While this does not affect any behavior, it more accurately describes
the accessibility. Since abstract classes can only be instantiated by
an instance of a derived type, the constructors should at most have
protected access.
If one of these fields is accidentally reassigned, it's possible for
threads to be oblivious to an existing lock. By making the fields
readonly, we will be notified at compile-time if we inadvertently
redefine one of these fields.
Kenneth Hsu