[LeafShi1]

Fixes #15308

Root cause

The callback queue is lazily initialized under an instance lock, but readers access the queue reference without acquiring that initialization lock. Consequently, the initialization lock alone does not establish safe publication to those readers.

Additionally, the disposal path reads the queue's Count outside the queue lock, even though other threads may modify the queue.

Proposed change

• Add volatile to the callback queue field so other threads can safely access the initialized queue
• Remove the queue-count check outside the lock, the count is already checked inside the lock

Customer Impact

• Addresses a potential concurrency issue when marshaling callbacks to the UI thread

Regression?

• No

Risk

• Minimal

Test environment(s)

• .net 12.0.100-alpha.1.26480.102

Microsoft Reviewers: Open in CodeFlow

[Copilot]

🟢 Approval recommended

The narrow changes address the identified synchronization gaps while preserving existing queue locking and callback behavior.

No open issues 🎉

What changed in this PR

This PR addresses #15308 by tightening synchronization around the WinForms Control callback queue used to marshal work to the UI thread.

Changes:

• Marks the lazily initialized queue reference volatile for safe publication.
• Removes the unlocked queue-count check during disposal handling, retaining the check inside the queue lock.

File	Description
src/​System.Windows.Forms/​System/​Windows/​Forms/​Control.cs	Ensures safe queue publication and removes an unsynchronized count read.

_{Review effort: Balanced}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.84861%. Comparing base (c37f09d) to head (6ffa6fc).

Additional details and impacted files

@@              Coverage Diff              @@
##                main      #15311   +/-   ##
=============================================
  Coverage   37.84861%   37.84861%           
=============================================
  Files            246         246           
  Lines           9789        9789           
  Branches        1031        1031           
=============================================
  Hits            3705        3705           
  Misses          5920        5920           
  Partials         164         164           

Flag	Coverage Δ
Debug	37.84861% <ø> (ø)
production	40.04608% <ø> (ø)
test	20.64923% <ø> (ø)
unit	40.04608% <ø> (ø)

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