C# How to detect an object is already locked

Question

How can I detect whether an object is locked or not?

Monitor.TryEnter (as described in Is there a way to detect if an object is locked?) does not work for me because it locks the object if it is not locked.

I only want to check if it is locked and somewhere else in my code I will use the Monitor class to lock the object.

I know it is possible to use for example an boolean field (for example private bool ObjectIsLocked) but what to detect it using the lock-object itself.

The example code below shows what I want to do:

private static object myLockObject = new object();

private void SampleMethod()
{
    if(myLockObject /*is not locked*/) // First check without locking it
    {
        ...
        // The object will be locked some later in the code
        if(!Monitor.TryEnter(myLockObject)) return;

        try
        {

            ....
        }
        catch(){...}
        finally
        {
            Monitor.Exit(myLockObject);
        }
    }
}

wouldn't it be a problem if the lock was instantly locked by some other thread after `if(myLockObject /*is not locked*/)`? — default, Aug 20 '12 at 07:41
Think this through carefully - as soon as the "is this object locked" method returns, the answer can flip - unless, as part of the test, *you* take the lock. You then also only know the answer "it's locked, by me", up until the point you release it. So the stand alone "is this object locked" method may as well always be hard coded to return `false` (or `true`, you pick). — Damien_The_Unbeliever, Aug 20 '12 at 07:42
@Damien_The_Unbeliever My thought exactly, although you expressed yourself more clearly :) — default, Aug 20 '12 at 07:45
@Damien_The_Unbeliever (I am not sure I understand your answer correctly but..) Pete's answer shows that in .net 4.5 the `Monitor.IsEntered` is added, which shows that it is possible to detect whether the object is locked or not. Also if an object is locked, other thread definitely can detect this otherwise the locking architecture will not work at all. If other threads cannot detect whether an object is locked or not it will ignore the lock. — hwcverwe, Aug 20 '12 at 08:02
@hwcverwe - the point is, if *other* threads can take the lock, then you've no way of knowing if they've changed from locked->unlocked or unlocked->locked, immediately after you perform your test, and before you act upon the information. — Damien_The_Unbeliever, Aug 20 '12 at 08:04
@Damien_The_Unbeliever I agree. I know I need to check twice (1: `is not locked`, 2: `Monitor.TryEnter()`) Unfortunately the design forces me to do something outside the lock but I want to do it only when it is not locked. Currently I am using just a simple flag but I was hoping there is a way to check it based on the lock-object itself — hwcverwe, Aug 20 '12 at 08:13
@Damien_The_Unbeliever I have changed the second check (`if(!Monitor.TryEnter(myLockObject)) return;`) to make clear I am checking twice — hwcverwe, Aug 20 '12 at 08:14
@hwcverwe - You may as well do that activity outside of the lock *anyway*, and then attempt to obtain the lock. You'll still have to write the code to cope with the lock not being obtainable at that time. So save some code and skip step 1. — Damien_The_Unbeliever, Aug 20 '12 at 08:20
'it is possible to detect whether the object is locked or not' - no. It is possible to detect whether the object WAS locked or not. Your design is flawed, as @Damien_The_Unbeliever says. — Martin James, Aug 20 '12 at 09:43
Here's a scenario for you: I am making a file manager. I am multi-threading the getting of custom FileItems because the construction is non-trivial. I want to get and construct these FileItems as much as possible, but I want to respond to the front-end as much as possible. So, if the thread that updates the UI is locked, just keep getting FileItems. If not, signal on the list that is currently outstanding. In this way, processing is not dependent on the lock. — Tom Padilla, Feb 16 '15 at 13:19
https://stackoverflow.com/questions/9209590/non-blocking-locking Is an alternative for skipping the que. — Tyler S. Loeper, Jan 24 '18 at 14:36

score 59 · Accepted Answer · edited Oct 03 '18 at 13:13

59

You're doing it wrong. If you don't have the lock on the object you can't check if it is locked (and if you have the lock you will know in advance). You can "ask" "is locked?" and get a "not" as response, then on the next nanosecond another thread can take the lock and your program will enter in a corrupted state. This simply is not the way to go on multithreaded apps and the reason why .NET does not have a Monitor.IsLocked method. If your code needs to check the lock before acquire it so you have a design problem. Trying to solve it with unprotected flags is a poor solution that is guaranteed by 100% of chance that will not work.

Anyway, do not use a bool var to signal multi-thread is locked state (because you can have the same problem, you read "false" and 1 nanosecond later another thread will write "true" to it). Use Interlock.CompareExchange.

private static int _lockFlag = 0; // 0 - free

if (Interlocked.CompareExchange(ref _lockFlag, 1, 0) == 0){
   // only 1 thread will enter here without locking the object/put the
   // other threads to sleep.

   Monitor.Enter(yourLockObject); 

   // free the lock.
   Interlocked.Decrement(ref _lockFlag);
}

You'll see that you'll need to change the _lockFlag on every place where a lock to your object could be aquired. In other words, you'll construct a custom lock system around the native one.

edited Oct 03 '18 at 13:13

Eric Labashosky

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answered Aug 20 '12 at 22:53

Marcelo De Zen

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Then what would you suggest for this scenario? I am making a file manager. I am multi-threading the getting of custom FileItems because the construction is non-trivial. I want to get and construct these FileItems as much as possible, but I want to respond to the front-end as much as possible. So, if the thread that updates the UI is locked, just keep getting FileItems. If not, signal on the list that is currently outstanding. In this way, processing is not dependent on the lock – Tom Padilla Feb 16 '15 at 13:20
@TomPadilla, the exact solution will depent on the underlying UI framework. One solution would be: a) create a service class that will hold the FileItems load. This service must provide synchronized Add/Remove methods (and methods you need to query the service's inner FileItems collection). You then go and build a multi-threaded FileItems loader. It must call the other service Add method whenever it loads a fresh FileItem. Last, your service must provide an OnUpdate event. The UI thread then subscribes to this event and take (or schedule) the necessary steps to handle the data... – Marcelo De Zen Feb 16 '15 at 23:36
... (query the service, update ui, etc.). The service's OnUpdate event will be fired from the worker thread, and your code must call the UI's Dispatch() to schedule the event handling code to be executed by the UI thread. – Marcelo De Zen Feb 16 '15 at 23:45
This is why I need more people on this project. Please consider: [FileTracker] (https://code.google.com/p/filetracker/) – Tom Padilla Feb 17 '15 at 12:40
@TomPadilla I`ll take a look and get back to you. – Marcelo De Zen Feb 17 '15 at 22:43
1

Agree. If you are having this problem, creating an "IsLocked" flag is a code smell and will result in random hard to re-produce bugs depending on what the user is doing. – rollsch Mar 04 '17 at 03:20
@rolls I'm not so sure. In any multi-threaded (or UWP multiTask ) application incorrect code will create unreproducible bugs. However I can see a task deciding 'what should I do next'. If I do (A) I will hit a lock but (B) is fine. I agree flags could solve it but that's also asking for trouble. You could write your own `bool Lock("some name",Action)` so you could code `if (Lock("Reports",()=>Report(this))` and the bool says if it is done – Paulustrious Oct 28 '17 at 16:51
For example, I have one loader thread and one ui thread. Loader, as it's name says, loads some stuff and it is a long time operation. Lets say loader thread locks some resource to load it. In ui thread i don't want simple lock same resource because it results in ui hanging. I want to just ask "is it locked?" and if "yes" i just say "nevermind, i'll try later" otherwise do some quick stuff with that resource. Why it is supposed to be bad? – BlackOverlord Apr 09 '18 at 07:11
@BlackOverlord, it could be done using Monitor.TryLock() and Monitor.Unlock(). This solution adds some overhead though, and is a potetial bottleneck if the UI checks the lock "too frequently". – Marcelo De Zen Jun 13 '18 at 18:52
@MarceloDeZen - why do the Interlocked.Decrement(ref _lockFlag) if only 1 thread can get into the if block? Could the code safely say _lockFlag = 0? – Jeff Maass Jun 11 '22 at 12:42
@JeffMaass I guess in some cases it would work. But it's better to always use Interlocked because it puts memory barriers in place to prevent speculation and reordering (from the JIT or Processor pipeline), thus ensuring the same behaviour in every OS/Hardware. – Marcelo De Zen Jul 14 '22 at 15:19
... there's also - I suppose - the issue of processor cache not being flushed. Memory barriers also take care of that (I'm not sure but declaring the variable as volatile should remediate this problem) – Marcelo De Zen Jul 14 '22 at 15:27

score 12 · Answer 2 · answered Aug 20 '12 at 11:29

There is no way to do this with the Monitor class in C#

Just use;

    var lockedBySomeoneElse = !Monitor.TryEnter(obj);
    if (!lockedBySomeoneElse) Monitor.Exit(obj);
    // the variable 'lockedBySomeoneElse' has the info you want

Other locks like readerwriterlockslim do not really help. That one can tell you how may readers there are, but not if there is a writer busy ;-(

also if you use your own suggestion 'private bool ObjectIsLocked', which is the route I would take I think, you should use

      private volatile bool ObjectIsLocked

This will make C# reflect changes to it better with multithread updates.

Because the lock is re-entrant, you may want to first check `Monitor.IsEntered`, in case this is being used as an Assert to verify that the lock is held, by anyone. `TryEnter` will succeed even if the lock is already entered, as long as it is called by the same thread. — jackmott, Feb 17 '17 at 03:36

Pete · Answer 3 · 2012-08-20T07:44:07.940

10

Monitor.IsEntered should do the trick.

Edit: I just re-read the documentation, and it says:

Determines whether the current thread holds the lock on the specified object.

So that is not sufficient, as you would probably like to know if a different thread holds a lock?

edited Aug 20 '12 at 07:44

answered Aug 20 '12 at 07:39

Pete

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According to the docs, that seems to be only for the current thread, correct? – default Aug 20 '12 at 07:42
1

that's nice. The problem is.. We are using 4.0. (I will change my question and add this restriction) – hwcverwe Aug 20 '12 at 07:44
To answer your question. Yes I want to know a different thread holds the lock. – hwcverwe Aug 20 '12 at 08:05

score 0 · Answer 4 · answered Aug 20 '12 at 07:36

0

If you want to ensure the object is still lockable later on, just call TryEnter and hold the lock the whole time. Otherwise, if you want to try to lock the object later, just call TryEnter and immediately unlock it if it's locked.

answered Aug 20 '12 at 07:36

David Schwartz

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score 0 · Answer 5 · answered Aug 20 '12 at 07:57

Techincally you can check for object's Sync Block Index field which has an index of associated lazily allocated structure in Sync Blocks array - every object has this field and every object, which used for synchronziation, has this field set. These structures are used to coordinate thread synchronization. However, I highly doubt that you'll be able to access this information without Profiling API.

score -3 · Answer 6 · answered Feb 02 '18 at 17:17

I can in no way advocate checking locks then entering code blocks. However, I found this thread while looking for a way to check a new function couldn't leave an object locked. A unit test based on Monitor.IsEntered got me exactly what I was looking for.

C# How to detect an object is already locked

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