Proper use of the IDisposable interface

Question

I know from reading the MSDN documentation that the "primary" use of the IDisposable interface is to clean up unmanaged resources http://msdn.microsoft.com/en-us/library/system.idisposable.aspx.

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, you gravy sucking pig dog!
    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?

Answer 1

Yep, that code is completely redundant and unnecessary and it doesn't make the garbage collector do anything it wouldn't otherwise do (once an instance of MyCollection goes out of scope, that is.) Especially the .Clear() calls.

Answer to your edit: Sort of. If I do this:

public void WasteMemory()
{
    var instance = new MyCollection(); // this one has no Dispose() method
    instance.FillItWithAMillionStrings();
}

// 1 million strings are in memory, but marked for reclamation by the GC

It's functionally identical to this for purposes of memory management:

public void WasteMemory()
{
    var instance = new MyCollection(); // this one has your Dispose()
    instance.FillItWithAMillionStrings();
    instance.Dispose();
}

// 1 million strings are in memory, but marked for reclamation by the GC

If you really really really need to free the memory this very instant, call GC.Collect(). There's no reason to do this here, though. The memory will be freed when it's needed.

Answer 2

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.");
        }
    }
}

Answer 3

If MyCollection is going to be garbage collected anyway, then you shouldn't need to dispose it. Doing so will just churn the CPU more than necessary, and may even invalidate some pre-calculated analysis that the garbage collector has already performed.

I use IDisposable to do things like ensure threads are disposed correctly, along with unmanaged resources.

EDIT In response to Scott's comment:

The only time the GC performance metrics are affected is when a call the [sic] GC.Collect() is made"

Conceptually, the GC maintains a view of the object reference graph, and all references to it from the stack frames of threads. This heap can be quite large and span many pages of memory. As an optimisation, the GC caches its analysis of pages that are unlikely to change very often to avoid rescanning the page unnecessarily. The GC receives notification from the kernel when data in a page changes, so it knows that the page is dirty and requires a rescan. If the collection is in Gen0 then it's likely that other things in the page are changing too, but this is less likely in Gen1 and Gen2. Anecdotally, these hooks were not available in Mac OS X for the team who ported the GC to Mac in order to get the Silverlight plug-in working on that platform.

Another point against unnecessary disposal of resources: imagine a situation where a process is unloading. Imagine also that the process has been running for some time. Chances are that many of that process's memory pages have been swapped to disk. At the very least they're no longer in L1 or L2 cache. In such a situation there is no point for an application that's unloading to swap all those data and code pages back into memory to 'release' resources that are going to be released by the operating system anyway when the process terminates. This applies to managed and even certain unmanaged resources. Only resources that keep non-background threads alive must be disposed, otherwise the process will remain alive.

Now, during normal execution there are ephemeral resources that must be cleaned up correctly (as @fezmonkey points out database connections, sockets, window handles) to avoid unmanaged memory leaks. These are the kinds of things that have to be disposed. If you create some class that owns a thread (and by owns I mean that it created it and therefore is responsible for ensuring it stops, at least by my coding style), then that class most likely must implement IDisposable and tear down the thread during Dispose.

The .NET framework uses the IDisposable interface as a signal, even warning, to developers that the this class must be disposed. I can't think of any types in the framework that implement IDisposable (excluding explicit interface implementations) where disposal is optional.

Answer 4

You're right, this code doesn't accomplish much.

Answer 5

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."

Answer 6

There are things that the Dispose() operation does in the example code that might have an effect that would not occur due to a normal GC of the MyCollection object.

If the objects referenced by _theList or _theDict are referred to by other objects, then that List<> or Dictionary<> object will not be subject to collection but will suddenly have no contents. If there were no Dispose() operation as in the example, those collections would still contain their contents.

Of course, if this were the situation I would call it a broken design - I'm just pointing out (pedantically, I suppose) that the Dispose() operation might not be completely redundant, depending on whether there are other uses of the List<> or Dictionary<> that are not shown in the fragment.

Answer 7

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 Framwork 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 can call, in order to clean up unmanaged resources. There is even a standardized name for this method you must create:

public void Dispose()

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

public interface class 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.gdiCursorBitmapStreamFileHandle);
}

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? Do you really want that 250MB of memory just sitting there, waiting for the garbage collector to eventually come along and call Dispose? Or what if you have a database DbConnection open? The user has told us they don't our object anymore, why not get rid of those wasteful Bitmaps and database connections?

So now you get rid of unmanaged resources (because you have to), and you also get rid of managed resources (because you want to be helpful):

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

   //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 using your object simply forgets to even call Dispose() when they're done with it? 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.

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

Note: The garbage collector will eventually free all managed objects. When it does it calls the Finalize method. 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 Garbage collector destroying our object 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:

public ~MyObject()
{
    //we're being 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 way for Finalize to tellDispose that it should not touch any managed resources (because they might not be there anymore).

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), and so its 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 freeManagedObjectsAlso)
{
   //Free unmanaged resources
   Win32.DestroyHandle(this.gdiCursorBitmapStreamFileHandle);

   //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 (freeManagedObjectsAlso)  
   {    
      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.Dipose() method to:

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

and your finalizer to:

public ~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 t call their base Dispose method when you overrode Dispose:

public 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 gdiCursorBitmapStreamFileHandle 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.gdiCursorBitmapStreamFileHandle); <--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 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
}

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.

---

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, that they're no longer needed. Otherwise the machine grinds to a swapping halt.

Answer 8

Scenarios I make use of IDisposable: clean up unmanaged resources, unsubscribe for events, close connections

The idiom I use for implementing IDisposable (not threadsafe):

class MyClass : IDisposable 
{
    // ...

#region IDisposable Members and Helpers
private bool disposed = false;
public void Dispose()
{
 Dispose(true);
 GC.SuppressFinalize(this);
}

private void Dispose(bool disposing)
{
 if (!this.disposed)
 {
  if (disposing)
  {
   // cleanup code goes here
  }
  disposed = true;
 }
}

~MyClass()
{
 Dispose(false);
}
#endregion
}

Answer 9

There should be no further calls to an object's methods after Dispose has been called on it (although an object should tolerate further calls to Dispose). Therefore the example in the question is silly. If Dispose is called, then the object itself can be discarded. So the user should just discard all references to that whole object (set them to null) and all the related objects internal to it will automatically get cleaned up.

As for the general question about managed/unmanaged and the discussion in other answers, I think any answer to this question has to start with a definition of an unmanaged resource.

What it boils down to is that there is a function you can call to put the system into a state, and there's another function you can call to bring it back out of that state. Now, in the typical example, the first one might be a function that returns a file handle, and the second one might be a call to CloseHandle.

But - and this is the key - they could be any matching pair of functions. One builds up a state, the other tears it down. If the state has been built but not torn down yet, then an instance of the resource exists. You have to arrange for the teardown to happen at the right time - the resource is not managed by the CLR. The only automatically managed resource type is memory. There are two kinds: the GC, and the stack. Value types are managed by the stack, and reference types are managed by the GC.

These functions may cause state changes that can be freely interleaved, or may need to be perfectly nested. The state changes may be threadsafe, or they might not.

Look at the example in Justice's question. Changes to the Log file's indentation must be perfectly nested, or it all goes wrong. Also they are unlikely to be threadsafe.

It is possible to hitch a ride with the garbage collector to get your unmanaged resources cleaned up. But only if the state change functions are threadsafe and two states can have lifetimes that overlap in any way. So Justice's example of a resource must NOT have a finalizer! It just wouldn't help anyone.

For those kinds of resources, you can just implement IDisposable, without a finalizer. The finalizer is absolutely optional - it has to be. This is glossed over or not even mentioned in many books.

You then have to use the using statement to have any chance of ensuring that Dispose is called. This is essentially like hitching a ride with the stack (so as finalizer is to the GC, using is to the stack).

The missing part is that you have to manually write Dispose and make it call onto your fields and your base class. C++/CLI programmers don't have to do that. The compiler writes it for them in most cases.

There is an alternative, which I prefer for states that nest perfectly and are not threadsafe (apart from anything else, avoiding IDisposable spares you the problem of having an argument with someone who can't resist adding a finalizer to every class that implements IDisposable).

Instead of writing a class, you write a function. The function accepts a delegate to call back to:

public static void Indented(this Log log, Action action)
{
    log.Indent();
    try
    {
        action();
    }
    finally
    {
        log.Outdent();
    }
}

And then a simple example would be:

Log.Write("Message at the top");
Log.Indented(() =>
{
    Log.Write("And this is indented");

    Log.Indented(() =>
    {
        Log.Write("This is even more indented");
    });
});
Log.Write("Back at the outermost level again");

The lambda being passed in serves as a code block, so it's like you make your own control structure to serve the same purpose as using, except that you no longer have any danger of the caller abusing it. There's no way they can fail to clean up the resource.

This technique is less useful if the resource is the kind that may have overlapping lifetimes, because then you want to be able to build resource A, then resource B, then kill resource A and then later kill resource B. You can't do that if you've forced the user to perfectly nest like this. But then you need to use IDisposable (but still without a finalizer, unless you have implemented threadsafety, which isn't free).

Answer 10

If anything, I'd expect the code to be less efficient than when leaving it out.

Calling the Clear() methods are unnecessary, and the GC probably wouldn't do that if the Dispose didn't do it...

Answer 11

In the example you posted, it still doesn't "free the memory now". All memory is garbage collected, but it may allow the memory to be collected in an earlier generation. You'd have to run some tests to be sure.

---

The Framework Design Guidelines are guidelines, and not rules. They tell you what the interface is primarily for, when to use it, how to use it, and when not to use it.

I once read code that was a simple RollBack() on failure utilizing IDisposable. The MiniTx class below would check a flag on Dispose() and if the Commit call never happened it would then call Rollback on itself. It added a layer of indirection making the calling code a lot easier to understand and maintain. The result looked something like:

using( MiniTx tx = new MiniTx() )
{
    // code that might not work.

    tx.Commit();
}

I've also seen timing / logging code do the same thing. In this case the Dispose() method stopped the timer and logged that the block had exited.

using( LogTimer log = new LogTimer("MyCategory", "Some message") )
{
    // code to time...
}

So here are a couple of concrete examples that don't do any unmanaged resource cleanup, but do successfully used IDisposable to create cleaner code.

Answer 12

I won't repeat the usual stuff about Using or freeing un-managed resources, that has all been covered. But I would like to point out what seems a common misconception.
Given the following code

Public Class LargeStuff
  Implements IDisposable
  Private _Large as string()

  'Some strange code that means _Large now contains several million long strings.

  Public Sub Dispose() Implements IDisposable.Dispose
    _Large=Nothing
  End Sub

I realise that the Disposable implementation does not follow current guidelines, but hopefully you all get the idea.
Now, when Dispose is called, how much memory gets freed?

Answer: None.
Calling Dispose can release unmanaged resources, it CANNOT reclaim managed memory, only the GC can do that. Thats not to say that the above isn't a good idea, following the above pattern is still a good idea in fact. Once Dispose has been run, there is nothing stopping the GC re-claiming the memory that was being used by _Large, even though the instance of LargeStuff may still be in scope. The strings in _Large may also be in gen 0 but the instance of LargeStuff might be gen 2, so again, memory would be re-claimed sooner.
There is no point in adding a finaliser to call the Dispose method shown above though. That will just DELAY the re-claiming of memory to allow the finaliser to run.

Answer 13

Joe Duffy has a lot of useful advice on using IDisposable and finalizers in this post.