Lock statement - does it always release the lock?

Question

I have recently read this post from Eric Lippert regarding the lock implementation in c# and still some questions remain.

In 4.0 implementation if a thread abort or any cross thread exception occurs just before the Monitor.Exit(temp) in the finally block is executed - would that keep the lock on the object?

Is there any possibility for an exception to occur at this level, leaving the object still in a locked state?

Answer 1

In 4.0 implementation if a thread abort or any cross thread exception occurs just before the Monitor.Exit(temp) in the finally block is executed - would that keep the lock on the object?

Let's take a look at that code, so that it is clear to other readers:

bool lockWasTaken = false;
var temp = obj;
try 
{ 
  Monitor.Enter(temp, ref lockWasTaken); 
  { 
    body 
  } 
}
finally 
{ 
  if (lockWasTaken) 
  {
    // What if a thread abort happens right here?
    Monitor.Exit(temp); 
  }
}

Your question is not answerable because it is based on a false assumption, namely, that thread aborts can happen in the middle of a finally block.

Thread aborts cannot happen in the middle of a finally block. That is just one reason amongst many reason why you should never attempt to abort a thread. The entire thread could be running in a finally block, and therefore be not-abortable.

Is there any possibility for an exception to occur at this level, leaving the object still in a locked state?

No. A thread abort will be delayed until control leaves the finally. Unlocking a valid lock does not allocate memory or throw another exception.