How to force full garbage collection in .NET 4.x?

Question

I've a problem with WeakReferences in .NET 4.x, I was running tests to make sure some objects were not referenced anymore (using WeakReferences) and I noticed the behavior is not consistent across framework versions:

using System;
using System.Text;
using NUnit.Framework;

[TestFixture]
public class WeakReferenceTests
{
    [Test]
    public void TestWeakReferenceIsDisposed()
    {
        WeakReference weakRef = new WeakReference(new StringBuilder("Hello"));

        GC.Collect();
        GC.WaitForPendingFinalizers();
        GC.WaitForFullGCComplete();
        GC.Collect();

        var retrievedSb = weakRef.Target as StringBuilder;
        Assert.That(retrievedSb, Is.Null);
    }
}

Results:

.NET 2.0  PASS
.NET 3.0  FAIL
.NET 3.5  PASS
.NET 4.0  FAIL
.NET 4.5  FAIL

Is this documented somewhere?

Is there a way to force the GC to collect that reference in .NET 4.5?

Thanks in advance.

Just a note: GC does not collect unreachable objects in debug mode. — tukaef, May 27 '13 at 10:38
Hmm, the code you've shown works for me on all versions of the framework. (Well, I don't have NUnit installed, so I'm just using `Debug.Assert` but that should not change the behavior.) — Cody Gray - on strike, May 27 '13 at 10:45
Not sure if .NET platform is the best solution if you need to precisely control disposing of objects... Every learning resource says quite clearly - you can't predict when GC happens nor should you try to force it... — walther, May 27 '13 at 10:49
Just to build on what @walther mentioned, even if you do invoke the garbage collector, there is no guarantee that it will execute. — Levi Botelho, May 27 '13 at 10:51
@Cody Gray: perhaps NUnit or NCrunch is doing something weird, I'll look into that. (walther: yes I know but it's the easiest way I found to check behavior of an external assembly.) — Guillaume86, May 27 '13 at 10:53
@Cody Gray You're right it's NUnit or my test runner that's messing with this. (Even compiled in DEBUG this test pass on all versions) — Guillaume86, May 27 '13 at 11:00
@Cody Gray post your comment as an answer since it's the solution, I'll accept it — Guillaume86, May 27 '13 at 11:01
I use NCrunch and it was instrumenting the output assembly, producing this behavior (disabling output instrumenting make the test pass on all platforms) — Guillaume86, May 27 '13 at 11:04

score 8 · Accepted Answer · edited May 27 '13 at 11:12

8

The problem here is related to NCrunch. The code works fine on my machine for all versions of the framework if I replace the test with a simple call to Debug.Assert:

using System;
using System.Text;
using System.Diagnostics;

public class WeakReferenceTests
{
    public void TestWeakReferenceIsDisposed()
    {
        WeakReference weakRef = new WeakReference(new StringBuilder("Hello"));

        GC.Collect();
        GC.WaitForPendingFinalizers();
        GC.WaitForFullGCComplete();
        GC.Collect();

        var retrievedSb = weakRef.Target as StringBuilder;
        Debug.Assert(retrievedSb == null);
    }
}

edited May 27 '13 at 11:12

Guillaume86

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answered May 27 '13 at 11:09

Cody Gray - on strike

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Thanks, it was NCrunch that instruments assemblies, I disabled the option and it pass. – Guillaume86 May 27 '13 at 11:10
I'm trying to get this done, but my application is built on Silverlight 4, and I don't have the WaitForFullGCComplete option. Would it be on SL5? – Ricardo Appleton Sep 17 '14 at 16:49

score 0 · Answer 2 · answered May 27 '13 at 11:08

0

Thanks to @ Cody Gray (see comments), I figured that out.

I use NCrunch to run my tests and it was instrumenting the output assembly, producing this behavior (disabling output instrumenting make the test pass on all platforms).

answered May 27 '13 at 11:08

Guillaume86

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How to force full garbage collection in .NET 4.x?

2 Answers2

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