Categories
.net c# garbage-collection idisposable

Proper use of the IDisposable interface

1823

I know from reading Microsoft documentation that the “primary” use of the IDisposable interface is to clean up unmanaged resources.

To me, “unmanaged” means things like database connections, sockets, window handles, etc. But, I’ve seen code where the Dispose() method is implemented to free managed resources, which seems redundant to me, since the garbage collector should take care of that for you.

For example:

public class MyCollection : IDisposable
{
    private List<String> _theList = new List<String>();
    private Dictionary<String, Point> _theDict = new Dictionary<String, Point>();

    // Die, clear it up! (free unmanaged resources)
    public void Dispose()
    {
        _theList.clear();
        _theDict.clear();
        _theList = null;
        _theDict = null;
    }
}

My question is, does this make the garbage collector free memory used by MyCollection any faster than it normally would?


Edit: So far people have posted some good examples of using IDisposable to clean up unmanaged resources such as database connections and bitmaps. But suppose that _theList in the above code contained a million strings, and you wanted to free that memory now, rather than waiting for the garbage collector. Would the above code accomplish that?

8

  • 40

    I like the accepted answer because it tell you the correct ‘pattern’ of using IDisposable, but like the OP said in his edit, it does not answer his intended question. IDisposable does not ‘call’ the GC, it just ‘marks’ an object as destroyable. But what is the real way to free memory ‘right now’ instead of waiting for GC to kick in? I think this question deserves more discussion.

    Oct 1, 2010 at 17:15

  • 48

    IDisposable doesn’t mark anything. The Dispose method does what it has to do to clean up resources used by the instance. This has nothing to do with GC.

    Oct 1, 2010 at 19:37

  • 5

    @John. I do understand IDisposable. And which is why I said that the accepted answer does not answer the OP’s intended question(and follow-up edit) about whether IDisposable will help in <i>freeing memory</i>. Since IDisposable has nothing to do with freeing memory, only resources, then like you said, there is no need to set the managed references to null at all which is what OP was doing in his example. So, the correct answer to his question is “No, it does not help free memory any faster. In fact, it does not help free memory at all, only resources”. But anyway, thanks for your input.

    Oct 7, 2010 at 15:17

  • 12

    @desigeek: if this is the case, then you should not have said “IDisposable does not ‘call’ the GC, it just ‘marks’ an object as destroyable”

    Oct 7, 2010 at 15:31

  • 6

    @desigeek: There is no guaranteed way of freeing memory deterministically. You could call GC.Collect(), but that is a polite request, not a demand. All running threads must be suspended for garbage collection to proceed – read up on the concept of .NET safepoints if you want to learn more, e.g. msdn.microsoft.com/en-us/library/678ysw69(v=vs.110).aspx . If a thread cannot be suspended, e.g. because there’s a call into unmanaged code, GC.Collect() may do nothing at all.

    Aug 14, 2015 at 5:15

2830

The point of Dispose is to free unmanaged resources. It needs to be done at some point, otherwise they will never be cleaned up. The garbage collector doesn’t know how to call DeleteHandle() on a variable of type IntPtr, it doesn’t know whether or not it needs to call DeleteHandle().

Note: What is an unmanaged resource? If you found it in the Microsoft .NET Framework: it’s managed. If you went poking around MSDN yourself, it’s unmanaged. Anything you’ve used P/Invoke calls to get outside of the nice comfy world of everything available to you in the .NET Framework is unmanaged – and you’re now responsible for cleaning it up.

The object that you’ve created needs to expose some method, that the outside world can call, in order to clean up unmanaged resources. The method can be named whatever you like:

public void Cleanup()

or

public void Shutdown()

But instead there is a standardized name for this method:

public void Dispose()

There was even an interface created, IDisposable, that has just that one method:

public interface IDisposable
{
   void Dispose()
}

So you make your object expose the IDisposable interface, and that way you promise that you’ve written that single method to clean up your unmanaged resources:

public void Dispose()
{
   Win32.DestroyHandle(this.CursorFileBitmapIconServiceHandle);
}

And you’re done. Except you can do better.


What if your object has allocated a 250MB System.Drawing.Bitmap (i.e. the .NET managed Bitmap class) as some sort of frame buffer? Sure, this is a managed .NET object, and the garbage collector will free it. But do you really want to leave 250MB of memory just sitting there – waiting for the garbage collector to eventually come along and free it? What if there’s an open database connection? Surely we don’t want that connection sitting open, waiting for the GC to finalize the object.

If the user has called Dispose() (meaning they no longer plan to use the object) why not get rid of those wasteful bitmaps and database connections?

So now we will:

  • get rid of unmanaged resources (because we have to), and
  • get rid of managed resources (because we want to be helpful)

So let’s update our Dispose() method to get rid of those managed objects:

public void Dispose()
{
   //Free unmanaged resources
   Win32.DestroyHandle(this.CursorFileBitmapIconServiceHandle);

   //Free managed resources too
   if (this.databaseConnection != null)
   {
      this.databaseConnection.Dispose();
      this.databaseConnection = null;
   }
   if (this.frameBufferImage != null)
   {
      this.frameBufferImage.Dispose();
      this.frameBufferImage = null;
   }
}

And all is good, except you can do better!


What if the person forgot to call Dispose() on your object? Then they would leak some unmanaged resources!

Note: They won’t leak managed resources, because eventually the garbage collector is going to run, on a background thread, and free the memory associated with any unused objects. This will include your object, and any managed objects you use (e.g. the Bitmap and the DbConnection).

If the person forgot to call Dispose(), we can still save their bacon! We still have a way to call it for them: when the garbage collector finally gets around to freeing (i.e. finalizing) our object.

Note: The garbage collector will eventually free all managed objects.
When it does, it calls the Finalize
method on the object. The GC doesn’t know, or
care, about your Dispose method.
That was just a name we chose for
a method we call when we want to get
rid of unmanaged stuff.

The destruction of our object by the Garbage collector is the perfect time to free those pesky unmanaged resources. We do this by overriding the Finalize() method.

Note: In C#, you don’t explicitly override the Finalize() method.
You write a method that looks like a C++ destructor, and the
compiler takes that to be your implementation of the Finalize() method:

~MyObject()
{
    //we're being finalized (i.e. destroyed), call Dispose in case the user forgot to
    Dispose(); //<--Warning: subtle bug! Keep reading!
}

But there’s a bug in that code. You see, the garbage collector runs on a background thread; you don’t know the order in which two objects are destroyed. It is entirely possible that in your Dispose() code, the managed object you’re trying to get rid of (because you wanted to be helpful) is no longer there:

public void Dispose()
{
   //Free unmanaged resources
   Win32.DestroyHandle(this.gdiCursorBitmapStreamFileHandle);

   //Free managed resources too
   if (this.databaseConnection != null)
   {
      this.databaseConnection.Dispose(); //<-- crash, GC already destroyed it
      this.databaseConnection = null;
   }
   if (this.frameBufferImage != null)
   {
      this.frameBufferImage.Dispose(); //<-- crash, GC already destroyed it
      this.frameBufferImage = null;
   }
}

So what you need is a way for Finalize() to tell Dispose() that it should not touch any managed resources (because they might not be there anymore), while still freeing unmanaged resources.

The standard pattern to do this is to have Finalize() and Dispose() both call a third(!) method; where you pass a Boolean saying if you’re calling it from Dispose() (as opposed to Finalize()), meaning it’s safe to free managed resources.

This internal method could be given some arbitrary name like “CoreDispose”, or “MyInternalDispose”, but is tradition to call it Dispose(Boolean):

protected void Dispose(Boolean disposing)

But a more helpful parameter name might be:

protected void Dispose(Boolean itIsSafeToAlsoFreeManagedObjects)
{
   //Free unmanaged resources
   Win32.DestroyHandle(this.CursorFileBitmapIconServiceHandle);

   //Free managed resources too, but only if I'm being called from Dispose
   //(If I'm being called from Finalize then the objects might not exist
   //anymore
   if (itIsSafeToAlsoFreeManagedObjects)  
   {    
      if (this.databaseConnection != null)
      {
         this.databaseConnection.Dispose();
         this.databaseConnection = null;
      }
      if (this.frameBufferImage != null)
      {
         this.frameBufferImage.Dispose();
         this.frameBufferImage = null;
      }
   }
}

And you change your implementation of the IDisposable.Dispose() method to:

public void Dispose()
{
   Dispose(true); //I am calling you from Dispose, it's safe
}

and your finalizer to:

~MyObject()
{
   Dispose(false); //I am *not* calling you from Dispose, it's *not* safe
}

Note: If your object descends from an object that implements Dispose, then don’t forget to call their base Dispose method when you override Dispose:

public override void Dispose()
{
    try
    {
        Dispose(true); //true: safe to free managed resources
    }
    finally
    {
        base.Dispose();
    }
}

And all is good, except you can do better!


If the user calls Dispose() on your object, then everything has been cleaned up. Later on, when the garbage collector comes along and calls Finalize, it will then call Dispose again.

Not only is this wasteful, but if your object has junk references to objects you already disposed of from the last call to Dispose(), you’ll try to dispose them again!

You’ll notice in my code I was careful to remove references to objects that I’ve disposed, so I don’t try to call Dispose on a junk object reference. But that didn’t stop a subtle bug from creeping in.

When the user calls Dispose(): the handle CursorFileBitmapIconServiceHandle is destroyed. Later when the garbage collector runs, it will try to destroy the same handle again.

protected void Dispose(Boolean iAmBeingCalledFromDisposeAndNotFinalize)
{
   //Free unmanaged resources
   Win32.DestroyHandle(this.CursorFileBitmapIconServiceHandle); //<--double destroy 
   ...
}

The way you fix this is tell the garbage collector that it doesn’t need to bother finalizing the object – its resources have already been cleaned up, and no more work is needed. You do this by calling GC.SuppressFinalize() in the Dispose() method:

public void Dispose()
{
   Dispose(true); //I am calling you from Dispose, it's safe
   GC.SuppressFinalize(this); //Hey, GC: don't bother calling finalize later
}

Now that the user has called Dispose(), we have:

  • freed unmanaged resources
  • freed managed resources

There’s no point in the GC running the finalizer – everything’s taken care of.

Couldn’t I use Finalize to cleanup unmanaged resources?

The documentation for Object.Finalize says:

The Finalize method is used to perform cleanup operations on unmanaged resources held by the current object before the object is destroyed.

But the MSDN documentation also says, for IDisposable.Dispose:

Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.

So which is it? Which one is the place for me to cleanup unmanaged resources? The answer is:

It’s your choice! But choose Dispose.

You certainly could place your unmanaged cleanup in the finalizer:

~MyObject()
{
   //Free unmanaged resources
   Win32.DestroyHandle(this.CursorFileBitmapIconServiceHandle);

   //A C# destructor automatically calls the destructor of its base class.
}

The problem with that is you have no idea when the garbage collector will get around to finalizing your object. Your un-managed, un-needed, un-used native resources will stick around until the garbage collector eventually runs. Then it will call your finalizer method; cleaning up unmanaged resources. The documentation of Object.Finalize points this out:

The exact time when the finalizer executes is undefined. To ensure deterministic release of resources for instances of your class, implement a Close method or provide a IDisposable.Dispose implementation.

This is the virtue of using Dispose to cleanup unmanaged resources; you get to know, and control, when unmanaged resource are cleaned up. Their destruction is “deterministic”.


To answer your original question: Why not release memory now, rather than for when the GC decides to do it? I have a facial recognition software that needs to get rid of 530 MB of internal images now, since they’re no longer needed. When we don’t: the machine grinds to a swapping halt.

Bonus Reading

For anyone who likes the style of this answer (explaining the why, so the how becomes obvious), I suggest you read Chapter One of Don Box’s Essential COM:

In 35 pages he explains the problems of using binary objects, and invents COM before your eyes. Once you realize the why of COM, the remaining 300 pages are obvious, and just detail Microsoft’s implementation.

I think every programmer who has ever dealt with objects or COM should, at the very least, read the first chapter. It is the best explanation of anything ever.

Extra Bonus Reading

When everything you know is wrong archiveby Eric Lippert

It is therefore very difficult indeed to write a correct finalizer,
and the best advice I can give you is to not try.

15

  • 43

    This is a great answer but I think it would however benefit from a final code listing for a standard case and for a case where the the class derives from a baseclass that already implements Dispose. e.g having read here (msdn.microsoft.com/en-us/library/aa720161%28v=vs.71%29.aspx) as well I have got confused about what I should do when deriving from the class that already implements Dispose (hey I’m new to this).

    Feb 9, 2012 at 12:42


  • What is the effect of setting the managed instances to null during the Dispose() call, other than ensuring that they won’t be disposed again because the != null check would fail? What about managed types that are not Disposable? Should they be handled in the Dispose method at all (e.g. Set to null)? Should it be done for all managed objects, or only those that we consider ‘heavy’ and Worth the effort of doing anything before GC kicks in? I expect its only meant for Disposable members of a class, but system.Drawing.Image mentioned as example doesnt seem to be disposable…

    – Bartosz

    May 30, 2021 at 18:11

  • @Bartosz You can set any variable you like to null inside your Dispose method. Setting a variable to null means it only might get collected sooner (since it has no outstanding references). If an object doesn’t implement IDisposable, then you don’t have to dispose of it. An object will only expose Dispose if it needs to be disposed of.

    – Ian Boyd

    May 30, 2021 at 19:17

  • 2

    @Ayce “If you write correct code, you never need the finalizer/Dispose(bool) thingy.” I’m not protecting against me; i’m protecing against the dozens, hundreds, thousands, or millions of other developers who might not get it right every time. Sometimes developers forget to call .Dispose. Sometimes you can’t use using. We’re following the .NET/WinRT approach of “the pit of success”. We pay our developer taxes, and write better and defensive code to make it resilient to these problems.

    – Ian Boyd

    Sep 13, 2021 at 13:53


  • 2

    “But you don’t always have to write code for “the public”.” But when trying to come up with best practices for a 2k+ upvoted answer, meant for general introduction to unmanaged memory, it’s best to provide the best code samples possible. We don’t want to leave it all out – and let people stumble into all this the hard way. Because that’s the reality – thousands of developers each year learning this nuance about Disposing. No need to make it needlessly harder for them.

    – Ian Boyd

    Sep 13, 2021 at 18:55


78

IDisposable is often used to exploit the using statement and take advantage of an easy way to do deterministic cleanup of managed objects.

public class LoggingContext : IDisposable {
    public Finicky(string name) {
        Log.Write("Entering Log Context {0}", name);
        Log.Indent();
    }
    public void Dispose() {
        Log.Outdent();
    }

    public static void Main() {
        Log.Write("Some initial stuff.");
        try {
            using(new LoggingContext()) {
                Log.Write("Some stuff inside the context.");
                throw new Exception();
            }
        } catch {
            Log.Write("Man, that was a heavy exception caught from inside a child logging context!");
        } finally {
            Log.Write("Some final stuff.");
        }
    }
}

0

    47

    The purpose of the Dispose pattern is to provide a mechanism to clean up both managed and unmanaged resources and when that occurs depends on how the Dispose method is being called. In your example, the use of Dispose is not actually doing anything related to dispose, since clearing a list has no impact on that collection being disposed. Likewise, the calls to set the variables to null also have no impact on the GC.

    You can take a look at this article for more details on how to implement the Dispose pattern, but it basically looks like this:

    public class SimpleCleanup : IDisposable
    {
        // some fields that require cleanup
        private SafeHandle handle;
        private bool disposed = false; // to detect redundant calls
    
        public SimpleCleanup()
        {
            this.handle = /*...*/;
        }
    
        protected virtual void Dispose(bool disposing)
        {
            if (!disposed)
            {
                if (disposing)
                {
                    // Dispose managed resources.
                    if (handle != null)
                    {
                        handle.Dispose();
                    }
                }
    
                // Dispose unmanaged managed resources.
    
                disposed = true;
            }
        }
    
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }
    }
    

    The method that is the most important here is the Dispose(bool), which actually runs under two different circumstances:

    • disposing == true: the method has been called directly or indirectly by a user’s code. Managed and unmanaged resources can be disposed.
    • disposing == false: the method has been called by the runtime from inside the finalizer, and you should not reference other objects. Only unmanaged resources can be disposed.

    The problem with simply letting the GC take care of doing the cleanup is that you have no real control over when the GC will run a collection cycle (you can call GC.Collect(), but you really shouldn’t) so resources may stay around longer than needed. Remember, calling Dispose() doesn’t actually cause a collection cycle or in any way cause the GC to collect/free the object; it simply provides the means to more deterministicly cleanup the resources used and tell the GC that this cleanup has already been performed.

    The whole point of IDisposable and the dispose pattern isn’t about immediately freeing memory. The only time a call to Dispose will actually even have a chance of immediately freeing memory is when it is handling the disposing == false scenario and manipulating unmanaged resources. For managed code, the memory won’t actually be reclaimed until the GC runs a collection cycle, which you really have no control over (other than calling GC.Collect(), which I’ve already mentioned is not a good idea).

    Your scenario isn’t really valid since strings in .NET don’t use any unamanged resources and don’t implement IDisposable, there is no way to force them to be “cleaned up.”

    0