How to unit test IDisposable?

Question

I'm working on a piece of library code around IDisposable. The managed path (via using) is easily testable. I'm wondering about the finalizer though: Is calling System.GC.Collect() sufficient to force the finalizer to run?

If you stick to the prescribed pattern for IDisposable, I'm sure that unit testing it is going to be that useful. — Mitch Wheat, Oct 29 '08 at 10:49
sorry, that should have been "I'm not sure that unit testing it is going to be that useful... — Mitch Wheat, Oct 29 '08 at 10:58
@Mitch: implementing IDisposable correctly, so that managed and unmanaged resources are disposed of at the right moment is not trivial. As the library code in question is responsible for exactly that I don't see a point in _not_ testing that ... — David Schmitt, Oct 29 '08 at 11:01

score 8 · Accepted Answer · answered Oct 29 '08 at 10:45

8

No, the GC.Collect() call is asynchronous, you would also need to call this:

System.GC.WaitForPendingFinalizers();

answered Oct 29 '08 at 10:45

RickL

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I think the GC.Collect() itself is synchronous (i.e. it will have freed any memory it can by the time it returns) but the finalizers themselves run separately. I could be entirely wrong though... – Jon Skeet Oct 29 '08 at 11:06
I'm doing both now and the test passed every time. That is, called the correct extension point from the finalizer. – David Schmitt Oct 29 '08 at 21:53
Note to self: also check that the initialisation code of the class in question doesn't "root" the object somewhere. – David Schmitt Oct 30 '08 at 21:39
@EricBurcham: an example where I use that is here: https://github.com/daszat/zetbox/blob/master/Tests/DisposableMockTemplate.cs – David Schmitt Feb 19 '14 at 14:54
this doesn't work for me. I called `GC.Collect()` and then `GC.WaitForPendingFinalizers();` and at no point is `disposing` ever `false` – Matthew Jul 24 '19 at 20:32

score 2 · Answer 2 · answered Feb 11 '09 at 18:45

I would take a look at Dispose, Finalization, and Resource Management its the best reference on the subject I know of. Using their pattern:

~ComplexCleanupBase()
{
    Dispose(false);
}

public void Dispose()
{
    Dispose(true);
    GC.SuppressFinalize(this);
}

protected override void Dispose(bool disposing)
{
   if (!disposed)
   {
        if (disposing)
        {
            // dispose-only, i.e. non-finalizable logic
        }

        // new shared cleanup logic
        disposed = true;
    }

    base.Dispose(disposing);
}

You wind up with dead simple Finalizer/Dispose() methods and a testable Dispose(bool). No need to force Finalization or anything using the GC class.

Yeah, I'm doing it quite like that. It's only a slight adaptation of what's recommended in the MSDN. — David Schmitt, Feb 12 '09 at 07:52

score 0 · Answer 3 · answered Oct 29 '08 at 10:43

0

Could you mock out an IDisposable interface and expect a call to Dispose? That would at least let you see when the object is actually disposed.

answered Oct 29 '08 at 10:43

brien

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Setting a breakpoint is sufficient to discover that. I'm testing the Dispose()'s implementation though, not whether it's called. – David Schmitt Jan 24 '09 at 20:12

score 0 · Answer 4 · answered Oct 29 '08 at 21:31

0

I think I would lean towards making Finalize() call another method, and test that the other method does what you want. You wouldn't get 100% code coverage, but at least you'd know that the method releases the object's resources properly.

answered Oct 29 '08 at 21:31

Ben Fulton

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How to unit test IDisposable?

4 Answers4