Detecting "leaked" IDisposable objects

Question

There are many questions SO asking how to detect IDisposable objects leak. It seems like the answer is "you can't".

I just checked with the most trivial test case, that FxCop 10.0 doesn't do it, ReSharper 4 with MSVS2010 doesn't do it.

This seems wrong to me, worse than memory leaks in C (for which at least we have established tools to detect).

I was thinking: Is it possible, using reflection and other obscure advanced techniques, that I can inject a check at runtime, in the finalizer to see whether Dispose has been called?

How about magic tricks with WinDBG+SOS?

Even if there aren't existing tools to do it, I'd like to know whether this is theoretically possible (my C# isn't very sharp).

Ideas?

NOTE This question's title might have been misleading. The real question here should be whether a IDisposable object has been Disposed() properly. Getting disposed by the GC doesn't count since I regard that as a mistake.

Edit: Solution: .NET Memory Profiler does the job. We just need to spam several GC.Collect() at the end of the program to enable our profiler to correctly pick up the stats.

The reason why tools exist for C++ but maybe not for C# is that resources in C# are fundamentally different since unmanaged resources are *no longer coupled to object lifetime*. What can be traced, both in C# and in C++, is object lifetime and whether an object has been properly disposed of. But disposable resources in C# are not in any way bound to object lifetime which makes tracking them much harder. For comparison, try tracking leaked GDI resources that are not bound via RAII to object lifetime in C++. Not as easy either. — Konrad Rudolph, Jan 19 '11 at 16:23
I have been pondering this a bit. I developed a habit of quickly checking types as I write code to see if they inherit from `IDisposable`. If they do I wrap them in `using` at the scope they need to live. It does nothing for existing code but I just thought I'd mention it. — Nick Strupat, Oct 31 '11 at 19:36
Have a look at this post where you can use Visual Studio code analysis to detect iDisposable issues at compile time: http://stackoverflow.com/a/6213977/2862 — John Dyer, May 09 '16 at 14:36

Justin Niessner · Accepted Answer · 2011-01-19T16:15:09.337

15

You didn't search hard enough. There are plenty of .NET Memory Profilers out there that will look at your program as it runs and let you know where/how your memory is used (and what is leaking it).

I would check out any of the following:

Microsoft's CLR Memory Profiler (free)
RedGate ANTS Memory Profiler
JetBrain's DotTrace (includes code profiler as well)
SciTech .NET Memory Profiler

Update

SciTech's .NET Memory Profiler has a feature called 'Dispose Tracker' that fits the bill for the OP's request of tracking only the Dispose calls in their application.

edited Jan 19 '11 at 16:15

answered Jan 19 '11 at 15:27

Justin Niessner

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Does that detect `Dispose` or just memory? What if my `IDisposable` object just has `Console.WriteLine("baz");` (poor example, I know, but you get the point) and I want to make sure it's actually called NOT by the GC? – kizzx2 Jan 19 '11 at 15:31
@kizzx2 - It will detect everything, but from there you can narrow it down to find what you're looking for. – Justin Niessner Jan 19 '11 at 15:31
I had best experiences with RedGate's ANTS Memory Profiler. – Uwe Keim Jan 19 '11 at 15:46
@Uwe Keim: does RedGate's ANTS Memory Profiler track what the question actually asked? (That's the only one in the list that requires sending email for evaluation -- too lazy to do that right now) – kizzx2 Jan 19 '11 at 16:02
3

.NET Memory Profiler happens to has this feature called "Dispose Tracker" that does exactly what I wanted. Unfortunately this answer as it stands now is not really useful to anyone because it just lists out Google search results (not that it is that not useful -- it forced me to go down and try each of them to prove that at least one is relevant). Can you edit it to at least include "Dispose Tracker" about .NET Memory Profiler so this is at least informative? – kizzx2 Jan 19 '11 at 16:11
@kizzx2 - I'll make the edit, but for what it's worth .NET Memory Profiler is the only one found via search result. The rest are from actual use. – Justin Niessner Jan 19 '11 at 16:14
@Justin Neissner: CLRProfiler for example, seems simple enough and I couldn't quite figure out how it can be used to track down undisposed (not necessarily leaking memory) objects? – kizzx2 Jan 19 '11 at 16:16

Frederik Gheysels · Answer 2 · 2011-01-19T19:24:36.617

you can do it, by adding a Finalizer to your IDisposable objects. In the finalizer, you can check whether the object has been disposed or not. If it hasn't been disposed, you can assert this, or write something to a log, or whatever.

 ~Disposable()
 {
#if DEBUG
            // In debug-builds, make sure that a warning is displayed when the Disposable object hasn't been
            // disposed by the programmer.

            if( _disposed == false )
            {
                System.Diagnostics.Debug.Fail ("There is a disposable object which hasn't been disposed before the finalizer call: {0}".FormatString (this.GetType ().Name));
            }
#endif
            Dispose (false);
 }

You can factor this functionality into a base class -Disposable- for instance, which can be used as a template to implement the Disposable pattern for instance.

Like this, for instance:

    /// <summary>
    /// Abstract base class for Disposable types.    
    /// </summary>
    /// <remarks>This class makes it easy to correctly implement the Disposable pattern, so if you have a class which should
    /// be IDisposable, you can inherit from this class and implement the DisposeManagedResources and the
    /// DisposeUnmanagedResources (if necessary).
    /// </remarks>
    public abstract class Disposable : IDisposable
    {
        private bool                    _disposed = false;

        /// <summary>
        /// Releases the managed and unmanaged resources.
        /// </summary>
        public void Dispose()
        {
            Dispose (true);
            GC.SuppressFinalize (this);
        }

        /// <summary>
        /// Releases the unmanaged and managed resources.
        /// </summary>
        /// <param name="disposing">When disposing is true, the managed and unmanaged resources are
        /// released.
        /// When disposing is false, only the unmanaged resources are released.</param>
        [System.Diagnostics.CodeAnalysis.SuppressMessage ("Microsoft.Design", "CA1063:ImplementIDisposableCorrectly")]
        protected void Dispose( bool disposing )
        {
            // We can suppress the CA1063 Message on this method, since we do not want that this method is 
            // virtual.  
            // Users of this class should override DisposeManagedResources and DisposeUnmanagedResources.
            // By doing so, the Disposable pattern is also implemented correctly.

            if( _disposed == false )
            {
                if( disposing )
                {
                    DisposeManagedResources ();
                }
                DisposeUnmanagedResources ();

                _disposed = true;
            }
        }

        /// <summary>
        /// Override this method and implement functionality to dispose the 
        /// managed resources.
        /// </summary>
        protected abstract void DisposeManagedResources();

        /// <summary>
        /// Override this method if you have to dispose Unmanaged resources.
        /// </summary>
        protected virtual void DisposeUnmanagedResources()
        {
        }

        /// <summary>
        /// Releases unmanaged resources and performs other cleanup operations before the
        /// <see cref="Disposable"/> is reclaimed by garbage collection.
        /// </summary>
        [System.Diagnostics.CodeAnalysis.SuppressMessage ("Microsoft.Design", "CA1063:ImplementIDisposableCorrectly")]
        ~Disposable()
        {
#if DEBUG
            // In debug-builds, make sure that a warning is displayed when the Disposable object hasn't been
            // disposed by the programmer.

            if( _disposed == false )
            {
                System.Diagnostics.Debug.Fail ("There is a disposable object which hasn't been disposed before the finalizer call: {0}".FormatString (this.GetType ().Name));
            }
#endif
            Dispose (false);
        }
    }

That's what Raymond at the Old New Thing [suggested](http://blogs.msdn.com/b/oldnewthing/archive/2010/08/13/10049634.aspx). I use it all the time for my own classes, now I was thinking if I could also catch e.g. `FileStream`. — kizzx2, Jan 19 '11 at 15:52
this is a poor approach as it isnt maintainable and adds far too much clutter and complexity. Use a profiler instead. I use Red Gate Memory Profiler all the time. — Dustin Davis, Jan 19 '11 at 17:42
Why is it a poor approach ? Why isn't it maintanable, and adds it complexity ? If you factor this code into a base class, which is called 'Disposable' for instance, i see no problem. Next to that, you can use this base class as a 'template' to correctly implement the 'Disposable pattern'. — Frederik Gheysels, Jan 19 '11 at 19:22
I don't think this is a poor approach. In fact, I think it is a _far_ superior approach than using a profiler for something so simple to implement. The only downside with this is that it can't check other people's classes, so a profiler is still needed. But why not opt for an easier way out when you have control over adding a `Debug.Assert` in your finalizer? Do you really enjoy running the program in a profiler all the time just to catch some careless mistakes? I wonder what kind of software development can make this trivial approach "far too much clutter and complex" in comparison :P — kizzx2, Jan 20 '11 at 14:24
Finalizers introduce performance penalties, which may not be acceptable given the implied goals of the question. Jeffrey Richter's CLR via C# 2nd Edition (p. 477) points out that objects with finalizers take longer to allocate, get promoted to older generations (so they are collected later), and require extra processing at collection time. — Neil, Jun 09 '11 at 16:59
@Neil Whitaker is correct. See [Framework Design Guidelines 2nd Ed.](http://www.amazon.com/Framework-Design-Guidelines-Conventions-Libraries/dp/0321545613) pp. 329-330. "AVOID making types finalizable." Quoting Joe Duffy, "On a heavily loaded server, you may find that one processor spends 100 percent of its time just running finalizers." — TrueWill, Aug 25 '11 at 17:48
@TrueWill @Neil Whittaker, while the performance concerns are true, the finalizer is in many cases inevitable when doing `IDisposable` because you need to stick a `Dispose` in the finalizer. — kizzx2, Aug 25 '11 at 23:09
@kizzx2 - finalizers are *not* inevitable. Most IDisposable classes do not need a finalizer. For many of the rest, SafeHandle descendants will suffice. Read Microsoft's guidelines on this topic. — TrueWill, Aug 26 '11 at 01:41
@TrueWill Of course, that's what I was aware when I said "in many cases". BTW this answer simply suggests adding assertions in DEBUG mode only so I think your comment a little irrelevant here — kizzx2, Aug 26 '11 at 01:45
@kizzx2 - but the sample code shows the `#if DEBUG` **inside** the finalizer, and the answer suggests using this "as a template to implement the Disposable pattern". — TrueWill, Aug 26 '11 at 02:43

score 3 · Answer 3 · answered May 22 '11 at 12:16

3

While @Justin Niessner's recommendation works, I find that using a full blown profiler too heavy.

I created my home-brew solution: EyeDisposable. It instruments assemblies to detect when Dispose have not been called.

answered May 22 '11 at 12:16

kizzx2

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Thanks, looks pretty cool! I would have loved *static* analysis for this though (perhaps as a R# plugin). One small comment, in the *Cloning* section of the readme one has to run `git submodule init` before `git submodule update`. – Ohad Schneider Oct 04 '14 at 11:41
1

@OhadSchneider Thanks for the heads up -- static analysis would be cool but bound to have many false positives for non-trivial cases -- and is honestly way too more complicated than the original scope of this _small_ utility :P – kizzx2 Oct 04 '14 at 12:10

Detecting "leaked" IDisposable objects

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