClrProperty().CustomAttributes;
+            if (attributes.All(attribute => attribute.Type.FullName != "Avalonia.Controls.ResolveByNameAttribute"))
+                return node;
 
-                if (propertyValueNode.Property is XamlAstClrProperty referenceNode &&
-                    referenceNode.Getter != null)
-                {
-                    attributes = attributes.Concat(referenceNode.Getter.CustomAttributes);
-                }
+            if (propertyValueNode.Values.Count != 1 || !(propertyValueNode.Values.First() is XamlAstTextNode))
+                return node;
 
-                foreach (var attribute in attributes)
-                {
-                    if (attribute.Type.FullName == "Avalonia.Controls.ResolveByNameAttribute")
-                    {
-                        if (propertyValueNode.Values.Count == 1 &&
-                            propertyValueNode.Values.First() is XamlAstTextNode)
-                        {
-                            if (XamlTransformHelpers.TryConvertMarkupExtension(context, new XamlAstObjectNode(
-                                    propertyValueNode.Values[0],
-                                    new XamlAstClrTypeReference(propertyValueNode.Values[0],
-                                    context.GetAvaloniaTypes().ResolveByNameExtension, true))
-                            {
-                                Arguments = new System.Collections.Generic.List<IXamlAstValueNode>
-                                    {
-                                        propertyValueNode.Values[0]
-                                    }
-                            }, out var extensionNode))
-                            {
-                                propertyValueNode.Values[0] = extensionNode;
-                            }
-                        }
-                        break;
-                    }
-                }
+            var newNode = new XamlAstObjectNode(
+                propertyValueNode.Values[0],
+                new XamlAstClrTypeReference(propertyValueNode.Values[0],
+                    context.GetAvaloniaTypes().ResolveByNameExtension, true))
+            {
+                Arguments = new List<IXamlAstValueNode> { propertyValueNode.Values[0] }
+            };
+            
+            if (XamlTransformHelpers.TryConvertMarkupExtension(context, newNode, out var extensionNode))
+            {
+                propertyValueNode.Values[0] = extensionNode;
             }
 
             return node;