Monitor vs lock

Question

When is it appropriate to use either the Monitor class or the lock keyword for thread safety in C#?

EDIT: It seems from the answers so far that lock is short hand for a series of calls to the Monitor class. What exactly is the lock call short-hand for? Or more explicitly,

class LockVsMonitor {     private readonly object LockObject = new object();     public void DoThreadSafeSomethingWithLock(Action action)     {         lock (LockObject)         {             action.Invoke();         }     }     public void DoThreadSafeSomethingWithMonitor(Action action)     {         // What goes here ?     } } 

Update

Thank you all for your help : I have posted a another question as a follow up to some of the information you all provided. Since you seem to be well versed in this area, I have posted the link: What is wrong with this solution to locking and managing locked exceptions?

Answer 1

Eric Lippert talks about this in his blog: Locks and exceptions do not mix

The equivalent code differs between C# 4.0 and earlier versions.

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In C# 4.0 it is:

bool lockWasTaken = false; var temp = obj; try {     Monitor.Enter(temp, ref lockWasTaken);     { body } } finally {     if (lockWasTaken) Monitor.Exit(temp); } 

It relies on Monitor.Enter atomically setting the flag when the lock is taken.

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And earlier it was:

var temp = obj; Monitor.Enter(temp); try {    body } finally {     Monitor.Exit(temp); } 

This relies on no exception being thrown between Monitor.Enter and the try. I think in debug code this condition was violated because the compiler inserted a NOP between them and thus made thread abortion between those possible.

Answer 2

lock is just shortcut for Monitor.Enter with try + finally and Monitor.Exit. Use lock statement whenever it is enough - if you need something like TryEnter, you will have to use Monitor.