Wait for a lock to be released

Question

I have a thread. At a certain point, what I want to do is check if a certain lock is free. If it is free, I want the thread to continue on its merry way. If it is not free, I want to wait until it is free, but then not actually acquire the lock.

Here is my code so far:

private object LockObject = new Object();

async void SlowMethod() {
  lock (LockObject) {
    // lotsa stuff
  }
}

void AppStartup() {
  this.SlowMethod();
  UIThreadStartupStuff();
  // now, I want to wait on the lock.  I don't know where/how the results
  // of SlowMethod might be needed.  But I do know that they will be needed.
  // And I don't want to check the lock every time.
}

See http://stackoverflow.com/questions/12033725/c-sharp-how-to-detect-an-object-is-already-locked — dizel3d, Sep 26 '14 at 20:29
@dizel3d That's a different operation. Determining if the lock is free is different from waiting until its free without acquiring the lock. — Servy, Sep 26 '14 at 20:30
If your example code is representative of your real problem, then you could just `Wait` on the task returned by `SlowMethod`. — Eren Ersönmez, Sep 26 '14 at 20:37
Pretty unlikely you are using that *lock* statement appropriately. Use AutoResetEvent for signaling. Use Task as a return value for async code so you can wait it. — Hans Passant, Sep 26 '14 at 20:39
Do you want to start only after `SlowMethod` completes? Or should they execute in parallel? — Yuval Itzchakov, Sep 26 '14 at 21:30
They should execute in parallel. The point is I'm trying to speed up my startup. — William Jockusch, Sep 26 '14 at 21:33
Any reason you're not offloading `SlowMethod` to a background thread? — Yuval Itzchakov, Sep 26 '14 at 22:39

score 2 · Answer 1 · edited Sep 26 '14 at 21:11

I think you have classical XY problem. I guess what you want is start a Task with you SlowMethod and then Continue it with the UIThreadStartupStuff is UI thread.

Task.Factory.StartNew(()=>SlowMethod())
     .ContinueWith(t=>UIThreadStartupStuff(), TaskScheduler.FromCurrentSynchronizationContext());

or with async/await (make your SlowMethod to return Task)

try
{
   await SlowMethod();
}
catch(...)
{}
UIThreadStartupStuff();

score 2 · Answer 2 · answered Sep 29 '14 at 16:23

You don't want to use a lock here. You need an event. Either ManualResetEvent or AutoResetEvent.

Remember, locks are used for mutual exclusion. Events are used for signaling.

You have your SlowMethod set the event when it's done. For example:

private ManualResetEvent DoneEvent = new ManualResetEvent(false);

async void SlowMethod() {
  // lotsa stuff
  // done with lotsa stuff. Signal the event.
  DoneEvent.Set();
}

void AppStartup() {
  this.SlowMethod();
  UIThreadStartupStuff();

  // Wait for the SlowMethod to set the event:
  DoneEvent.WaitOne();
}

score -1 · Answer 3 · answered Sep 29 '14 at 14:07

I might not get what you want to achieve, but why not wait on the lock "properly"? After all, it is a clear sign of the lock being free if you can take it. Also, you can release it immediately if it is important.

void AppStartup() {
    this.SlowMethod();
    UIThreadStartupStuff();

    // now, I want to wait on the lock.  I don't know where/how the results
    // of SlowMethod might be needed.  But I do know that they will be needed.
    // And I don't want to check the lock every time.

    lock (LockObject) {
        // nothing, you now know the lock was free
    }

    // continue...

}

Wait for a lock to be released

3 Answers3