CODEWITHSUNDEEP
c#
Don Vince
Don Vince

Reputation: 1330

How do I track down where I've been leaking IDisposable objects from?

I've been debugging some code recently that was a bit memory leaky. It's a long running program that runs as a Windows service.

If you find a class wearing an IDisposable interface, it is telling you that some of the resources it uses are outside the abilities of the garbage collector to clean up for you.

The reason it is telling you this is that you, the user of this object, are now responsible for when these resources are cleaned up. Congratulations!

As a conscientious developer, you are nudged towards calling the .Dispose() method when you've finished with the object in order to release those unmanaged resources.

There is the nice using() pattern to help clean up these resources once they are finished with. Which just leaves finding which exact objects are causing the leakyness?

In order to aid tracking down these rogue unmanaged resources, is there any way to query what objects are loitering around waiting to be Disposed at any given point in time?

Upvotes: 3

Views: 1072

Answers (9)

STW
STW

Reputation: 46434

Can I ask how you're certain that it's specifically objects which implement IDisposable? In my experience the most-likely zombie objects are objects which have not properly had all their event handlers removed (thereby leaving a reference to them from another 'live' object and not qualifying them as unreachable during garbage collection).

There are tools which can help track these down by taking a snapshot of the managed heap and stacks and allowing you to see what objects are considered in-use at a given point in time. A freebie is windbg using sos.dll; it'll take some googling for tutorials to show you the commands you need--but it works and it's free. A more user-friendly (don't confused that with "simple") option is Red Gate's ANTS Profiler running in Memory Profiling mode--it's a slick tool.

Edit: Regarding the usefulness of calling Dispose--it provides a deterministic way to cleanup objects. Garbage Collection only runs when your app has ran out of its allocated memory--it's an expensive task which basically stops your application from executing and looks at all objects in existance and builds a tree of "reachable" (in-use) objects, then cleans up the unreachable objects. Manually cleaning up an object frees it before GC ever has to run.

Upvotes: 1

jrista
jrista

Reputation: 33010

Determining when and where to call Dispose() is a very subjective thing, dependent on the nature of the program and how it uses disposable objects. Subjective problems are not something compilers are very good at. Instead, this is more a job for static analysis, which is the arena of tools like FxCop and StyleCop, or perhaps more advanced compilers like Spec#/Sing#. Static analysis uses rules to determine if subjective requirements, such as "Always ensure .Dispose() is called at some point.", are met.

I am honestly not sure if any static analyzers exist that are capable of checking whether .Dispose() is called. Even for static analysis as it exists today, that might be a bit on the too-subjective side of things. If you need a place to start looking, however, "Static Analysis for C#" is probably the best place.

Upvotes: 0

Szymon Rozga
Szymon Rozga

Reputation: 18178

"Is there any way to detect at the end of the program which objects are loitering around waiting to be Disposed?"

Well, if all goes well, at the end of the program the CLR will call all object's finalizers, which, if the IDisposable pattern was implemented properly, will call the Dispose() methods. So at the end, everything will be cleared up properly.

The problem is that if you have a long running program, chances are some of your IDiposable instances are locking some resources that shouldn't be locked. For cases like this, user code should use the using block or call Dispose() as soon as it is done with an object, but there's really no way for a anyone except the code author to know that.

Upvotes: 3

user13276
user13276

Reputation: 4883

IDisposable is more for making use of the using keyword. It's not there to force you to call Dispose() - it's there to enable you to call it in a slick, non-obtrusive way:

class A : IDisposable {}

/// stuff

using(var a = new A()) {
  a.method1();
}

after you leave the using block, Dispose() is called for you.

Upvotes: 4

Norman Ramsey
Norman Ramsey

Reputation: 202735

What if the disposable object is created in one class/module (say a factory) and is handed off to a different class/module to be used for a while before being disposed of? That use case should be OK, and the compiler shouldn't badger you about it. I suspect that's why there's no compile-time warning---the compiler assumes the Dispose call is in another file.

Upvotes: 0

Marc Charbonneau
Marc Charbonneau

Reputation: 40513

A good example is the .NET 2.0 Ping class, which runs asynchronously. Unless it throws an exception, you don't actually call Dispose until the callback method. Note that this example has some slightly weird casting due to the way Ping implements the IDisposable interface, but also inherits Dispose() (and only the former works as intended).

private void Refresh( Object sender, EventArgs args )
{
    Ping ping = null;

    try
    {
        ping = new Ping();
        ping.PingCompleted += PingComplete;
        ping.SendAsync( defaultHost, null );
    }
    catch ( Exception )
    {
        ( (IDisposable)ping ).Dispose();
        this.isAlive = false;
    }
}

private void PingComplete( Object sender, PingCompletedEventArgs args )
{
    this.isAlive = ( args.Error == null && args.Reply.Status == IPStatus.Success );
    ( (IDisposable)sender ).Dispose();
}

Upvotes: 1

Ishmaeel
Ishmaeel

Reputation: 14403

There shouldn't be any cases where you don't want to call Dispose, but the compiler cannot tell you where you should call dispose.

Suppose you write a factory class which creates and returns disposable objects. Should the compiler bug you for not calling Dispose when the cleanup should be the responsibility of your callers?

Upvotes: 6

tvanfosson
tvanfosson

Reputation: 532765

Because the method creating the disposable object may be legitimately returning it as a value, that is, the compiler can't tell how the programming is intending to use it.

Upvotes: 0

Otávio Décio
Otávio Décio

Reputation: 74320

You are not required to call the Dispose method. Implementing the IDisposable interface is a reminder that your class probably is using resources such as a database connection, a file handle, that need to be closed, so GC is not enough. The best practice AFAIK is to call Dispose or even better, put the object in a using statement.

Upvotes: 1

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