Addressed feedback on PR #10608

src/Avalonia.Controls/Carousel.cs

@@ -1,7 +1,6 @@
 using Avalonia.Animation;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
-using Avalonia.Controls.Utils;
 
 namespace Avalonia.Controls
 {
@@ -20,8 +19,8 @@ namespace Avalonia.Controls
         /// The default value of <see cref="ItemsControl.ItemsPanelProperty"/> for 
         /// <see cref="Carousel"/>.
         /// </summary>
-        private static readonly ITemplate<Panel> PanelTemplate =
-            new FuncTemplate<Panel>(() => new VirtualizingCarouselPanel());
+        private static readonly FuncTemplate<Panel?> DefaultPanel =
+            new(() => new VirtualizingCarouselPanel());
 
         private IScrollable? _scroller;
 
@@ -31,7 +30,7 @@ namespace Avalonia.Controls
         static Carousel()
         {
             SelectionModeProperty.OverrideDefaultValue<Carousel>(SelectionMode.AlwaysSelected);
-            ItemsPanelProperty.OverrideDefaultValue<Carousel>(PanelTemplate);
+            ItemsPanelProperty.OverrideDefaultValue<Carousel>(DefaultPanel);
         }
 
         /// <summary>

src/Avalonia.Controls/ComboBox.cs

@@ -1,13 +1,10 @@
 using System;
-using System.Diagnostics;
 using System.Linq;
 using Avalonia.Automation.Peers;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
-using Avalonia.Controls.Selection;
 using Avalonia.Controls.Shapes;
 using Avalonia.Controls.Templates;
-using Avalonia.Controls.Utils;
 using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.Layout;
@@ -29,8 +26,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// The default value for the <see cref="ItemsControl.ItemsPanel"/> property.
         /// </summary>
-        private static readonly FuncTemplate<Panel> DefaultPanel =
-            new FuncTemplate<Panel>(() => new VirtualizingStackPanel());
+        private static readonly FuncTemplate<Panel?> DefaultPanel =
+            new(() => new VirtualizingStackPanel());
 
         /// <summary>
         /// Defines the <see cref="IsDropDownOpen"/> property.

src/Avalonia.Controls/ContextMenu.cs

@@ -4,7 +4,6 @@ using System.ComponentModel;
 using Avalonia.Automation.Peers;
 using System.Linq;
 using Avalonia.Controls.Diagnostics;
-using Avalonia.Controls.Generators;
 using Avalonia.Controls.Platform;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Primitives.PopupPositioning;
@@ -84,8 +83,8 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<Control?> PlacementTargetProperty =
             Popup.PlacementTargetProperty.AddOwner<ContextMenu>();
 
-        private static readonly ITemplate<Panel> DefaultPanel =
-            new FuncTemplate<Panel>(() => new StackPanel { Orientation = Orientation.Vertical });
+        private static readonly FuncTemplate<Panel?> DefaultPanel =
+            new(() => new StackPanel { Orientation = Orientation.Vertical });
         private Popup? _popup;
         private List<Control>? _attachedControls;
         private IInputElement? _previousFocus;

src/Avalonia.Controls/ItemsControl.cs

@@ -2,9 +2,7 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
-using System.Diagnostics.CodeAnalysis;
 using Avalonia.Automation.Peers;
-using Avalonia.Collections;
 using Avalonia.Controls.Generators;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Presenters;
@@ -29,8 +27,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// The default value for the <see cref="ItemsPanel"/> property.
         /// </summary>
-        private static readonly FuncTemplate<Panel> DefaultPanel =
-            new FuncTemplate<Panel>(() => new StackPanel());
+        private static readonly FuncTemplate<Panel?> DefaultPanel =
+            new(() => new StackPanel());
 
         /// <summary>
         /// Defines the <see cref="Items"/> property.
@@ -58,8 +56,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="ItemsPanel"/> property.
         /// </summary>
-        public static readonly StyledProperty<ITemplate<Panel>> ItemsPanelProperty =
-            AvaloniaProperty.Register<ItemsControl, ITemplate<Panel>>(nameof(ItemsPanel), DefaultPanel);
+        public static readonly StyledProperty<ITemplate<Panel?>> ItemsPanelProperty =
+            AvaloniaProperty.Register<ItemsControl, ITemplate<Panel?>>(nameof(ItemsPanel), DefaultPanel);
 
         /// <summary>
         /// Defines the <see cref="ItemsSource"/> property.
@@ -202,7 +200,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the panel used to display the items.
         /// </summary>
-        public ITemplate<Panel> ItemsPanel
+        public ITemplate<Panel?> ItemsPanel
         {
             get => GetValue(ItemsPanelProperty);
             set => SetValue(ItemsPanelProperty, value);

src/Avalonia.Controls/ListBox.cs

@@ -1,13 +1,10 @@
 using System.Collections;
-using Avalonia.Controls.Generators;
 using Avalonia.Controls.Metadata;
-using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Selection;
 using Avalonia.Controls.Templates;
 using Avalonia.Input;
 using Avalonia.Input.Platform;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Controls
 {
@@ -20,8 +17,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// The default value for the <see cref="ItemsControl.ItemsPanel"/> property.
         /// </summary>
-        private static readonly FuncTemplate<Panel> DefaultPanel =
-            new FuncTemplate<Panel>(() => new VirtualizingStackPanel());
+        private static readonly FuncTemplate<Panel?> DefaultPanel =
+            new(() => new VirtualizingStackPanel());
 
         /// <summary>
         /// Defines the <see cref="Scroll"/> property.

src/Avalonia.Controls/Menu.cs

@@ -1,7 +1,6 @@
 using Avalonia.Automation;
 using Avalonia.Automation.Peers;
 using Avalonia.Controls.Platform;
-using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
 using Avalonia.Input;
 using Avalonia.Interactivity;
@@ -14,9 +13,8 @@ namespace Avalonia.Controls
     /// </summary>
     public class Menu : MenuBase, IMainMenu
     {
-        private static readonly ITemplate<Panel> DefaultPanel =
-            new FuncTemplate<Panel>(() => new StackPanel { Orientation = Orientation.Horizontal });
-
+        private static readonly FuncTemplate<Panel?> DefaultPanel =
+            new (() => new StackPanel { Orientation = Orientation.Horizontal });
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Menu"/> class.

src/Avalonia.Controls/MenuItem.cs

@@ -4,7 +4,6 @@ using System.Linq;
 using Avalonia.Reactive;
 using System.Windows.Input;
 using Avalonia.Automation.Peers;
-using Avalonia.Controls.Generators;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Mixins;
 using Avalonia.Controls.Primitives;
@@ -107,8 +106,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// The default value for the <see cref="ItemsControl.ItemsPanel"/> property.
         /// </summary>
-        private static readonly ITemplate<Panel> DefaultPanel =
-            new FuncTemplate<Panel>(() => new StackPanel());
+        private static readonly FuncTemplate<Panel?> DefaultPanel =
+            new(() => new StackPanel());
 
         private bool _commandCanExecute = true;
         private bool _commandBindingError;

src/Avalonia.Controls/Presenters/ItemsPresenter.cs

@@ -16,7 +16,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Defines the <see cref="ItemsPanel"/> property.
         /// </summary>
-        public static readonly StyledProperty<ITemplate<Panel>> ItemsPanelProperty =
+        public static readonly StyledProperty<ITemplate<Panel?>> ItemsPanelProperty =
             ItemsControl.ItemsPanelProperty.AddOwner<ItemsPresenter>();
 
         private PanelContainerGenerator? _generator;
@@ -68,7 +68,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets or sets a template which creates the <see cref="Panel"/> used to display the items.
         /// </summary>
-        public ITemplate<Panel> ItemsPanel
+        public ITemplate<Panel?> ItemsPanel
         {
             get => GetValue(ItemsPanelProperty);
             set => SetValue(ItemsPanelProperty, value);
@@ -166,6 +166,12 @@ namespace Avalonia.Controls.Presenters
                 }
 
                 Panel = ItemsPanel.Build();
+
+                if (Panel is null)
+                {
+                    return;
+                }
+
                 Panel.TemplatedParent = TemplatedParent;
                 Panel.IsItemsHost = true;
                 _scrollSnapPointsInfo = Panel as IScrollSnapPointsInfo;
@@ -183,7 +189,7 @@ namespace Avalonia.Controls.Presenters
                 else
                     CreateSimplePanelGenerator();
 
-                if(Panel is IScrollSnapPointsInfo scrollSnapPointsInfo)
+                if (Panel is IScrollSnapPointsInfo scrollSnapPointsInfo)
                 {
                     scrollSnapPointsInfo.VerticalSnapPointsChanged += (s, e) =>
                     {

src/Avalonia.Controls/Primitives/TabStrip.cs

@@ -1,15 +1,13 @@
-using Avalonia.Controls.Generators;
 using Avalonia.Controls.Templates;
 using Avalonia.Input;
 using Avalonia.Layout;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Controls.Primitives
 {
     public class TabStrip : SelectingItemsControl
     {
-        private static readonly FuncTemplate<Panel> DefaultPanel =
-            new FuncTemplate<Panel>(() => new WrapPanel { Orientation = Orientation.Horizontal });
+        private static readonly FuncTemplate<Panel?> DefaultPanel =
+            new(() => new WrapPanel { Orientation = Orientation.Horizontal });
 
         static TabStrip()
         {

src/Avalonia.Controls/TabControl.cs

@@ -1,8 +1,6 @@
-using System.ComponentModel;
 using System.Linq;
 using Avalonia.Collections;
 using Avalonia.Automation.Peers;
-using Avalonia.Controls.Generators;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
@@ -12,7 +10,6 @@ using Avalonia.LogicalTree;
 using Avalonia.VisualTree;
 using Avalonia.Automation;
 using Avalonia.Controls.Metadata;
-using Avalonia.Data;
 
 namespace Avalonia.Controls
 {
@@ -61,8 +58,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// The default value for the <see cref="ItemsControl.ItemsPanel"/> property.
         /// </summary>
-        private static readonly FuncTemplate<Panel> DefaultPanel =
-            new FuncTemplate<Panel>(() => new WrapPanel());
+        private static readonly FuncTemplate<Panel?> DefaultPanel =
+            new(() => new WrapPanel());
 
         /// <summary>
         /// Initializes static members of the <see cref="TabControl"/> class.

src/Avalonia.Controls/Templates/FuncTemplate`1.cs

@@ -7,7 +7,7 @@ namespace Avalonia.Controls.Templates
     /// Creates a control from a <see cref="Func{TControl}"/>.
     /// </summary>
     /// <typeparam name="TControl">The type of control.</typeparam>
-    public class FuncTemplate<TControl> : ITemplate<TControl> where TControl : Control
+    public class FuncTemplate<TControl> : ITemplate<TControl> where TControl : Control?
     {
         private readonly Func<TControl> _func;
 
@@ -31,6 +31,6 @@ namespace Avalonia.Controls.Templates
             return _func();
         }
 
-        object ITemplate.Build() => Build();
+        object? ITemplate.Build() => Build();
     }
 }

src/Avalonia.Controls/TreeViewItem.cs

@@ -40,8 +40,8 @@ namespace Avalonia.Controls
             AvaloniaProperty.RegisterDirect<TreeViewItem, int>(
                 nameof(Level), o => o.Level);
 
-        private static readonly ITemplate<Panel> DefaultPanel =
-            new FuncTemplate<Panel>(() => new StackPanel());
+        private static readonly FuncTemplate<Panel?> DefaultPanel =
+            new(() => new StackPanel());
 
         private TreeView? _treeView;
         private Control? _header;

src/Markup/Avalonia.Markup.Xaml/Extensions.cs

@@ -31,9 +31,9 @@ namespace Avalonia.Markup.Xaml
         [RequiresUnreferencedCode(TrimmingMessages.XamlTypeResolvedRequiresUnreferenceCodeMessage)]
         public static Type ResolveType(this IServiceProvider ctx, string? namespacePrefix, string type)
         {
-            var tr = ctx.GetService<IXamlTypeResolver>();
+            var tr = ctx.GetRequiredService<IXamlTypeResolver>();
             string name = string.IsNullOrEmpty(namespacePrefix) ? type : $"{namespacePrefix}:{type}";
-            return tr?.Resolve(name) ?? throw new XamlLoadException($"Could not find type '{name}'.");
+            return tr.Resolve(name);
         }
         
         public static object? GetDefaultAnchor(this IServiceProvider provider)

src/Markup/Avalonia.Markup/Markup/Parsers/ExpressionParser.cs

@@ -13,7 +13,7 @@ namespace Avalonia.Markup.Parsers
     internal class ExpressionParser
     {
         private readonly bool _enableValidation;
-        private readonly Func<string, string, Type>? _typeResolver;
+        private readonly Func<string?, string, Type>? _typeResolver;
         private readonly INameScope? _nameScope;
 
         public ExpressionParser(bool enableValidation, Func<string?, string, Type>? typeResolver, INameScope? nameScope)
@@ -88,7 +88,7 @@ namespace Avalonia.Markup.Parsers
             Type? ancestorType = null;
             var ancestorLevel = node.Level;
 
-            if (!(node.Namespace is null) && !(node.TypeName is null))
+            if (!string.IsNullOrEmpty(node.TypeName))
             {
                 if (_typeResolver == null)
                 {
@@ -104,7 +104,7 @@ namespace Avalonia.Markup.Parsers
         private TypeCastNode ParseTypeCastNode(BindingExpressionGrammar.TypeCastNode node)
         {
             Type? castType = null;
-            if (!(node.Namespace is null) && !(node.TypeName is null))
+            if (!string.IsNullOrEmpty(node.TypeName))
             {
                 if (_typeResolver == null)
                 {