Add code coverage for ImageListConverter & RelatedImageListAttribute

[Zheng-Li01]

Related #13442

Proposed changes
Add unit test file: ImageListConverterTests.cs & RelatedImageListAttributeTests for public/Internal methods of the ImageListConverter.cs & RelatedImageListAttribute.cs

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.59784%. Comparing base (5821493) to head (20655b6).
Report is 16 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                  @@
##                main      #13511          +/-   ##
====================================================
+ Coverage   62.21083%   76.59784%   +14.38701%     
====================================================
  Files           3213        3232          +19     
  Lines         638302      639176         +874     
  Branches       47201       47297          +96     
====================================================
+ Hits          397093      489595       +92502     
+ Misses        234153      146000       -88153     
+ Partials        7056        3581        -3475     

Flag	Coverage Δ
Debug	76.59784% <100.00000%> (+14.38701%)	⬆️
integration	18.77731% <ø> (-0.01147%)	⬇️
production	51.00311% <ø> (+32.21433%)	⬆️
test	97.40419% <100.00000%> (+0.00007%)	⬆️
unit	48.40165% <ø> (?)

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[Copilot]

Pull Request Overview

Adds unit tests to improve code coverage for ImageListConverter and RelatedImageListAttribute.

• Introduce RelatedImageListAttributeTests covering constructor behavior
• Introduce ImageListConverterTests covering base type assignment and GetPropertiesSupported

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/test/unit/System.Windows.Forms/System/Windows/Forms/RelatedImageListAttributeTests.cs	Add test verifying that constructor value is returned by RelatedImageList property
src/test/unit/System.Windows.Forms/System/Windows/Forms/ImageListConverterTests.cs	Add tests for default constructor base type and GetPropertiesSupported

Comments suppressed due to low confidence (4)

src/test/unit/System.Windows.Forms/System/Windows/Forms/RelatedImageListAttributeTests.cs:4

• Test namespace 'System.Windows.Forms.Controls.Tests' does not align with the project’s folder and other test namespaces. Consider using 'System.Windows.Forms.Tests' to match existing patterns.

namespace System.Windows.Forms.Controls.Tests;

src/test/unit/System.Windows.Forms/System/Windows/Forms/RelatedImageListAttributeTests.cs:14

• Consider adding tests for Equals, GetHashCode, and default attribute behavior to cover the full RelatedImageListAttribute API surface.

RelatedImageListAttribute attr = new(value);

src/test/unit/System.Windows.Forms/System/Windows/Forms/ImageListConverterTests.cs:6

• Test namespace 'System.Windows.Forms.Controls.Tests' should be updated to 'System.Windows.Forms.Tests' to align with the assembly under test.

namespace System.Windows.Forms.Controls.Tests;

src/test/unit/System.Windows.Forms/System/Windows/Forms/ImageListConverterTests.cs:21

• Add tests for GetProperties, ConvertTo, and ConvertFrom methods of ImageListConverter to ensure functionality beyond support flags is covered.

Converter.GetPropertiesSupported(null).Should().BeTrue();