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I need to ensure thread safety when disposing an IDisposable resource. The resource is cached multiple times in memory. When entries are evicted from the cache we have a callback that calls Dispose().

Therefore if the resource is cached three times, we'll call Dispose() three times.

Is it the responsibility of the IDisposable resource to ensure thread safety or the caller?

Ben Foster
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  • The `IDisposable` interface makes no guarantee about thread-safety. Do not assume a function is thread-safe unless explicitly specified. – SLaks Feb 06 '14 at 15:05
  • An object must only ever be disposed when no code can use it again. Which automatically means it is thread-safe. – Hans Passant Feb 06 '14 at 15:48
  • It may *mean* it is thread safe but it does not guarantee it. @SLaks I'm not making any assumption about thread safety - I just wanted to know whose responsibility it is to make it thread safe. – Ben Foster Feb 06 '14 at 15:54
  • @HansPassant: Calling `Dispose` on an object will disrupt any operations using that object, but such disruption is sometimes necessary and appropriate, especially when using blocking I/O (e.g. if a worker thread is blocked waiting for some data to arrive, and a user clicks "cancel", disposing the connection will often be the cleanest way to let the worker thread proceed immediately). The connection type will need to include code to ensure disposal is thread-safe, but that's often not too difficult. – supercat Feb 06 '14 at 21:20

4 Answers4

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It is the responsibility of the class implementing the IDisposable interface to ensure that the method can be called multiple times without throwing exception.

To help ensure that resources are always cleaned up appropriately, a Dispose method should be callable multiple times without throwing an exception.

http://msdn.microsoft.com/en-us/library/fs2xkftw(v=vs.110).aspx

After an object is disposed, calls that fail because the object is disposed can and likely should throw an ObjectDisposedException (to aid in future debugging). If it is important that external objects know whether or not an object is disposed before making calls (because the object is shared), then it is customary to add a public boolean property (IsDisposed/Disposed) that indicates the state of the object.

EDIT: To more clearly cast this answer to the phrasing of the question, the class implementing IDisposable should implementing thread-safety if it is expected the class will be used in a cross-threaded environment. The link I posted shows an example of this at the bottom of the page.

Rob Ocel
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Either

  1. The system which is evicting the values and calling Dispose must use synchronization to ensure the calls don't overlap
  2. The object itself must implement Dispose in such a way that it can be safely called from multiple threads

Both of these are completely valid solutions to the problem. Which one is better will depend a lot on your system.

All other things being equal I would opt for #2. I prefer to have my objects be self sufficient and require as little help as possible to successfully execute in the environment they are designed for. Making it thread safe reduces the knowledge the rest of the system needs to use it correctly

JaredPar
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Is it the responsibility of the IDisposable resource to ensure thread safety or the caller?

It is up to the caller to guarantee thread safety through lock primitive or Monitor class IDisposable has nothing to do with thread safety. It's just a signal that the object's instance is ready to be garbage collected.

BRAHIM Kamel
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The answer depends on whether the objects you want to dispose adhere to the guidelines or not.

If they do some things get simpler, from IDisposable.Dispose:

If an object's Dispose method is called more than once, the object must ignore all calls after the first one. The object must not throw an exception if its Dispose method is called multiple times. Instance methods other than Dispose can throw an ObjectDisposedException when resources are already disposed.

So if calling Dispose multiple times has no effect other than the first time it's called you only have to ensure that you don't call them at the same time. And that should absolutely be the responsibility of the caller.

Stephen Cleary wrote an article on CodeProject about how to implement IDisposable and the possible problems which I find extremely helpful.

Dirk
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  • This, however, is different when being called from multiple threads. If the object does something as simple as setting a bool to indicate it has been disposed then you have a race condition where `Dispose` is called twice and both methods get past the `isDisposed` check before either sets it to `true`. Most `IDisposable` objects won't have the appropriate synchronization tools needed to prevent such a case. – Servy Feb 06 '14 at 15:12
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    @Servy yes, that's why I say it's the responsibility of the caller to ensure thread-safety. – Dirk Feb 06 '14 at 15:13
  • @Dirk: In some contexts it may not be practical for the caller to ensure thread safety. If requests to abort operations on an I/O stream may be received asynchronously (e.g. from a user clicking "cancel"), having `Dispose` be thread-safe (causing any pending or future operations to fail with deterministic exceptions) may be much easier than requiring the caller to ensure thread safety (if code uses blocking I/O, it may be necessary to kill off a connection while an operation is "in progress" [blocking] on it; that's hard to make thread-safe in the caller). – supercat Feb 06 '14 at 21:39