New Rule Idea: locked field should not be public nor internal nor protected internal

[antonioaversa]

What

When a lock statement is on a field, the field should not be

• public
• internal
• protected internal

How

• The new rule would cover the part of S2445 left out from the implementation during review.
• This proposal extends the scope of "S2445 leftover" to visibility modifiers other than private: private protected, protected are also considered acceptable, because exposing fields to derived classes, in the same assembly or not, could be a valid approach when the synchronization should happen between methods of the base classes and methods of the derived class.
• file accessibility should be considered as acceptable for locking, because of the fairly limited scope.

Background

• Spawned from New Rule S2445: Blocks should be synchronized on read-only fields #6701.
• S2445 was originally defined as "Blocks should be synchronized on "private final" fields".
• After discussions happened in New Rule S2445: Blocks should be synchronized on read-only fields #6750, the scope was narrowed for C#.
• The rule became "Blocks should be synchronized on read-only fields", for reasons explained here.