Fix 13538: Update method Drop of DropTarget.cs

[LeafShi1]

Fixes #13538

Proposed changes

• Update method Drop to delete *pdwEffect = DROPEFFECT.DROPEFFECT_NONE before CreateDragEventArgs

Customer Impact

• The component controls can be moved in design time

Regression?

• Yes

Risk

• Minimal

Screenshots

Before

Unable move component controls in DemoConsole application.

cannot.move.mp4

After

Component controls can be moved normally

Test methodology

• Manually

Test environment(s)

• .net 10.0.0-preview.6.25278.103

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull Request Overview

This PR addresses issue #13538 by updating the Drop method in DropTarget.cs to properly handle default drag effects and null drag events, restoring design-time component movement.

• Removed the redundant initial reset of *pdwEffect before creating drag event args
• Added an else branch to reset *pdwEffect when CreateDragEventArgs returns null

Comments suppressed due to low confidence (2)

src/System.Windows.Forms/System/Windows/Forms/OLE/DropTarget.cs:215

• The pointer *pdwEffect is used as input to CreateDragEventArgs without being initialized, which could lead to undefined behavior. Consider reintroducing a default assignment (e.g., *pdwEffect = DROPEFFECT.DROPEFFECT_NONE) before this call or passing a local default value instead.

if (CreateDragEventArgs(pDataObj, grfKeyState, pt, *pdwEffect) is { } dragEvent)

src/System.Windows.Forms/System/Windows/Forms/OLE/DropTarget.cs:228

• No automated tests cover the scenario where CreateDragEventArgs returns null. Add a unit or integration test to verify that *pdwEffect is correctly set to DROPEFFECT_NONE in this branch.

else

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 76.59551%. Comparing base (2a0c79b) to head (53fcbb3).
Report is 3 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13542         +/-   ##
===================================================
+ Coverage   76.59184%   76.59551%   +0.00366%     
===================================================
  Files           3230        3230                 
  Lines         639149      639151          +2     
  Branches       47295       47295                 
===================================================
+ Hits          489536      489561         +25     
+ Misses        146037      146007         -30     
- Partials        3576        3583          +7     

Flag	Coverage Δ
Debug	76.59551% <0.00000%> (+0.00366%)	⬆️
integration	18.78908% <0.00000%> (-0.00329%)	⬇️
production	50.99758% <0.00000%> (+0.00836%)	⬆️
test	97.40411% <ø> (ø)
unit	48.40087% <0.00000%> (+0.00802%)	⬆️

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[JeremyKuhne]

Thanks!