Add code coverage for DesignerUtils (#13341)

* Add code coverage for DesignerUtils

* Address the FeedBacks

Author: Zheng-Li01

src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/DesignerUtilsTests.cs

@@ -0,0 +1,147 @@
+// 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.Drawing.Imaging;
+
+namespace System.Windows.Forms.Design.Tests;
+
+public class DesignerUtilsTests :IDisposable
+{
+    private readonly Bitmap _bitmap;
+    private readonly Graphics _graphics;
+    private readonly Rectangle _bounds;
+
+    public DesignerUtilsTests()
+    {
+        _bitmap = new(20, 20);
+        _graphics = Graphics.FromImage(_bitmap);
+        _bounds = new(5, 5, 20, 20);
+    }
+
+    public void Dispose()
+    {
+        _graphics.Dispose();
+        _bitmap.Dispose();
+    }
+
+    [WinFormsFact]
+    public void BoxImage_ShouldReturnNonNullImage() =>
+        DesignerUtils.BoxImage.Should().BeOfType<Bitmap>();
+
+    [WinFormsFact]
+    public void BoxImage_ShouldHaveExpectedDimensions()
+    {
+        DesignerUtils.BoxImage.Width.Should().Be(DesignerUtils.s_boxImageSize);
+        DesignerUtils.BoxImage.Height.Should().Be(DesignerUtils.s_boxImageSize);
+    }
+
+    [WinFormsFact]
+    public void BoxImage_ShouldHaveExpectedPixelFormat() =>
+        ((Bitmap)DesignerUtils.BoxImage).PixelFormat.Should().Be(PixelFormat.Format32bppPArgb);
+
+    [WinFormsFact]
+    public void BoxImage_ShouldBeCached()
+    {
+        Image firstCall = DesignerUtils.BoxImage;
+        Image secondCall = DesignerUtils.BoxImage;
+        firstCall.Should().BeSameAs(secondCall);
+    }
+
+    [Fact]
+    public void HoverBrush_ShouldBeSolidBrush() =>
+        DesignerUtils.HoverBrush.Should().BeOfType<SolidBrush>();
+
+    [Fact]
+    public void HoverBrush_ShouldHaveExpectedColor() =>
+        ((SolidBrush)DesignerUtils.HoverBrush).Color.Should().Be(Color.FromArgb(50, SystemColors.Highlight));
+
+    [Fact]
+    public void MinDragSize_ShouldReturnNonEmptySize()
+    {
+        Size minDragSize = DesignerUtils.MinDragSize;
+        minDragSize.Should().NotBe(Size.Empty);
+    }
+
+    [Fact]
+    public void MinDragSize_ShouldBeCached()
+    {
+        Size firstCall = DesignerUtils.MinDragSize;
+        Size secondCall = DesignerUtils.MinDragSize;
+        firstCall.Should().Be(secondCall);
+    }
+
+    [WinFormsTheory]
+    [MemberData(nameof(ResizeBorderTestData))]
+    public void DrawResizeBorder_ShouldUseCorrectBrushBasedOnBackColor(Color backColor, Color expectedColor)
+    {
+        using Region region = new(new Rectangle(0, 0, _bounds.Width, _bounds.Height));
+        DesignerUtils.DrawResizeBorder(_graphics, region, backColor);
+        Color pixelColor = _bitmap.GetPixel(_bounds.Width / 2, _bounds.Height / 2);
+        pixelColor.ToArgb().Should().Be(expectedColor.ToArgb());
+    }
+
+    public static TheoryData<Color, Color> ResizeBorderTestData => new()
+    {
+        { Color.White, SystemColors.ControlDarkDark },
+        { Color.Black, SystemColors.ControlLight }
+    };
+
+    [WinFormsFact]
+    public void DrawGrabHandle_ShouldNotThrow_WhenCalledWithValidParameters()
+    {
+        Exception exception = Record.Exception(() => DesignerUtils.DrawGrabHandle(_graphics, _bounds, isPrimary: true));
+        exception.Should().BeNull();
+    }
+
+    [WinFormsTheory]
+    [BoolData]
+    public void DrawGrabHandle_ShouldDrawHandle_BasedOnSelectionType(bool isPrimary)
+    {
+        DesignerUtils.DrawGrabHandle(_graphics, _bounds, isPrimary);
+
+        Color pixelColor = _bitmap.GetPixel(6, 6);
+        pixelColor.ToArgb().Should().NotBe(Color.Empty.ToArgb());
+    }
+
+    [WinFormsTheory]
+    [BoolData]
+    public void DrawLockedHandle_ShouldDrawHandle_BasedOnSelectionType(bool isPrimary)
+    {
+        DesignerUtils.DrawLockedHandle(_graphics, _bounds, isPrimary);
+
+        Color pixelColor = _bitmap.GetPixel(6, 6);
+        pixelColor.ToArgb().Should().NotBe(Color.Empty.ToArgb());
+    }
+
+    [WinFormsFact]
+    public void DrawLockedHandle_ShouldDrawUpperRect_WhenCalledWithPrimarySelection()
+    {
+        DesignerUtils.DrawLockedHandle(_graphics, _bounds, isPrimary: true);
+        Color pixelColor = _bitmap.GetPixel(_bounds.Left + 1, _bounds.Top + 1);
+        pixelColor.ToArgb().Should().NotBe(Color.Empty.ToArgb());
+    }
+
+    [WinFormsFact]
+    public void DrawLockedHandle_ShouldDrawLowerRect_WhenCalledWithNonPrimarySelection()
+    {
+        DesignerUtils.DrawLockedHandle(_graphics, _bounds, isPrimary: false);
+        Color pixelColor = _bitmap.GetPixel(_bounds.Left + 1, _bounds.Top + DesignerUtils.s_lockedHandleLowerOffset + 1);
+        pixelColor.ToArgb().Should().NotBe(Color.Empty.ToArgb());
+    }
+
+    [WinFormsFact]
+    public void DrawSelectionBorder_ShouldNotThrow_WhenCalledWithValidParameters()
+    {
+        Exception exception = Record.Exception(() => DesignerUtils.DrawSelectionBorder(_graphics, _bounds));
+        exception.Should().BeNull();
+    }
+
+    [WinFormsFact]
+    public void DrawSelectionBorder_ShouldFillRectangle_WhenCalled()
+    {
+        DesignerUtils.DrawSelectionBorder(_graphics, _bounds);
+        Color pixelColor = _bitmap.GetPixel(6, 6);
+        pixelColor.ToArgb().Should().NotBe(Color.Empty.ToArgb());
+    }
+}