Is it safe to call BeginXXX without calling EndXXX if the instance is already disposed

Question

When using the Asynchronous Programming Model it is usually recommended to match every BeginXXX with an EndXXX, otherwise you risk leaking resources until the asynchronous operation completes.

Is that still the case if the class implements IDisposable and the instance was disposed by calling Dispose?

If for example I use UdpClient.BeginReceive in a UdpListener:

class UdpListener : IDisposable
{
    private bool _isDisposed;
    private readonly IPAddress _hostIpAddress;
    private readonly int _port;
    private UdpClient _udpClient;
    public UdpListener(IPAddress hostIpAddress, int port)
    {
        _hostIpAddress = hostIpAddress;
        _port = port;
    }
    public void Start()
    {
        _udpClient.Connect(_hostIpAddress, _port);
        _udpClient.BeginReceive(HandleMessage, null);
    }
    public void Dispose()
    {
        if (_isDisposed)
        {
            throw new ObjectDisposedException("UdpListener");
        }
        ((IDisposable) _udpClient).Dispose();
        _isDisposed = true;
    }
    private static void HandleMessage(IAsyncResult asyncResult)
    {
        // handle...
    }
}

Do I still need to make sure UdpClient.EndReceive is called on the disposed _udpClient (which will just result in an ObjectDisposedException)?

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Edit:

It isn't uncommon to dispose a UdpClient (and other IDisposables) before all asynchronous operations completed as a way to cancel or implement a timeout, especially over operations that will never complete. This is also what's recommended throughout this site.

Simplism

You're (potentially) disposing of your client before it has actually completed the operation. You almost certainly don't want to be doing that. The code has correctness problems, not performance problems. – Schistosome

@Schistosome this is just an example, not actual code. Imagine a timeout over some network operation. – Simplism

What Servy is trying to say is you don't want to use APM inside a using statement where you escape the block before the APM call has a chance to even start. The issue is using APM inside a short lived using, the client object in your case needs to have the same lifetime as the entire Begin/End, not just the Begin one. – Dish

@RonBeyer yes, I understand his comment. As I said, this isn't actual code. I changed the example to a more believable (and complicated) one. – Simplism

The purpose of the EndOperationName method is not to free resources alone, but to also guarantee the Async method finishes. If the asynchronous operation represented by the IAsyncResult object has not completed when EndOperationName is called, EndOperationName blocks the calling thread until the asynchronous operation is complete. That is an excerpt from the link you provided. Did you actually read it entirely? – Beckerman

If you were to call EndReceive on the disposed _udpClient instance it would simply throw an ObjectDisposedException without doing anything else. So no, you should not do that, although I am having a hard time imagining when/where you would call EndReceive in such a case. – Wheelock

@EBrown I did. Did you? This excerpt has nothing to do with the question. Calling EndXXX won't block the thread if the instance is disposed. It will get an ObjectDisposedException. – Simplism

That excerpt has everything to do with the question. Another excerpt, from the document you claim to understand: For each call to BeginOperationName, the application should also call EndOperationName to get the results of the operation. Your application should always call EndOperationName for every BeginOperationName call. This guarantees that the operation described actually finished, otherwise it's anyone's guess as to whether or not it completed, or whether it had an error. (Omitting EndOperationName will also omit the tracking of errors, FYI.) – Beckerman

@EBrown you won't get any error, or any result out of calling EndXXX on a disposed object. You also wouldn't know if the operation completed or not. All you'll ever get is an ObjectDisposedException. – Simplism

That's why you call it before it's disposed. – Beckerman

@PrestonGuillot Timeout (or cancellation) – Simplism

@PrestonGuillot Indeed. The purpose of marking something IDisposable is not to get out of calling an EndOperationName method. (#538560) The point of Dispose is to free unmanaged resources. – Beckerman

In your concrete example, where no resources are disposed in the call to EndReceive, or as the result of EndReceive you might as well omit it. The native overlapped & GC pinned buffers will be freed on the IAsyncResult completion callback anyway. So if you don't need to wait for the result in this specific case (and potentially others), no harm is done. – Wheelock

@EBrown If you call it before your thread will block until the operation completes, which may never come. Also, the question clearly states that the object is disposed. – Simplism

If you were to read the API regarding the object you are using, you would see the following: The asynchronous BeginReceive operation must be completed by calling the EndReceive method. You never actually let the operation complete, or give it a chance to. This will amost always have unintended side-effects. msdn.microsoft.com/en-us/library/… So, to answer your question, there is no point to calling EndOperationName on a disposed object, but whatever side-effects are there from disposing it before that call will likely remain. – Beckerman

@EBrown If you were to read the code you would see that it's perfectly safe to call Dispose before letting the operation complete as it frees up any resources as Dispose is supposed to do. Moreover this is exactly what you are supposed to do when the operation can't complete. – Simplism

Then why did you ask the question of you already had an answer? (Even though that answer does not specify what you imply it does.) That answer does not even mention disposing an object, but instead says to close the socket. (Which is implicitly done by disposing it, but not the scope of the question.) Disposing and closing the socket are not the same thing. (They may be behind the scenes, but should you really rely on that?) – Beckerman

@EBrown I asked because there wasn't a question, and I would have answered if no one did. I asked a general question about IDisposable, not about UdpClient or Socket. And I asked to get people's informed opinion. Too bad most people don't understand the question and don't bother trying to. And all Close does on Socket is call Dispose, which is obviously also true for UdpClient and TcpClient. – Simplism

It's too bad that you want to rewrite the Asynchronous Programming Model for your situation. This is what CancellationTokens are for (msdn.microsoft.com/en-us/library/…). – Beckerman

@EBrown CancellationToken is never used with the APM as APM predates it. It is however used with async-await but UdpClient.RecieveAsync doesn't accept one. What you should do is use WithCancellation, and since you usually do that in using scope you end up calling Dispose. Even if you don't the only way to clear these resources is calling Dispose. – Simplism

@Simplism Please note that from your referenced "throughout this site" recommendations, this and this and this, i.e. all of them, except your own referenced answer, recommend calling Close, not disposing the client/connection and omitting the EndReceive call as you suggest they do. – Wheelock

@Wheelock All Close does on Socket and UdpClient (and TcpClient for that matter) is call Dispose. Obviously there's no general IClosable interface and the IDisposable in these classes is implemented explicitly. " CONSIDER providing method Close(), in addition to the Dispose(), if close is standard terminology in the area. When doing so, it is important that you make the Close implementation identical to Dispose and consider implementing the IDisposable.Dispose method explicitly." – Simplism

Answer 1

When using the Asynchronous Programming Model it is usually recommended to match every BeginXXX with an EndXXX, otherwise you risk leaking resources kept while the asynchronous operation is still "running".

Is that still the case if the class implements IDisposable and Dispose was called on the instance?

This has nothing to do with a class implementing IDisposable or not.

Unless you can be sure that the async completion will free up any resources tied up with the async operation initiated through BeginXXX, and no cleanup is performed in, or as a result of the EndXXX call, you need to ensure that you match your calls. The only way to be certain of this, is to inspect the implementation of a specific async operation.

For the UdpClient example you chose, it happens to be the case that:

1. Calling EndXXX after disposing the UDPClient instance will result in it directly throwing an ObjectDisposedException.
2. No resources are disposed in or as a result of the EndXXX call.
3. The resources tied up with this operation (native overlapped and pinned unmanaged buffers), will be recycled on the async operation completion callback.

So in this case it is perfectly safe, without leakage.

As a general approach

I don't believe this approach is correct as a general approach, because:

1. The implementation could change in the future, breaking your assumptions.
2. There are better ways to do this, using cancellation and time-outs for your async (I/O) operations (e.g. by calling Close on the _udpClient instance to force an I/O failure).

Also, I would not want to rely on me inspecting the entire call stack (and not making a mistake in doing so) to ensure that no resources will be leaked.

Recommended and documented approach

Please note the following from the documentation for the UdpClient.BeginReceive method:

The asynchronous BeginReceive operation must be completed by calling the EndReceive method. Typically, the method is invoked by the requestCallback delegate.

And the following for the underlying Socket.BeginReceive method:

The asynchronous BeginReceive operation must be completed by calling the EndReceive method. Typically, the method is invoked by the callback delegate.

To cancel a pending BeginReceive, call the Close method.

I.e. this is the "by design" documented behavior. You can argue whether the design is very good, but it is clear in what the expected approach to cancellation is, and the behavior that you can expect as the result of doing so.

For your specific example (updated to do something useful with the async result), and other situations similar to it, the following would be an implementation that follows the recommended approach:

public class UdpListener : IDisposable
{
    private readonly IPAddress _hostIpAddress;
    private readonly int _port;
    private readonly Action<UdpReceiveResult> _processor;
    private TaskCompletionSource<bool> _tcs = new TaskCompletionSource<bool>();
    private CancellationTokenSource _tokenSource = new CancellationTokenSource();
    private CancellationTokenRegistration _tokenReg;
    private UdpClient _udpClient;

    public UdpListener(IPAddress hostIpAddress, int port, Action<UdpReceiveResult> processor)
    {
        _hostIpAddress = hostIpAddress;
        _port = port;
        _processor = processor;
    }

    public Task ReceiveAsync()
    {
        // note: there is a race condition here in case of concurrent calls 
        if (_tokenSource != null && _udpClient == null)
        {
            try 
            {
                _udpClient = new UdpClient();
                _udpClient.Connect(_hostIpAddress, _port);
                _tokenReg = _tokenSource.Token.Register(() => _udpClient.Close());
                BeginReceive();
            }
            catch (Exception ex)
            {
                _tcs.SetException(ex);
                throw;
            }
        }
        return _tcs.Task;
    }

    public void Stop()
    {
        var cts = Interlocked.Exchange(ref _tokenSource, null);
        if (cts != null)
        {
            cts.Cancel();
            if (_tcs != null && _udpClient != null)
                _tcs.Task.Wait();
            _tokenReg.Dispose();
            cts.Dispose();
        }
    }

    public void Dispose()
    {
        Stop();
        if (_udpClient != null) 
        {
            ((IDisposable)_udpClient).Dispose();
            _udpClient = null;
        }
        GC.SuppressFinalize(this);
    }

    private void BeginReceive()
    {
        var iar = _udpClient.BeginReceive(HandleMessage, null);
        if (iar.CompletedSynchronously)
            HandleMessage(iar); // if "always" completed sync => stack overflow
    }

    private void HandleMessage(IAsyncResult iar)
    {
        try
        {
            IPEndPoint remoteEP = null;
            Byte[] buffer = _udpClient.EndReceive(iar, ref remoteEP);
            _processor(new UdpReceiveResult(buffer, remoteEP));
            BeginReceive(); // do the next one
        }
        catch (ObjectDisposedException)
        {
            // we were canceled, i.e. completed normally
            _tcs.SetResult(true);
        }
        catch (Exception ex)
        {
            // we failed.
            _tcs.TrySetException(ex); 
        }
    }
}

Wheelock

1. How is "being sure that the async completion will free up any resources" unrelated to IDisposable if IDisposable is meant exactly for freeing up resources and disposed objects throw ObjectDisposedException for instance methods (like EndXXX) – Simplism

2. The resources in UdpClient tied with the async operation are indeed freed when the operation completes and the callback is called. The point is that these callbacks are also called on Dispose. That's the case for UdpClient, TcpClient, Socket, etc. That's exactly how Dispose frees resources. – Simplism

3. You suggest using cancellation or timeouts with Close as a better way, which is surprising as all Close does is (on UdpClient, TcpClient, Socket, etc.) is call Dispose. Your suggestion is exactly what the question suggests only it means getting an ObjectDisposedException. – Simplism

@Simplism you are relying on the results of inspection of the current code implementation, instead of the published documentation for these classes. The documentation is explicit about how one should cancel these operations and what the expected behavior is when you do. – Wheelock

Well, I do think the implementation speaks louder but nevermind. The documentation is not at all explicit about how you should cancel these operations (or what the outcome would be). It is however implicit about the fact that these operations are uncancellable. The only way you can cancel these operations (e.g. the async version of Receive on UdpClient) is to call Dispose on the instance. – Simplism

@Simplism Then you may want to explain how these are not explicit: "The asynchronous BeginReceive operation must be completed by calling the EndReceive" ,"To cancel a pending BeginReceive, call the Close method.", "To cancel a pending call to the BeginConnect() method, close the Socket. ... A subsequent call to the EndConnect(IAsyncResult) method will throw an ObjectDisposedException to indicate that the operation has been cancelled" – Wheelock

So you agree with me now? That there's no way to cancel these operations other than disposing the instance itself? – Simplism

@Simplism No. It appears you prefer reading what you want to read instead of what it actually says: "call Close" and "must be completed by calling EndReceive" is not the same as "call Dispose and do not call EndReceive". – Wheelock

I would say that you are the one reading what you want as "A subsequent call to the EndConnect method will throw an ObjectDisposedException to indicate that the operation has been cancelled" means that is what will happen and not what must. Calling Socket.EndConnect on a disposed socket will do nothing other than raise the exception. It would be equivalent to throw new ObjectDisposedException("Socket");. – Simplism

@i3arnon: The proper solution would probably be for the documentation to explicitly specify whether or not EndConnect is required after Dispose, and also to indicate whether a Dispose that squeaks in between the completion of the connection and the call to EndConnect will cause the latter to throw an exception, or if the exception will only be thrown if the Dispose prevents the expected action from completing. It may be reasonable to require clients to call EndConnect, or for require that implementations not require anything beyond Dispose, but is either requirement stated? – Glutamate

@Glutamate I haven't found any explicit requirement on what to do in this exact situation. However, each implementation I actually look at show calling EndXXX will only result in an exception. – Simplism

@i3arnon: What happens in scenarios where the action is completed prior to Dispose, and EndXX is called right around the time of Dispose [quite possibly in a different thread]? Does Dispose kill the evidence of the action's having completed? – Glutamate

@Glutamate If the call to Dispose already completed you get ObjectDisposedException. If it started but haven't completed yet there may be a scenario in which you get the result but it's more likely you get a NullReferenceException on some internal member that was already disposed/cleared/closed. – Simplism

@i3arnon: A properly designed class should guarantee that either the Dispose will beat the completion or not, and should make it possible to discover which occurred. If EndXXX is expected to wait for an action to complete (as is often the case), then it should be expected that Dispose is going to happen on another thread [it can't happen on the thread which is blocking in the EndXXX call!], and thus properly-written code must include enough thread-safety logic to guard against that case even if nothing else is thread-safe. – Glutamate

@Glutamate It is expected that Dispose will be called from another thread and it is possible to discover whether the completion happened before the instance was disposed (in which you'll get an actual result back) or whether it didn't and you'll get an exception. – Simplism

@Glutamate The point of this issue is when you already decided to dispose the instance and the disposal was complete there's no point in calling EndXXX as it's too late to get something other than an exception. – Simplism

@i3arnon: By "the completion happened" do you mean "the EndXX call finished" or "the action finished even if the EndXX hadn't been performed"? If the action completes before Dispose, I would think that it would be reasonable for EndXX to relate the results of the successfully-performed exception; even if present implementations throw an exception in that case, I wouldn't think they should be contractually required to do so. To my mind, the purpose of ObjectDisposed exception should be to say "Disposal of the object has made it impossible to satisfy the contract for this action." – Glutamate

If the contract for the action can be completed despite the object's having been disposed, I would think that in many cases that would be preferable to throwing an exception, though there are some interesting corner cases, somewhat analogous to timeouts. There are three ways a blocking "read 1024" bytes request might sensibly be handled: (1) Either return 1024 bytes or throw an exception; (2) Wait until some data is available, and immediately return up to 1024 bytes of it; (3) Return whatever data arrive before timeout. If code is doing the first type of read and the object is disposed... – Glutamate

...after 700 bytes were received, the method should discard the data and throw an exception. If it's doing one of the other types of read, however, it may make more sense to regard the Dispose as triggering an immediate timeout, in which case Dispose would cause the second style of read to throw an exception if no data was pending or else return normally with that pending data. In the third scenario, if a timeout with no data would cause a successful 0-bytes return, it may make sense to say that the first such read after Dispose would yield 0 bytes, and later ones throw an exception. – Glutamate

@Glutamate IIUC you're saying that an exception should be raised after Dispose only if it must. This isn't the case with these classes (and every other one that I checked) but more to the point I think that if you already called Dispose you have decided to let go of that operation, and while you may still call EndXXX and figure out if a result is there it's not worth it as in most (or all) cases you'll get back an exception which hurts performance. – Simplism

@i3arnon: There are many cases where an out-of-thread "Dispose" on a service means "start shutting yourself down gracefully". Transactions which are being performed when a dispose occurs should either be rolled back or completed, and disposal should not prevent completed transactions from being reported as such. If one views the meaning of Dispose as "Consider yourself notified that your owner is no longer interested in having you perform your job", and ownership of an IDisposable or other resource as an obligation to notify it when you're no longer interested in it, then... – Glutamate

...calling Dispose on a service while transactions are in progress would basically say that the owner isn't interested in having the object accept any more work, but entities from whom the service has accepted work would still have an interest until such time as they are notified that their jobs have either been completed or rolled back. – Glutamate

@Glutamate That isn't really the meaning of IDisposable. Its meaning is to have unmanaged resources waiting for Dispose to be released. APM waits for EndXXX to release resources. What I am saying is that when you combine the two you don't need EndXXX after Dispose. Can you still call it and maybe get a result? Yes. Extremely unlikey, but possible. But even then it's still safe not to call it as you already timed out. – Simplism

Answer 2

Considering the facts that Dispose (which should be identical to Close¹) releases any unmanaged resources (the GC releases the managed ones) and methods throw ObjectDisposedException when called on a disposed instance² it should be safe to not call EndXXX.

That behavior of course depends on the specific implementation but it should be safe and it is indeed the case in UdpClient, TcpClient, Socket and more...

Since APM predates the TPL and the CancelationToken that came with it you usually can't use CancelationToken to cancel these asynchronous operations. That's why you also can't pass a CancelationToken on the equivalent async-await methods (e.g. UdpClient.RecieveAsync) as they are just a wrapper over the BeginXXX/EndXXX methods with a call to Task.Factory.FromAsync. Moreover timeouts (like Socket.ReceiveTimeout) usually only affect the synchronous options and not the asynchronous ones.

The only way to cancel this type of operations is by disposing the instance itself³ which releases all resources and invokes all the waiting callbacks which in turn usually call EndXXX and get the appropriate ObjectDisposedException. This exception is usually raised from the very first line of these methods when the instance is disposed.

From what we know about APM and IDisposable calling Dispose should be enough to clear out any hanging resources and adding a call to EndXXX will just raise an unhelpful ObjectDisposedException and nothing more. Calling EndXXX may protect you where the developer didn't follow the guidelines (it may not, it depends on the faulty implementation) but not calling it would be safe in many if not all of .Net's implementations and should be safe in the rest.

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1. "Consider providing method Close(), in addition to the Dispose(), if close is standard terminology in the area. When doing so, it is important that you make the Close implementation identical to Dispose and consider implementing the IDisposable.Dispose method explicitly"

2. "Do throw an ObjectDisposedException from any member that cannot be used after the object has been disposed of".

3. "To cancel a pending call to the BeginConnect method, close the Socket. When the Close method is called while an asynchronous operation is in progress, the callback provided to the BeginConnect method is called."

Simplism