Should IDisposable.Dispose() implementations be idempotent?

Question

The Microsoft.NET framework provides the IDisposable interface which requires an implementation of void Dispose() method. Its purpose is to enable manual, or scope-based releasing of expensive resources an IDisposable implementation may have allocated. Examples include database collections, streams and handles.

My question is, should the implementation of the Dispose() method be idempotent - when called more than once on the same instance, the instance to be 'disposed of' only once, and subsequent calls not to throw exceptions. In Java, most of the objects that have similar behavior (again streams and database connections come to my mind as examples) are idempotent for their close() operation, which happens to be the analogue for the Dispose() method.

However, my personal experience with .NET (and Windows Forms in particular), shows that not all implementations (that are part of the .NET framework itself) are idempotent, so that subsequent calls to these throw an ObjectDisposedException. This really confuses me on how a disposable object's implementation should be approached. Is there a common answer for the scenario, or is it dependent on the concrete context of the object and its usage?

Answer 1

should the implementation of the Dispose() method be idempotent

Yes, it should. There is no telling how many times it will be called.

From Implementing a Dispose Method on MSDN:

a Dispose method should be callable multiple times without throwing an exception.

An object with a good implementation of IDispose will have a boolean field flag indicating if it has been disposed of already and on subsequent calls do nothing (as it was already disposed).

Answer 2

Yes, also make sure the other methods of the class respond correctly when they are called when the object has already been disposed.

public void SomeMethod()
{
     if(_disposed)
     {
         throw new ObjectDisposedException();
     }
     else
     {
         // ...
     }

}

Answer 3

From MSDN:

Allow a Dispose method to be called more than once without throwing an exception. The method should do nothing after the first call.

Answer 4

Personally - Yes - I always make Dispose() idempotent.

During the usual life-cyle of an object in a given application it may not be necessary - the life-cyle from creation to disposal may be deterministic and well known.

However, equally, in some applications it might not be so clear.

For example, in a decorator scenario: I may have a disposable object A, decorated by another disposable object B. I may want to explicitly dispose A, and yet Dispose on B may also dispose the instance it wraps (think: streams).

Given it is relatively easy to make Dispose idempotent (ie if already disposed, do nothing), it seems silly not to.