[C] avoid concurrent access to _values

[StephaneDelcroix]

Issues Fixed

• should fix Collection was modified after the enumerator was instantiated. #23597

[mattleibow]

Just so the machines can stop nagging me about this PR, I think locking around the setting of the field is also worth doing just to make sure that there is no code path that also allows double setting.

Unless that is somehow impossible because of something else.

[MartyIX]

Locks are typically readonly.

[MartyIX]

I wonder why the lock is necessary in the first place. Is SetterSpecificityList.cs to be accessed from multiple threads?

If access from multiple threads is expected, then why _values is locked but not

maui/src/Controls/src/Core/SetterSpecificityList.cs

Lines 13 to 14 in a3b0f4e

	KeyValuePair<SetterSpecificity, object>? _first;
	KeyValuePair<SetterSpecificity, object>? _second;

?

[rmarinho]

Seems this could help with the issue. Without a way to reproduce with a test is not easy to make sure it will actually work. But lets try

[MartyIX]

The original issue mentions #23597

*CurrentNode is extended form StackLayout which has some hierarchy childs.

My understanding is that StackLayout should not be used in the first place but rather {Horizontal|Vertical}StackLayout and it seems suspicious that it happens on a customized control. There is a good chance that there is a bug in that custom control.

Anyway, just my 50c.

[StephaneDelcroix]

@MartyIX we agree that this shouldn't happen, if used correctly. but if this fix prevents a crash while the user abuses the API, it's worth it