IDisposable and Thread Safety

Locking is the correct solution here as we're talking about stuff far more advanced than something like a simple getter/setter. It's imperative that the disposable stuff only gets called from one thread at a time.

I am always curious to know something about the IDispose interface. How does the compiler know that we have to implement a protected virtual Dispose(bool) method? Or make the class sealed?

This is a good point to highlight, but I do not believe it is the most efficient.

I personally use an Interlocked-based method to good effect here. There is really no need for something as "heavy" as locking here.