diff --git a/src/Avalonia.Base/StyledElement.cs b/src/Avalonia.Base/StyledElement.cs
index 731cb97161..5881efce1e 100644
--- a/src/Avalonia.Base/StyledElement.cs
+++ b/src/Avalonia.Base/StyledElement.cs
@@ -78,7 +78,7 @@ namespace Avalonia
         private static readonly ControlTheme s_invalidTheme = new ControlTheme();
         private int _initCount;
         private string? _name;
-        private readonly Classes _classes = new Classes();
+        private Classes? _classes;
         private ILogicalRoot? _logicalRoot;
         private IAvaloniaList<ILogical>? _logicalChildren;
         private IResourceDictionary? _resources;
@@ -183,21 +183,7 @@ namespace Avalonia
         /// collection.
         /// </para>
         /// </remarks>
-        public Classes Classes
-        {
-            get
-            {
-                return _classes;
-            }
-
-            set
-            {
-                if (_classes != value)
-                {
-                    _classes.Replace(value);
-                }
-            }
-        }
+        public Classes Classes => _classes ??= new();
 
         /// <summary>
         /// Gets or sets the control's data context.
diff --git a/src/Avalonia.Controls/DateTimePickers/DateTimePickerPanel.cs b/src/Avalonia.Controls/DateTimePickers/DateTimePickerPanel.cs
index 9774b603e8..69b0ffe9a6 100644
--- a/src/Avalonia.Controls/DateTimePickers/DateTimePickerPanel.cs
+++ b/src/Avalonia.Controls/DateTimePickers/DateTimePickerPanel.cs
@@ -454,7 +454,7 @@ namespace Avalonia.Controls.Primitives
                 children.Add(new ListBoxItem
                 {
                     Height = ItemHeight,
-                    Classes = new Classes($"{PanelType}Item"),
+                    Classes = { $"{PanelType}Item" },
                     VerticalContentAlignment = Avalonia.Layout.VerticalAlignment.Center,
                     Focusable = false
                 });
diff --git a/src/Avalonia.Controls/Flyouts/Flyout.cs b/src/Avalonia.Controls/Flyouts/Flyout.cs
index cebcbb6562..8ec5cfa50a 100644
--- a/src/Avalonia.Controls/Flyouts/Flyout.cs
+++ b/src/Avalonia.Controls/Flyouts/Flyout.cs
@@ -18,17 +18,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets the Classes collection to apply to the FlyoutPresenter this Flyout is hosting
         /// </summary>
-        public Classes FlyoutPresenterClasses
-        {
-            get => _classes ??= new Classes();
-            set
-            {
-                if (_classes is null)
-                    _classes = value;
-                else if (_classes != value)
-                    _classes.Replace(value);
-            }
-        }
+        public Classes FlyoutPresenterClasses => _classes ??= new Classes();
 
         /// <summary>
         /// Defines the <see cref="FlyoutPresenterTheme"/> property.
diff --git a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlCompiler.cs b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlCompiler.cs
index 197815f9a0..5ca2b09eba 100644
--- a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlCompiler.cs
+++ b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlCompiler.cs
@@ -43,7 +43,8 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
                 new AvaloniaXamlIlTransformSyntheticCompiledBindingMembers());
             InsertAfter<PropertyReferenceResolver>(
                 new AvaloniaXamlIlAvaloniaPropertyResolver(),
-                new AvaloniaXamlIlReorderClassesPropertiesTransformer()
+                new AvaloniaXamlIlReorderClassesPropertiesTransformer(),
+                new AvaloniaXamlIlClassesTransformer()
             );
 
             InsertBefore<ContentConvertTransformer>(
diff --git a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlLanguageParseIntrinsics.cs b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlLanguageParseIntrinsics.cs
index 4068caac21..d8524cfd88 100644
--- a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlLanguageParseIntrinsics.cs
+++ b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/AvaloniaXamlIlLanguageParseIntrinsics.cs
@@ -221,15 +221,6 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions
                 return ConvertDefinitionList(node, text, types, types.RowDefinitions, types.RowDefinition, "row definitions", out result);
             }
 
-            if (type.Equals(types.Classes))
-            {
-                var classes = text.Split(' ');
-                var classNodes = classes.Select(c => new XamlAstTextNode(node, c, type: types.XamlIlTypes.String)).ToArray();
-
-                result = new AvaloniaXamlIlAvaloniaListConstantAstNode(node, types, types.Classes, types.XamlIlTypes.String, classNodes);
-                return true;
-            }
-
             if (types.IBrush.IsAssignableFrom(type))
             {
                 if (Color.TryParse(text, out Color color))
diff --git a/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXAmlIlClassesTransformer.cs b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXAmlIlClassesTransformer.cs
new file mode 100644
index 0000000000..ed42247d29
--- /dev/null
+++ b/src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXAmlIlClassesTransformer.cs
@@ -0,0 +1,46 @@
+using System.Linq;
+using XamlX.Ast;
+using XamlX.Transform;
+
+namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
+{
+    /// <summary>
+    /// Converts an attribute syntax property value assignment to a collection syntax property
+    /// assignment.
+    /// </summary>
+    /// <remarks>
+    /// Converts the property assignment `Classes="foo bar"` to:
+    /// 
+    /// <code>
+    ///     <StyledElement.Classes>
+    ///         <x:String>foo</String>
+    ///         <x:String>bar</String>
+    ///     </StyledElement.Classes>
+    /// </code>
+    /// </remarks>
+    class AvaloniaXamlIlClassesTransformer : IXamlAstTransformer
+    {
+        public IXamlAstNode Transform(AstTransformationContext context, IXamlAstNode node)
+        {
+            var types = context.GetAvaloniaTypes();
+
+            if (node is XamlAstXamlPropertyValueNode propertyValue &&
+                propertyValue.IsAttributeSyntax &&
+                propertyValue.Property is XamlAstClrProperty property &&
+                property.Getter?.ReturnType.Equals(types.Classes) == true &&
+                propertyValue.Values.Count == 1 &&
+                propertyValue.Values[0] is XamlAstTextNode value)
+            {
+                var classes = value.Text.Split(' ');
+                var stringType = context.Configuration.WellKnownTypes.String;
+                return new XamlAstXamlPropertyValueNode(
+                    node,
+                    property,
+                    classes.Select(x => new XamlAstTextNode(node, x, type: stringType)),
+                    false);
+            }
+
+            return node;
+        }
+    }
+}
diff --git a/tests/Avalonia.Markup.Xaml.UnitTests/Xaml/BasicTests.cs b/tests/Avalonia.Markup.Xaml.UnitTests/Xaml/BasicTests.cs
index 25c0516744..5e30198d00 100644
--- a/tests/Avalonia.Markup.Xaml.UnitTests/Xaml/BasicTests.cs
+++ b/tests/Avalonia.Markup.Xaml.UnitTests/Xaml/BasicTests.cs
@@ -1,24 +1,20 @@
-using System;
+using System.Collections;
+using System.ComponentModel;
+using System.Linq;
+using System.Xml;
 using Avalonia.Collections;
 using Avalonia.Controls;
+using Avalonia.Controls.Documents;
 using Avalonia.Controls.Presenters;
 using Avalonia.Data;
 using Avalonia.Data.Converters;
-using Avalonia.Markup.Data;
-using Avalonia.Markup.Xaml.Styling;
 using Avalonia.Markup.Xaml.Templates;
 using Avalonia.Media;
 using Avalonia.Media.Immutable;
+using Avalonia.Metadata;
 using Avalonia.Styling;
 using Avalonia.UnitTests;
-using System.Collections;
-using System.ComponentModel;
-using System.Linq;
-using System.Xml;
 using Xunit;
-using Avalonia.Controls.Documents;
-using Avalonia.Metadata;
-using Avalonia.Themes.Simple;
 
 namespace Avalonia.Markup.Xaml.UnitTests.Xaml
 {
@@ -920,6 +916,22 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
             Assert.Equal(new[] { "foo", "bar" }, target.Classes);
         }
 
+        [Fact]
+        public void Can_Specify_Button_Classes_Longform()
+        {
+            var xaml = @"
+<Button xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+  <Button.Classes>
+    <x:String>foo</x:String>
+    <x:String>bar</x:String>
+  </Button.Classes>
+</Button>";
+            var target = (Button)AvaloniaRuntimeXamlLoader.Load(xaml);
+
+            Assert.Equal(new[] { "foo", "bar" }, target.Classes);
+        }
+
         [Fact]
         public void Can_Specify_Flyout_FlyoutPresenterClasses()
         {