Garbage Collection should have removed object but WeakReference.IsAlive still returning true

Question

I have a test that I expected to pass but the behavior of the Garbage Collector is not as I presumed:

[Test]
public void WeakReferenceTest2()
{
    var obj =         


        

Answer 1

Hit the same issue as you - my test was passing everywhere, except for under NCrunch (could be any other instrumentation in your case). Hm. Debugging with SOS revealed additional roots held on a call stack of a test method. My guess is that they were a result of code instrumentation that disabled any compiler optimizations, including those that correctly compute object reachability.

The cure here is quite simple - don't ever hold strong references from a method that does GC and tests for aliveness. This can be easily achieved with a trivial helper method. The change below made your test case pass with NCrunch, where it was originally failing.

[TestMethod]
public void WeakReferenceTest2()
{
    var wRef2 = CallInItsOwnScope(() =>
    {
        var obj = new object();
        var wRef = new WeakReference(obj);

        wRef.IsAlive.Should().BeTrue(); //passes

        GC.Collect();

        wRef.IsAlive.Should().BeTrue(); //passes
        return wRef;
    });

    GC.Collect();

    wRef2.IsAlive.Should().BeFalse(); //used to fail, now passes
}

private T CallInItsOwnScope(Func getter)
{
    return getter();
}