Question

Let’s suppose I have a thread that locks on an object reference

Thread #1

lock(myObj) { ... }

later in code I have myObj = new XYZObj();

and then Thread #2 locks on it

lock(myObj) { ... }

Will this code be thread safe, if the object reference has changed? When the object reference changes, the first lock is still valid?

Answer 1

Locks work on instances, not variables.
The lock statement will hold its own reference to the instance so that it will only exit the instance you entered.

The spec says:

where x is an expression of a reference-type, is precisely equivalent
to

System.Threading.Monitor.Enter(x);
try {
   ...
}
finally {
   System.Threading.Monitor.Exit(x);
}

except that x is only evaluated once.

If you re-assign the variable between the two locks, you will get two valid locks on two different instances.

In general, however, you should never do that; it’s a recipe for subtle bugs and race conditions.
You should only lock on dedicated readonly lock objects.