.NET version of Java's assertGC: Unit tesing memory leaks

Question

I am trying to write a unit test using .NET 4 to ensure that an object can be garbage collected after some code is run. In Java, I would use assertGC to ensure that a weak reference is collected. How can I write this type of test for .NET?

I have tried keeping a WeakReference to the object and calling GC.Collect(), but as you'd expect, sometimes my object is collected and sometimes it is not. Note that this is for a unit test, not production code. I would not want GC.Collect() in my real code base.

I'm using C# but the same answer will be good for VB.NET too.

Answer 1

Try this:

Running for 5 minutes solid, still no exception... Are you sure your code isn't referencing your object somewhere?

using System;

internal class Program
{
    private static void Main(string[] args)
    {
        int cnt = 0;
        while (true)
        {
            ++cnt;
            bool gced = false;
            Action handler = () => gced = true;
            new Foo(handler);
            GC.Collect();
            GC.WaitForPendingFinalizers();
            Console.Out.WriteLine("{0} : {1}", cnt, gced);
            if (!gced)
            {
                throw new Exception("WTF?");
            }
        }
    }
}
class Foo
{
    private readonly Action _onFinalized;

    public Foo(Action finalized)
    {
        _onFinalized = finalized;
    }

    ~Foo()
    {
        if (_onFinalized != null) _onFinalized();
    }
}

Answer 2

It looks like this isn't possible at the moment with .NET 4. Though it is possible to know whether an object cannot be garbage collected (e.g. have a reference to it), there is no way to deterministically know whether an object can be garbage collected.

Using a WeakReference or a finaliser (and calling GC.Collect()) may collect the object, but unless it is in generation 0, there is a fair chance that it will not.

References:

• http://blogs.msdn.com/b/oldnewthing/archive/2010/08/11/10048629.aspx (found by http://stackoverflow.com/users/73070/joey)
• http://www.abhisheksur.com/2010/07/garbage-collection-algorithm-with-use.html