Fix issue 11956: [Dark Mode] CheckBox in TreeView/ListView is not in dark mode after enabled SystemColorMode.Dark

[SimonZhao888]

Fixes #11956

Proposed changes

Fixed an issue where the checkboxes in TreeView/ListView were not rendered correctly in dark theme. The main changes are as follows:

• TreeView: Creates/uses UpdateCheckBoxImages to generate a state imagelist of checkboxes that fits the dark system color, and refreshes and applies it when creating control code, enabling CheckBoxes, and setting WM_SYSCOLORCHANGE.
• ListView: Generates and applies a dark state imagelist to the checkboxes in ApplyDarkModeOnDemand, and refreshes the project state images to ensure the checkboxes display correctly in dark mode.

Customer Impact

• The checkboxes in TreeView/ListView render correctly in dark theme.

Regression?

• No

Risk

• Minimal

Screenshots

Before

After

Test methodology

• Manually

Test environment(s)

• .net 11.0.100-preview.1.26078.121

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

❌ Patch coverage is 33.69565% with 61 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.30967%. Comparing base (ada1700) to head (c0dd9db).
⚠️ Report is 33 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #14288         +/-   ##
===================================================
+ Coverage   77.22797%   77.30967%   +0.08169%     
===================================================
  Files           3279        3265         -14     
  Lines         645138      644539        -599     
  Branches       47730       47644         -86     
===================================================
+ Hits          498227      498291         +64     
+ Misses        143226      142558        -668     
- Partials        3685        3690          +5     

Flag	Coverage Δ
Debug	77.30967% <33.69565%> (+0.08169%)	⬆️
integration	19.12300% <2.17391%> (+0.05010%)	⬆️
production	52.27906% <33.69565%> (+0.11715%)	⬆️
test	97.41916% <ø> (+0.01436%)	⬆️
unit	49.63815% <33.69565%> (+0.12406%)	⬆️

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[Copilot]

Pull request overview

Fixes WinForms dark mode rendering for TreeView/ListView checkboxes when Application.SetColorMode(SystemColorMode.Dark) is enabled, aligning checkbox visuals with other controls in dark mode.

Changes:

• TreeView: generate and apply a dark-mode-aware state imagelist for checkboxes, and refresh it on handle creation, CheckBoxes enablement, and WM_SYSCOLORCHANGE.
• ListView: generate and apply a dark-mode-aware state imagelist for checkboxes during dark mode theme application, and refresh item state images to ensure correct visuals.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/System.Windows.Forms/System/Windows/Forms/Controls/TreeView/TreeView.cs	Adds UpdateCheckBoxImages() to supply themed checkbox state images and refresh them on relevant lifecycle/system events.
src/System.Windows.Forms/System/Windows/Forms/Controls/ListView/ListView.cs	Adds a dark-mode state imagelist for checkbox rendering during ApplyDarkModeOnDemand().

[KlausLoeffelmann]

Hi @SimonZhao888, thanks for tackling the TreeView/ListView checkbox dark-mode rendering — nice work on the visual-styles approach! A few items I'd like us to address before merging:

1. Bitmap resource leak in ListView.UpdateDarkModeCheckBoxImages — The bitmaps returned by CreateDarkModeCheckBoxBitmap are passed directly to imageList.Images.Add() but are never disposed. ImageList.Images.Add copies the image data into the native HIMAGELIST; the managed Bitmap still needs to be cleaned up. Compare with the TreeView code where you correctly use using Bitmap uncheckedBitmap = new(...). Please wrap or dispose the returned bitmaps after adding them.

2. Bitmap resource leak in TreeView.UpdateCheckBoxImages (placeholder) — The placeholder bitmap (new Bitmap(size, size)) added at index 0 is also never disposed. Consider wrapping it in a using declaration.

3. TreeView.UpdateCheckBoxImages runs in light mode too — The method has no Application.IsDarkModeEnabled guard, so it replaces the native checkbox images on every OnHandleCreated, WM_SYSCOLORCHANGE, and CheckBoxes setter call regardless of theme. Since CheckBoxRenderer.DrawCheckBoxWithVisualStyles should produce theme-correct glyphs this may be intentional, but please confirm it doesn't regress the default light-mode appearance.

4. ListView.UpdateDarkModeCheckBoxImages is only called from ApplyDarkModeOnDemand — If the user toggles CheckBoxes while the control is already in dark mode, will this method be invoked? The TreeView wires it up in the CheckBoxes setter; the ListView does not appear to.

(Copilot co-authored on Klaus' behalf.)