Add unit tests

src/System.Windows.Forms.Primitives/src/System/LocalAppContextSwitches/LocalAppContextSwitches.cs

@@ -11,7 +11,7 @@ namespace System.Windows.Forms.Primitives
     internal static partial class LocalAppContextSwitches
     {
         private const string ScaleTopLevelFormMinMaxSizeForDpiSwitchName = "System.Windows.Forms.ScaleTopLevelFormMinMaxSizeForDpi";
-        private const string EnableImprovedAnchorLayoutSwitchName = "System.Windows.Forms.EnableImprovedAnchorLayout";
+        internal const string EnableImprovedAnchorLayoutSwitchName = "System.Windows.Forms.EnableImprovedAnchorLayout";
 
         private static int s_scaleTopLevelFormMinMaxSizeForDpi;
         private static int s_enableImprovedAnchorLayout;

src/System.Windows.Forms/src/System/Windows/Forms/Layout/DefaultLayout.cs

@@ -717,13 +717,6 @@ internal static void UpdateAnchorInfo(IArrangedElement element, bool enableImpro
                 return;
             }
 
-            AnchorInfo anchorInfo = GetAnchorInfo(element);
-            if (anchorInfo is null)
-            {
-                anchorInfo = new AnchorInfo();
-                SetAnchorInfo(element, anchorInfo);
-            }
-
             Debug.WriteLineIf(CompModSwitches.RichLayout.TraceInfo, "Update anchor info");
             Debug.Indent();
             Debug.WriteLineIf(CompModSwitches.RichLayout.TraceInfo, element.Container is null ? "No parent" : "Parent");
@@ -735,16 +728,23 @@ internal static void UpdateAnchorInfo(IArrangedElement element, bool enableImpro
 
             if (enableImprovedAnchorLayout)
             {
-                ComputeAnchorInfo(element, anchorInfo);
+                ComputeAnchorInfo(element);
             }
             else
             {
-                ComputeLegacyAnchorInfo(element, anchorInfo);
+                ComputeLegacyAnchorInfo(element);
             }
         }
 
-        private static void ComputeLegacyAnchorInfo(IArrangedElement element, AnchorInfo anchorInfo)
+        private static void ComputeLegacyAnchorInfo(IArrangedElement element)
         {
+            AnchorInfo anchorInfo = GetAnchorInfo(element);
+            if (anchorInfo is null)
+            {
+                anchorInfo = new AnchorInfo();
+                SetAnchorInfo(element, anchorInfo);
+            }
+
             Rectangle bounds = GetCachedBounds(element);
             AnchorInfo oldAnchorInfo = new AnchorInfo
             {
@@ -833,8 +833,15 @@ private static void ComputeLegacyAnchorInfo(IArrangedElement element, AnchorInfo
             Debug.Unindent();
         }
 
-        private static void ComputeAnchorInfo(IArrangedElement element, AnchorInfo anchorInfo)
+        private static void ComputeAnchorInfo(IArrangedElement element)
         {
+            AnchorInfo anchorInfo = GetAnchorInfo(element);
+            if (anchorInfo is null)
+            {
+                anchorInfo = new AnchorInfo();
+                SetAnchorInfo(element, anchorInfo);
+            }
+
             Rectangle displayRect = element.Container.DisplayRectangle;
             Rectangle elementBounds = element.Bounds;
 
@@ -1042,7 +1049,7 @@ private static void SetCachedBounds(IArrangedElement element, Rectangle bounds)
             }
         }
 
-        private static AnchorInfo GetAnchorInfo(IArrangedElement element)
+        internal static AnchorInfo GetAnchorInfo(IArrangedElement element)
         {
             return (AnchorInfo)element.Properties.GetObject(s_layoutInfoProperty);
         }

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/Layout/AnchorLayoutTests.cs

@@ -0,0 +1,114 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Xunit;
+using System.Drawing;
+using System.Windows.Forms.Primitives;
+
+namespace System.Windows.Forms.Layout.Tests
+{
+    public class AnchorLayoutTests : IClassFixture<ThreadExceptionFixture>
+    {
+        private static (Form, Button) GetFormWithAnchoredButton()
+        {
+            Form form = new ();
+            form.Size = new Size(200, 300);
+            Button button = new ();
+            button.Location = new Point(20, 30);
+            button.Size = new Size(20, 30);
+            button.Anchor = AnchorStyles.Left | AnchorStyles.Top | AnchorStyles.Right | AnchorStyles.Bottom;
+
+            return (form, button);
+        }
+
+        private static void Dispose(Form form, Button button)
+        {
+            button?.Dispose();
+            form?.Dispose();
+        }
+
+        [WinFormsFact]
+        public void ControlNotParented_NoAnchorsComputed()
+        {
+            var (form, button) = GetFormWithAnchoredButton();
+            DefaultLayout.AnchorInfo anchorInfo = DefaultLayout.GetAnchorInfo(button);
+            Assert.Null(anchorInfo);
+            _ = button.Handle; // Force handle creation;
+            Assert.Null(anchorInfo);
+            Dispose(form, button);
+        }
+
+        [WinFormsFact]
+        public void ControlHandleNotCreated_NoAnchorsComputed()
+        {
+            var (form, button) = GetFormWithAnchoredButton();
+            DefaultLayout.AnchorInfo anchorInfo = DefaultLayout.GetAnchorInfo(button);
+            Assert.Null(anchorInfo);
+            form.Controls.Add(button);
+            Assert.Null(anchorInfo);
+            Dispose(form, button);
+        }
+
+        [WinFormsFact]
+        public void ControlParentHandleNotCreated_NoAnchorsComputed()
+        {
+            var (form, button) = GetFormWithAnchoredButton();
+            DefaultLayout.AnchorInfo anchorInfo = DefaultLayout.GetAnchorInfo(button);
+            Assert.Null(anchorInfo);
+            _ = button.Handle; // Force handle creation;
+            form.Controls.Add(button);
+            Assert.Null(anchorInfo);
+            Dispose(form, button);
+        }
+
+        [WinFormsFact]
+        public void ConfigSwitchDisabled_NoHanldeCreated_AnchorsComputed()
+        {
+            AppContext.SetSwitch(LocalAppContextSwitches.EnableImprovedAnchorLayoutSwitchName, false);
+            var (form, button) = GetFormWithAnchoredButton();
+            form.Controls.Add(button);
+            DefaultLayout.AnchorInfo anchorInfo = DefaultLayout.GetAnchorInfo(button);
+            Assert.NotNull(anchorInfo);
+            Dispose(form, button);
+        }
+
+        [WinFormsTheory]
+        [InlineData(AnchorStyles.Top, 44, 30, 20, 30)]
+        [InlineData(AnchorStyles.Left, 20, 68, 20, 30)]
+        [InlineData(AnchorStyles.Right, 220, 68, 20, 30)]
+        [InlineData(AnchorStyles.Bottom, 44, 330, 20, 30)]
+        [InlineData(AnchorStyles.Top | AnchorStyles.Left, 20, 30, 20, 30)]
+        [InlineData(AnchorStyles.Top | AnchorStyles.Right, 220, 30, 20, 30)]
+        [InlineData(AnchorStyles.Top | AnchorStyles.Bottom, 44, 30, 20, 330)]
+        [InlineData(AnchorStyles.Left | AnchorStyles.Right, 20, 68, 220, 30)]
+        [InlineData(AnchorStyles.Bottom | AnchorStyles.Right | AnchorStyles.Top | AnchorStyles.Left, 20, 30, 220, 330)]
+        public void ResizeAnchoredControlsParent_HanldeCreated_AnchorsApplied(AnchorStyles anchor, int x, int y, int width, int height)
+        {
+            AppContext.SetSwitch(LocalAppContextSwitches.EnableImprovedAnchorLayoutSwitchName, true);
+            var (form, button) = GetFormWithAnchoredButton();
+            button.Anchor = anchor;
+            DefaultLayout.AnchorInfo anchorInfo = DefaultLayout.GetAnchorInfo(button);
+            Assert.Null(anchorInfo);
+            form.Controls.Add(button);
+            form.Shown += OnFormShown;
+            form.ShowDialog();
+            Dispose(form, button);
+
+            void OnFormShown(object sender, EventArgs e)
+            {
+                Form formLocal = (Form)sender;
+                Control button1 = formLocal.Controls[0];
+
+                //Resize Form to compute button anchors.
+                formLocal.Size = new Size(400, 600);
+
+                Assert.Equal(x, button1.Bounds.X);
+                Assert.Equal(y, button1.Bounds.Y);
+                Assert.Equal(width, button1.Bounds.Width);
+                Assert.Equal(height, button1.Bounds.Height);
+                form.Close();
+            }
+        }
+    }
+}