Fix #12661 Null Reference Exception: System.Windows.Forms.TabControl.<WmSelChange>

[John-Qiao]

Fixes #12661

Proposed changes

• Update the WmSelChange method in TabControl.cs to check if the AccessibilityObject is null before calling the method
• Add a unit test to verify the fix

Customer Impact

• Throw exception in Application.Run()

Regression?

• No

Risk

• Minimal

Screenshots

Before

Throw exception in Application.Run()

After

No exception in runtime

Test methodology

• Manually
• Unit test

Test environment(s)

• 10.0.0-alpha.1.24622.1

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.02571%. Comparing base (084e6d0) to head (098eabe).
Report is 26 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                  @@
##                main      #12683          +/-   ##
====================================================
+ Coverage   76.03186%   97.02571%   +20.99385%     
====================================================
  Files           3181        1191        -1990     
  Lines         639670      355480      -284190     
  Branches       47215        5446       -41769     
====================================================
- Hits          486353      344907      -141446     
+ Misses        149797        9770      -140027     
+ Partials        3520         803        -2717     

Flag	Coverage Δ
Debug	97.02571% <100.00000%> (+20.99385%)	⬆️
integration	?
production	?
test	97.02571% <100.00000%> (-0.00344%)	⬇️
unit	?

Flags with carried forward coverage won't be shown. Click here to find out more.

[Tanya-Solyanik]

Looks good! I added a comment about a test

[Tanya-Solyanik]

Thank you!

Please work with Leaf to get this ported to the release/9 and release/8 branches.

[LeafShi1]

/backport to release/9.0

[github-actions]

Started backporting to release/9.0: https://github.com/dotnet/winforms/actions/runs/12664765466

[LeafShi1]

/backport to release/8.0

[github-actions]

Started backporting to release/8.0: https://github.com/dotnet/winforms/actions/runs/12664783112

[github-actions]

@LeafShi1 backporting to release/8.0 failed, the patch most likely resulted in conflicts:

$ git am --3way --ignore-whitespace --keep-non-patch changes.patch

Applying: Fix issue 12661 and add unit test
Using index info to reconstruct a base tree...
A	src/System.Windows.Forms/src/System/Windows/Forms/Controls/TabControl/TabControl.cs
M	src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/TabControlTests.cs
Falling back to patching base and 3-way merge...
Auto-merging src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/TabControlTests.cs
Auto-merging src/System.Windows.Forms/src/System/Windows/Forms/TabControl.cs
CONFLICT (content): Merge conflict in src/System.Windows.Forms/src/System/Windows/Forms/TabControl.cs
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config advice.mergeConflict false"
Patch failed at 0001 Fix issue 12661 and add unit test
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!