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

Attention: Patch coverage is 90.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 76.03363%. Comparing base (084e6d0) to head (43f7faa).
Report is 1 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #12683         +/-   ##
===================================================
+ Coverage   76.03186%   76.03363%   +0.00177%     
===================================================
  Files           3181        3181                 
  Lines         639670      639709         +39     
  Branches       47215       47218          +3     
===================================================
+ Hits          486353      486394         +41     
+ Misses        149797      149791          -6     
- Partials        3520        3524          +4     

Flag	Coverage Δ
Debug	76.03363% <90.00000%> (+0.00177%)	⬆️
integration	18.16430% <0.00000%> (-0.01198%)	⬇️
production	49.82409% <100.00000%> (+0.00210%)	⬆️
test	97.02835% <89.74359%> (-0.00080%)	⬇️
unit	47.05064% <100.00000%> (+0.00773%)	⬆️

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[Tanya-Solyanik]

We should also test if (IsAccessibilityObjectCreated && SelectedTab?.ParentInternal is TabControl) inside the method invoker delegate in case the control had been disposed before the delegate was invoked.

[LeafShi1]

In the BeginInvoke delegate, should we consider adding conditional checks for IsAccessibilityObjectCreated and SelectedTab?.ParentInternal is TabControl to ensure the state is valid at execution time?

BeginInvoke((MethodInvoker)(() =>
{
     if (IsAccessibilityObjectCreated && SelectedTab?.ParentInternal is TabControl && 
          !SelectedTab.IsDisposed && SelectedTab.TabAccessibilityObject != null)
     {
        SelectedTab.TabAccessibilityObject.RaiseAutomationEvent(UIA_EVENT_ID.UIA_AutomationFocusChangedEventId);
     }
}));

[Tanya-Solyanik]

Is SelectedTab.IsDisposed possible? Please double-check if we are un-selecting tabs before disposing them.

It's ok to create accessibility object for a non-disposed child control, if the parent has an accessible object. The child accessible object will be created to support some other user action anyway if accessibility tools are being used. And we know that accessibility tools are used because they initiated creation of the parent accessible object.

[Tanya-Solyanik]

In this scenario, exception happens after WmSelChange had been executed, the delegate is only scheduled for execution with BeginInvoke.

    control.TestAccessor().Dynamic.WmSelChange();
    Application.DoEvents();
    Thread.Sleep(100);
   // no exception

[LeafShi1]

So here we need to ensure that SelectedTab is diaposed after WmSelChange is executed and before the event UIA_AutomationFocusChangedEventId is executed.

 Action act = () => control.TestAccessor().Dynamic.WmSelChange();
 act.Should().NotThrow();

 control.TabPages.Clear();

 var exception = Record.Exception(() =>
 {
     Application.DoEvents();
     Thread.Sleep(100);
 });

 exception.Should().BeNull();