39

Imagine an implementation of the IDisposable interface, that has some public methods.

If an instance of that type is shared between multiple threads and one of the threads may dispose it, what is the best way to ensure the other threads do not attempt to work with the instance after disposed? In most cases, after the object is disposed, its methods must be aware of it and throw the ObjectDisposedException or maybe InvalidOperationException or at least inform the calling code for doing something wrong. Do I need synchronization for every method - particularly around the check if it is disposed? Do all IDisposable implementations with other public methods need to be thread-safe?


Here is an example:

public class DummyDisposable : IDisposable
{
    private bool _disposed = false;

    public void Dispose()
    {
        _disposed = true;
        // actual dispose logic
    }

    public void DoSomething()
    {
        // maybe synchronize around the if block?
        if (_disposed)
        {
            throw new ObjectDisposedException("The current instance has been disposed!");
        }

        // DoSomething logic
    }

    public void DoSomethingElse()
    {
         // Same sync logic as in DoSomething() again?
    }
}
Ivaylo Slavov
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  • I think you are mixing up Dispose vs finalizers. The docs are explaining the desired behaviour quite well:http://msdn.microsoft.com/de-de/library/system.idisposable.dispose.aspx – weismat Jan 19 '12 at 14:42
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    no, I am not, but since you mentioned - `Finalize()` is called from a separate GC thread as far as I remember. However, Finalize is called when there are no alive referrers to the object, so thread-safety should not be a consideration then. In MS best practices it is recommended to call Dispose inside Finalize, or at least use almost the same logic - see [link](http://msdn.microsoft.com/en-us/library/fs2xkftw%28v=vs.100%29.aspx). This could lead to calling thread-safety-aware code inside the `Finalize()` method anyway. – Ivaylo Slavov Jan 19 '12 at 14:48
  • I added sample implementation to clarify my question – Ivaylo Slavov Jan 19 '12 at 14:54
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    It should be up to the thread which initialized a disposable object to perform its cleanup after it's **no longer needed**. Why do you let some other thread dispose it? You can only get yourself in trouble that way. – vgru Jan 19 '12 at 15:17
  • I wanted some general guidance on that matter, not having a particular example. You are right, in most cases the creation thread should be cleaning up its resources, but this may not be the case in some scenarios. – Ivaylo Slavov Jan 19 '12 at 15:34
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    @Groo: Sometimes the last use of an object may be on a thread other than the one which creates it. For example, suppose one wants to asynchronously play a sound from a file, closing the file after playback is complete. The thread which opens the file may have better things to do than wait around for the sound to finish. – supercat Jan 19 '12 at 19:10
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    @supercat: if no other thread is going to use the file, then I guess it's ok. But in that case, I see on point in main thread opening that same file, either. Open it, play it, close it. And then do the entire job on a different thread, if you want. I don't think I ever encountered a scenario where I would let some other thread dispose an object instantiated elsewhere. It breaks SRP, if nothing else. – vgru Jan 19 '12 at 22:27
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    @Groo: While it's possible to dedicate a thread to a long-running operation, and have it wait for each part of the operation to complete before proceeding to the next part, a more efficient pattern in many cases is to use asynchronous callbacks fired from Threadpool threads. If one uses a "playback complete" callback to close the audio file, it's entirely possible that the file might get closed on a thread which didn't even exist when the file was opened, or that the thread that opened the file might cease to exist before playback is complete and the file is closed. – supercat Jan 19 '12 at 23:28
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    @Groo: Also, as I commented on another answer, there are some cases where the most natural way to abort a blocking I/O operation is to dispose the resource from underneath it. If that seems icky, consider that some objects allow operations to be aborted without disrupting the overall system state, but the objects whose operations are aborted will become useless. If doing something to an object will render it useless, that action may as well Dispose the object. If an object is disposed while one of its methods is waiting for something to happen, it makes little sense for it to keep waiting. – supercat Jan 19 '12 at 23:38
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    @Groo: If aborting an operation on an object will render it useless, and if Disposing an object should cause a blocking operation on it to be aborted, then Disposing the object would seem a good way to abort an operation on it. – supercat Jan 19 '12 at 23:40
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    I know it's an old topic, but I have a correction to add to your comment: "Finalize is called when there are no alive referrers to the object, so thread-safety should not be a consideration then." Unfortunately, finalizers mean you must care about thread-safety even if your class is not meant to be thread-safe on the outside. This is because the finalizer may run even while another method of its class is still executing! For more information, see https://blogs.msdn.microsoft.com/oldnewthing/20100810-00/?p=13193/ and expecially https://blogs.msdn.microsoft.com/oldnewthing/20100813-00/?p=13153. – relatively_random Nov 15 '17 at 09:17
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    The best practice is having only one owner of IDisposable type. In that case you always have one owner and only the owner can dispose the it. If you need to pass instance to other places the best solution is passing it as non Disposable interface – isxaker Mar 08 '23 at 13:54

6 Answers6

20

I tend to use an integer rather than a boolean as your field for storing the disposed status, because then you can use the thread-safe Interlocked class to test if Dispose has already been called.

Something like this:

private int _disposeCount;

public void Dispose()
{
    if (Interlocked.Increment(ref _disposeCount) == 1)
    {
        // disposal code here
    }
}

This ensures that the disposal code is called only once not matter how many times the method is called, and is totally thread safe.

Then each method can quite simply use call this method as a barrier check:

private void ThrowIfDisposed()
{
   if (_disposeCount > 0) throw new ObjectDisposedException(GetType().Name);
}

With regard to synchronising every method - are you saying a simple barrier check won't do - that you want to stop other threads that might be already executing code in the instance. This is a more complex problem. I don't know what your code is doing, but consider if you really need that - will a simple barrier check not do?

If you just meant with regard to the disposed check itself - my example above is fine.

EDIT: to answer the comment "What's the difference between this and a volatile bool flag? It's slightly confusing to have a field named somethingCount and allow it to hold 0 and 1 values only"

Volatile is related to ensuring the read or write operation operation is atomic and safe. It doesn't make the process of assigning and checking a value thread safe. So, for instance, the following is not thread safe despite the volatile:

private volatile bool _disposed;

public void Dispose()
{
    if (!_disposed)
    {
        _disposed = true

        // disposal code here
    }
}

The problem here is that if two threads were close together, the first could check _disposed, read false, enter the code block and get switched out before setting _disposed to true. The second then checks _disposed, sees false and also enters the code block.

Using Interlocked ensures both the assignment and subsequent read are a single atomic operation.

Rob Levine
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    What's the difference between this and a volatile bool flag? It's slightly confusing to have a field named `somethingCount` and allow it to hold 0 and 1 values only. – vgru Jan 19 '12 at 15:31
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    @Groo - see edit. It is a pity there aren't any Interlocked overloads for bool - but I'd prefer this thread-safe approach over a non-safe one. – Rob Levine Jan 19 '12 at 15:40
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    That makes sense in case multiple threads are trying to dispose an object simultaneously (which would be quite bizarre). But it still makes no guarantees that it will not be disposed in the middle of a different method, right after you've exited `ThrowIfDisposed`. This would be a more common case IMHO, if you have your object passed to multiple threads. – vgru Jan 19 '12 at 15:51
  • Yes - "That makes sense in case multiple threads are trying to dispose an object simultaneously" - that is the issue that the Interlocked part is meant to prevent. It is nothing more than a thread safe version of "if (isDisposed)" in Dispose(). The issue of disposal while another thread is *inside* another method is more complex (as mentioned in my reply). There is nothing to stop you putting more barrier checks inside your critical methods to make them exit early if disposed on another thread. It often only matters if it is using a resource that you need to free. – Rob Levine Jan 19 '12 at 15:54
  • @Groo Also - the point of doing this, is to ensure that Dispose() is idempotent. It doesn't matter how many times you call it (or whether it is called by two threads very close together, or whatever) - the code guarantees you will always get the same orderly result - a single execution of the disposal code. – Rob Levine Jan 19 '12 at 16:04
  • Using interlocked with an integer would allow one to perform some other checks as well. For example, if code which uses the object uses Interlocked.CompareExchange to change the state from "Idle" to "InUse" (throwing an exception if not "Idle") and then uses Interlocked.Exchange to set it to "Idle" on exit, one can throw an exception if one tries to use a non-threadsafe object in unsafe fashion (including attempting to dispose it when it's in use). If an attempt is made to dispose an object that's in use, one could throw an exception at either the dispose, the user, or both, or one could... – supercat Jan 19 '12 at 19:19
  • ...defer the Dispose until the pending operation was complete without throwing exception, let the Dispose proceed despite the in-progress operation with the expectation that it would kill it, etc. Many choices. – supercat Jan 19 '12 at 19:21
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    @Groo: One circumstance where simultaneous `Dispose` may occur "naturally" is when aborting I/O operations. If one thread is performing a blocking read on a `Socket`, for example, and another thread decides the first thread shouldn't wait any longer (e.g. because a user clicked "cancel") the proper way for the second thread to unblock the first thread is to Dispose the socket (which will cause the blocking read to stop waiting and immediately throw an exception). If the first thread was going to Dispose the socket after it finished its operations, both threads might dispose simultaneously. – supercat Jan 19 '12 at 19:27
  • Do you need really `volatile` for `_disposeCount`? `Interlocked` is atomic. – joe Aug 27 '19 at 01:44
  • Since this does not actually provide thread safety for the methods you're trying to protect from running while disposed (brushed off as "complex") and since you'll have to synchronize those methods anyway, why not just use 'lock' in the dispose method as well? I mean, Consider this, if you're concerned that multiple threads will cause dispose, then you're definitely not coordinating the disposal (i.e. when object no longer used), and if you're not coordinating that, you're not coordinate object usage, meaning it may be accessed during the dispose. You really have to do both, so just use lock. – Triynko Oct 07 '19 at 16:23
18

The simplest thing you can do is mark the private disposed variable as volatile and inspect it at the beginning of your methods. You can then throw an ObjectDisposedException if the object has already been disposed.

There are two caveats to this:

  1. You shouldn't throw an ObjectDisposedExceptionif the method is an event handler. Instead you should just gracefully exit from the method if that is possible. The reason being is that there exists a race condition where events can be raised after you unsubscribe from them. (See this article by Eric Lippert for more information.)

  2. This doesn't stop your class from being disposed while you are in the middle of executing one of your class methods. So if your class has instance members that can't be accessed after disposal, you're going to need to setup some locking behaviour to ensure access to these resources are controlled.

Microsoft's guidance around IDisposable says you should check for disposed on all methods, but I haven't personally found this necessary. The question really is, is something going to throw an exception or cause unintended side effects if you allow a method to execute after the class is disposed. If the answer is yes, you need to do some work to make sure that doesn't happen.

In terms of whether all IDisposable classes should be thread safe: No. Most of the use cases for disposable classes involve them only ever being accessed by a single thread.

That being said, you may want to investigate why you need your disposable class to be thread safe as it adds a lot of additional complexity. There may be an alternate implementation that allows you to not have to worry about thread safety issues in your disposable class.

Dan Rigby
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  • I think it might be wiser to implement either a Singleton or access the object via a proxy if you want to keep things simple. – weismat Jan 19 '12 at 15:02
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    Making it volatile won't prevent any of the race conditions. And there are several here. – H H Jan 19 '12 at 15:10
  • @Henk Yeah, it only solves one race condition at method entry (getting a stale value for the disposed private field). Like I pointed out in #2 above, you still have to deal with the fact that the object can still be disposed at any point in the method execution. – Dan Rigby Jan 19 '12 at 15:24
  • @weismat, A singleton disposable object will never be re-used in an application lifetime once disposed. I have always considered singletons pattern incompatible with disposing. Could you clarify if you had something else in mind? – Ivaylo Slavov Jan 19 '12 at 15:39
  • According to the MSDN https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/implementing-dispose "a Dispose method should be idempotent, such that it is callable multiple times without throwing an exception" – TheWizard Aug 27 '21 at 11:39
17

Most BCL implementations of Dispose are not thread-safe. The idea is that it's up to the caller of Dispose to make sure nobody else is using the instance anymore before it is Disposed. In other words, it pushes the synchronization responsibility upwards. This makes sense, as otherwise now all your other consumers need to handle the boundary case where the object was Disposed while they were using it.

That said, if you want a thread-safe Disposable class, you can just create a lock around every public method (including Dispose) with a check for _disposed at the top. This may become more complicated if you have long-running methods where you don't want to hold the lock for the entire method.

Dan Bryant
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    Yes, best solution for Dispose-from-multiple threads is: **Don't do that**. – H H Jan 19 '12 at 15:02
  • @Henk, to be fair, you really shouldn't ever see ObjectDisposedException either, if you don't have any bugs. Given that this is a debugging tool, though, I think it's reasonable that it not have guaranteed thread-safe behavior (i.e. you could run into other types of exceptions if you're using an object from multiple threads immediately around the time it's being Disposed.) – Dan Bryant Jan 19 '12 at 15:05
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    @DanBryant it's not about disposing in multithreaded environment, but execute a call while disposing is in process. We have to throw `ObjectDisposedException` in this case – Alex Zhukovskiy Jul 03 '17 at 10:11
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    Best solution is to implement thread-safety properly, and not be lazy or attempt to "push the responsibility up", which is ridiculous. Dispose should be thread-safe, like all the other methods. That means locks must be used in both Dispose and all other methods. My genius idea is to use a ReaderWriterLock, where Dispose takes out a write lock, and all other methods take out a read lock. As long as the class isn't disposed, there is no contention for the read locks, because there's no limit on how many can be held at once. The write lock in Dispose ensures a single thread executes it. – Triynko Aug 26 '19 at 04:32
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    The critical piece here is that when thread safety is implemented properly, you'll get an ObjectDisposedException, which you can catch and handle, rather than a non-deterministic exception that could result from using a non-thread-safe version while it's being disposed. It's a shame the BCL implementations don't implement thread safety for dispose. – Triynko Aug 26 '19 at 04:36
  • @Triynko, I like this approach, you should write it as an answer. However, now after having more experienced eyes on the matter, I'd usually stick to what BCL has -- and keep things simple. For critical resources which are disposable, and can be accessed/disposed by different threads, your approach is very very reasonable one. The calling code's developer, however, should be well informed of what type of disposable resources s/he is dealing with, as there is responsibility for him/her to properly dispose it. – Ivaylo Slavov Jul 01 '20 at 04:33
4

I prefer to use integers and Interlocked.Exchange or Interlocked.CompareExchange on an integer-type object "disposed" or "state" variable; I'd use enum if Interlocked.Exchange or Interlocked.CompareExchange could handle such types, but alas they cannot.

One point which most discussions of IDisposable and finalizers fail to mention is that while an object's finalizer shouldn't run while IDisposable.Dispose() is in progress, there's no way for a class to prevent objects of its type from being declared dead and then resurrected. To be sure, if outside code allows that to happen there obviously can't be any requirement that the object "work normally", but the Dispose and finalize methods should be well-enough protected to ensure that they won't corrupt any other objects' state, which will in turn generally require using either locks or Interlocked operations on object state variables.

supercat
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2

You have to lock every access to the ressource you are going to dispose. I also added the Dispose pattern I normally use.

public class MyThreadSafeClass : IDisposable
{
    private readonly object lockObj = new object();
    private MyRessource myRessource = new MyRessource();

    public void DoSomething()
    {
        Data data;
        lock (lockObj)
        {
            if (myResource == null) throw new ObjectDisposedException("");
            data = myResource.GetData();
        }
        // Do something with data
    }

    public void DoSomethingElse(Data data)
    {
        // Do something with data
        lock (lockObj)
        {
            if (myRessource == null) throw new ObjectDisposedException("");
            myRessource.SetData(data);
        }
    }

    ~MyThreadSafeClass()
    {
        Dispose(false);
    }
    public void Dispose() 
    { 
        Dispose(true); 
        GC.SuppressFinalize(this);
    }
    protected void Dispose(bool disposing) 
    {
        if (disposing)
        {
            lock (lockObj)
            {
                if (myRessource != null)
                {
                    myRessource.Dispose();
                    myRessource = null;
                }
            }
            //managed ressources
        }
        // unmanaged ressources
    }
}
Marc Messing
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    can you explain why you call `GC.SuppressFinalize(this)` in the finalizer - isn't it too late for that if the GC is already disposing the instance? Maybe call this after successful dispose to ease the GC? – Ivaylo Slavov Jan 19 '12 at 15:09
  • You are right. It was a oversight and should be in the Dispose(). – Marc Messing Jan 19 '12 at 15:35
1

FWIW, your sample code matches how my co-workers and I typically deal with this issue. We generally define a private CheckDisposed method on the class:

private volatile bool isDisposed = false; // Set to true by Dispose

private void CheckDisposed()
{
    if (this.isDisposed)
    {
        throw new ObjectDisposedException("This instance has already been disposed.");
    }
}

Then we call the CheckDisposed() method at the top of all public methods.

If thread contention over disposal is considered likely, rather than an error condition, I will also add a public IsDisposed() method (Similar to Control.IsDisposed).


Update: Based on the comments with respect to the value of making isDisposed volatile, note that the "fence" issue is rather trivial given how I use the CheckDisposed() method. It is essentially a troubleshooting tool for quickly catching the case where code calls a public method on the object after it has already been disposed. Calling CheckDisposed() at the start of a public method in no way guarantees that the object won't be disposed within that method. If I consider that to be a risk inherent in my class's design, as opposed to an error condition I failed to account for, then I use the aforementioned IsDisposed method along with appropriate locking.

dgvid
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    You also have to ensure that isDisposed is marked as volatile or you're open to a race condition. – Dan Rigby Jan 19 '12 at 15:02
  • Excellent point. Sample edited to include declaration of isDisposed. – dgvid Jan 19 '12 at 15:08
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    @Dan, I don't think a fence is going to help much here, as instruction reordering isn't the issue; the real issue is that Dispose could be called at any point while in the body of the method. – Dan Bryant Jan 19 '12 at 15:13
  • @Dan B Yes, it's just a piece of the problem. I covered your point in my answer to the question though. – Dan Rigby Jan 19 '12 at 15:30
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    @DanRigby: `volatile` only creates a fence. It doesn't change anything if another thread disposes the object right after the `if` condition. – vgru Jan 19 '12 at 15:32
  • @Groo Correct. It just prevents getting a stale value of isDisposed. See Caveat #2 in my answer to this question. – Dan Rigby Jan 19 '12 at 15:39
  • @DanRigby, instead of `volatile`, `Interlocked.CompareExchange()` might be a better approach, I believe it takes care of fencing. – Ivaylo Slavov Jan 19 '12 at 15:43