Added class selector style benchmarks.

tests/Avalonia.Benchmarks/Styling/Style_ClassSelector.cs

@@ -0,0 +1,85 @@
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using Avalonia.Controls;
+using Avalonia.Styling;
+using BenchmarkDotNet.Attributes;
+
+#nullable enable
+
+namespace Avalonia.Benchmarks.Styling
+{
+    [MemoryDiagnoser]
+    public class Style_ClassSelector
+    {
+        private Style _style = null!;
+
+        public Style_ClassSelector()
+        {
+            RuntimeHelpers.RunClassConstructor(typeof(TestClass).TypeHandle);
+        }
+
+        [GlobalSetup]
+        public void Setup()
+        {
+            _style = new Style(x => x.OfType<TestClass>().Class("foo"))
+            {
+                Setters = { new Setter(TestClass.StringProperty, "foo") }
+            };
+        }
+
+        [Benchmark(OperationsPerInvoke = 50)]
+        public void Apply()
+        {
+            var target = new TestClass();
+
+            target.BeginBatchUpdate();
+
+            for (var i = 0; i < 50; ++i)
+                _style.TryAttach(target, null);
+
+            target.EndBatchUpdate();
+        }
+
+        [Benchmark(OperationsPerInvoke = 50)]
+        public void Apply_Toggle()
+        {
+            var target = new TestClass();
+
+            target.BeginBatchUpdate();
+
+            for (var i = 0; i < 50; ++i)
+                _style.TryAttach(target, null);
+
+            target.EndBatchUpdate();
+
+            target.Classes.Add("foo");
+            target.Classes.Remove("foo");
+        }
+
+        [Benchmark(OperationsPerInvoke = 50)]
+        public void Apply_Detach()
+        {
+            var target = new TestClass();
+
+            target.BeginBatchUpdate();
+
+            for (var i = 0; i < 50; ++i)
+                _style.TryAttach(target, null);
+
+            target.EndBatchUpdate();
+
+            target.DetachStyles();
+        }
+
+        private class TestClass : Control
+        {
+            public static readonly StyledProperty<string?> StringProperty =
+                AvaloniaProperty.Register<TestClass, string?>("String");
+            public void DetachStyles() => InvalidateStyles();
+        }
+
+        private class TestClass2 : Control
+        {
+        }
+    }
+}