How do I unit test a finalizer?

Question

I have the following class which is a decorator for an IDisposable object (I have omitted the stuff it adds) which itself implements IDisposable using

Answer 1

It's not easy to test finalization, but it can be easier to test if an object is a subject to garbage collection.

This can be done with a weak references.

In a test, it's important to for the local variables to run out of scope before calling GC.Collect(). The easiest way to make sure is a function scope.

    class Stuff
    {
        ~Stuff()
        {
        }
    }

    WeakReference CreateWithWeakReference(Func factory)
    {
        return new WeakReference(factory());
    }

    [Test]
    public void TestEverythingOutOfScopeIsReleased()
    {
        var tracked = new List();

        var referer = new List();

        tracked.Add(CreateWithWeakReference(() => { var stuff = new Stuff(); referer.Add(stuff); return stuff; }));

        // Run some code that is expected to release the references
        referer.Clear();

        GC.Collect();

        Assert.IsFalse(tracked.Any(o => o.IsAlive), "All objects should have been released");
    }

    [Test]
    public void TestLocalVariableIsStillInScope()
    {
        var tracked = new List();

        var referer = new List();

        for (var i = 0; i < 10; i++)
        {
            var stuff = new Stuff();
            tracked.Add(CreateWithWeakReference(() => { referer.Add(stuff); return stuff; }));
        }

        // Run some code that is expected to release the references
        referer.Clear();

        GC.Collect();

        // Following holds because of the stuff variable is still on stack!
        Assert.IsTrue(tracked.Count(o => o.IsAlive) == 1, "Should still have a reference to the last one from the for loop");
    }