fix #13305 Baseline SnapLines do not appear in DesignSurface

[Epica3055]

fix #13305

The cause of this issue

This issue is caused by PR10279

These are two of all changes make in PR10279

BehaviorService, DragAssistanceManager use SnapLines to add some effects during design time.

ControlDesigner has a virtual property SnapLines, but most designers override the property, in other words, most designers have customized SnapLines

Code flow before PR10279

when BehaviorService or DragAssistanceManager accesses SnapLines , customized SnapLines were returned.

Code flow after PR10279

ControlDesigner.cs

when BehaviorService or DragAssistanceManager accesses SnapLinesInternal , customized SnapLines were skipped.

Proposed changes

• Use SnapLines

Screenshots

Before

After

Test methodology

• manually

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 62.58290%. Comparing base (88c3a11) to head (4ebcd6a).
Report is 171 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13324         +/-   ##
===================================================
+ Coverage   61.34314%   62.58290%   +1.23976%     
===================================================
  Files           1547        1560         +13     
  Lines         158479      159689       +1210     
  Branches       14751       14906        +155     
===================================================
+ Hits           97216       99938       +2722     
+ Misses         60561       58983       -1578     
- Partials         702         768         +66     

Flag	Coverage Δ
Debug	62.58290% <50.00000%> (-12.58918%)	⬇️
integration	11.32692% <50.00000%> (-7.54506%)	⬇️
production	40.77283% <50.00000%> (-34.39925%)	⬇️
test	95.69057% <ø> (∅)
unit	38.15207% <0.00000%> (-36.10495%)	⬇️

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[LeafShi1]

LGTM!

[Tanya-Solyanik]

Nice investigation!
Please add a test where a test custom designer derived from the ControlDesigner class, modifies the overridden SnapLines collection and this collection can be correctly accessed vis the base class virtual property ControlDesigner.Snaplines.

[Tanya-Solyanik]

Looks good, please add unit test.

[Epica3055]

@Tanya-Solyanik
We already have tests for snaplines in SnapLineTests.cs , and in specific designers like LabelDesigner.cs

Besides, I think the root of this issue resides with the misuse of property (SnapLinesInternal for SnapLines)

[Tanya-Solyanik]

Besides, I think the root of this issue resides with the misuse of property (SnapLinesInternal for SnapLines)

Yes, could you please add a test for the rootcause - a test that ensures that the virtual method is called. I'll merger this PR to include this fix in the next preview, but please add a test in the next PR.

[Epica3055]

/backport to release/9.0

[github-actions]

Started backporting to release/9.0: https://github.com/dotnet/winforms/actions/runs/14608188769