C# 5.0 Programmer's Reference

502 ❘ CHAPTER 22 Parallel Programming
{
try
{
// Critical code goes here ...
}
finally
{
Monitor.Exit(UpdateLock);
}
}
else
{
// Do something if we time out.
}
The code creates the lock object UpdateLock as before. Later it uses Monitor.TryEnter to try to
enter the critical region of code. If TryEnter succeeds, the thread enters its critical region and does
whatever it needs to do with the resources. The finally section of the try finally block ensures
that the code calls Monitor.Exit when it is done with the critical region.
If the call to TryEnter returns false, the thread timed out and failed to obtain the lock. In that
case the code should do something to recover, possibly trying again to obtain the lock.
Limited Do-Overs
If your code fails to obtain a lock, you may want to limit the number of times it can
try again. If a thread becomes permanently stuck inside the critical region, it will
block any other thread that must access that region.
Of course, if the program is in that situation, it may be impossible to get anything
done, and you may want close it and start over.
The following table lists the most useful Monitor class methods.
Method
Enter
Exit
IsEntered
TryEnter
Wait
Purpose
Acquires an exclusive lock on an object.
Releases an exclusive lock on an object.
Returns true if the current thread has a lock on a specific object.
Tries to acquire an exclusive lock on an object.
Releases the lock on an object and then blocks until it reacquires a
lock on that object. This is useful if the thread needs to allow another
thread to take some action before it can continue.
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