Is this Singleton implementation correct and thread-safe?

Technically, your version should work. However, I would not recommend exposing a public field within your Singleton class, and prefer using a Property (with a getter only). This will help future-proof your API if you need to make changes later.

I also recommend sealing any singleton implementation, as subclassing a singleton class is almost always a bad idea and problematic.

Yes. I would also make the class 'sealed' to avoid any future confusion.

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