Add code coverage for CheckBoxFlatAdapter (#13751)

* Add code coverage for CheckBoxFlatAdapter

* Address FeedBacks

Author: Zheng-Li01

src/test/unit/System.Windows.Forms/System/Windows/Forms/CheckBoxFlatAdapterTests.cs

@@ -0,0 +1,130 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Drawing;
+using System.Windows.Forms.ButtonInternal;
+
+namespace System.Windows.Forms.Tests;
+
+public class CheckBoxFlatAdapterTests : IDisposable
+{
+    private TestCheckBox? _checkBox;
+
+    private (TestCheckBoxFlatAdapter, TestCheckBox) CreateAdapter(Appearance appearance, bool enabled)
+    {
+        _checkBox = new TestCheckBox
+        {
+            Appearance = appearance,
+            Enabled = enabled
+        };
+
+        TestCheckBoxFlatAdapter checkBoxFlatAdapter = new(_checkBox);
+
+        return (checkBoxFlatAdapter, _checkBox);
+    }
+
+    public void Dispose() => _checkBox?.Dispose();
+
+    private class TestCheckBox : CheckBox
+    {
+        public new bool Enabled
+        {
+            get => base.Enabled;
+            set => base.Enabled = value;
+        }
+
+        public new Appearance Appearance
+        {
+            get => base.Appearance;
+            set => base.Appearance = value;
+        }
+
+        public new CheckState CheckState
+        {
+            get => base.CheckState;
+            set => base.CheckState = value;
+        }
+    }
+
+    private class TestCheckBoxFlatAdapter : CheckBoxFlatAdapter
+    {
+        public TestCheckBoxFlatAdapter(ButtonBase control) : base(control) { }
+
+        public void CallPaintDown(PaintEventArgs e, CheckState state) => PaintDown(e, state);
+
+        public bool PaintFlatWorkerCalled { get; private set; }
+
+        protected void PaintFlatWorker() => PaintFlatWorkerCalled = true;
+    }
+
+    [WinFormsTheory]
+    [InlineData(Appearance.Button, true)]
+    [InlineData(Appearance.Button, false)]
+    [InlineData(Appearance.Normal, true)]
+    [InlineData(Appearance.Normal, false)]
+    public void PaintDown_DoesNotThrow(Appearance appearance, bool enabled)
+    {
+        (TestCheckBoxFlatAdapter checkBoxFlatAdapter, TestCheckBox checkBox) = CreateAdapter(appearance, enabled);
+        checkBox.CheckState = CheckState.Checked;
+
+        using Bitmap bitmap = new(20, 20);
+        using Graphics graphics = Graphics.FromImage(bitmap);
+        using PaintEventArgs e = new(graphics, new Rectangle(0, 0, 20, 20));
+
+        Exception? exception = Record.Exception(() =>
+            checkBoxFlatAdapter.CallPaintDown(e, checkBox.CheckState));
+
+        exception.Should().BeNull();
+    }
+
+    [WinFormsTheory]
+    [InlineData(Appearance.Button, true)]
+    [InlineData(Appearance.Button, false)]
+    [InlineData(Appearance.Normal, true)]
+    [InlineData(Appearance.Normal, false)]
+    public void PaintOver_DoesNotThrow(Appearance appearance, bool enabled)
+    {
+        (TestCheckBoxFlatAdapter checkBoxFlatAdapter, TestCheckBox checkBox) = CreateAdapter(appearance, enabled);
+        checkBox.CheckState = CheckState.Indeterminate;
+
+        using Bitmap bitmap = new(20, 20);
+        using Graphics graphics = Graphics.FromImage(bitmap);
+        using PaintEventArgs e = new(graphics, new Rectangle(0, 0, 20, 20));
+
+        Exception? exception = Record.Exception(() =>
+            checkBoxFlatAdapter.PaintOver(e, checkBox.CheckState));
+
+        exception.Should().BeNull();
+    }
+
+    [WinFormsTheory]
+    [InlineData(Appearance.Button, true)]
+    [InlineData(Appearance.Button, false)]
+    [InlineData(Appearance.Normal, true)]
+    [InlineData(Appearance.Normal, false)]
+    public void PaintUp_DoesNotThrow(Appearance appearance, bool enabled)
+    {
+        (TestCheckBoxFlatAdapter checkBoxFlatAdapter, TestCheckBox checkBox) = CreateAdapter(appearance, enabled);
+        checkBox.CheckState = CheckState.Unchecked;
+
+        using Bitmap bitmap = new(20, 20);
+        using Graphics graphics = Graphics.FromImage(bitmap);
+        using PaintEventArgs e = new(graphics, new Rectangle(0, 0, 20, 20));
+
+        Exception? exception = Record.Exception(() =>
+            checkBoxFlatAdapter.PaintUp(e, checkBox.CheckState));
+
+        exception.Should().BeNull();
+    }
+
+    [WinFormsFact]
+    public void CreateButtonAdapter_ReturnsButtonFlatAdapter()
+    {
+        (TestCheckBoxFlatAdapter checkBoxFlatAdapter, _) = CreateAdapter(Appearance.Normal, true);
+
+        ButtonBaseAdapter result = checkBoxFlatAdapter.TestAccessor().Dynamic.CreateButtonAdapter();
+
+        result.Should().NotBeNull();
+        result.Should().BeOfType<ButtonFlatAdapter>();
+    }
+}