Proper Use of the Idisposable Interface

Proper use of the IDisposable interface

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()

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:

Direct link: Chapter 1 sample by Pearson Publishing
magnet: 84bf0b960936d677190a2be355858e80ef7542c0

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.

When should I implement IDisposable?

If you mean unmanaged objects then yes, you should be implementing it whenever you have one or more unmanaged resource you are handling in your class. You should also be using the pattern when you are possibly holding on to objects that are IDisposable themselves, and make sure to dispose of them when your class is disposed.

(agreed that this question has already been asked enough times as to run a small printer out of ink should they be printed...)

What's the purpose of implementing the IDisposable interface?

If your class creates unmanaged resources, then you can implement IDisposable so that these resources will be cleaned up properly when the object is disposed of. You override Dispose and release them there.

The primary use of IDisposable interface

The underlying connections to db's are not managed, as are file handles and a number of other low-level o/s objects. They are unmanaged. Implementing an IDisposable interface implies that you are not just relying on the garbage collector to release those resources; but you are closing those resources using what ever low-level API that you have available.

Also, I think Eric Lippert's answer (2nd one down) to a similar question is a very good explanation on why you would use IDisposable.

General Rule for When to Implement IDisposable

Implement IDisposable when you have a class what wraps an unmanaged resource or when your class has a field that implements IDisposable.

What is the relationship between the using keyword and the IDisposable interface?

If you use the using statement the enclosed type must already implement IDisposable otherwise the compiler will issue an error. So consider IDisposable implementation to be a prerequisite of using.

If you want to use the using statement on your custom class, then you must implement IDisposable for it. However this is kind of backward to do because there's no sense to do so for the sake of it. Only if you have something to dispose of like an unmanaged resource should you implement it.

// To implement it in C#:
class MyClass : IDisposable {

    // other members in you class 

    public void Dispose() {
        // in its simplest form, but see MSDN documentation linked above
    }
}

This enables you to:

using (MyClass mc = new MyClass()) {

    // do some stuff with the instance...
    mc.DoThis();  //all fake method calls for example
    mc.DoThat();

}  // Here the .Dispose method will be automatically called.

Effectively that's the same as writing:

MyClass mc = new MyClass();
try { 
    // do some stuff with the instance...
    mc.DoThis();  //all fake method calls for example
    mc.DoThat();
}
finally { // always runs
    mc.Dispose();  // Manual call. 
}

Implementing IDisposable C#

You're on the right track. Anytime you have a class level variable that is disposable the containing class should also be disposable. And you're handling it properly from what I can tell. I don't see class name line so I can't tell if you have the IDisposable interface included but I imagine you do since you have implemented the methods. If not make sure you add it.

IDisposable is a chain reaction type of implementation. If the variable is disposable, and it's only local to a method call then you dispose of it at the end of the call, but if it's at the class level you implement IDisposable and dispose of it with your class as you're doing. That way anyone using your class can dispose of it properly.

So for example: Say I have a file open in my class...

public class MyClass
{
     private File txtFile = File.Create(...)
}

Now someone uses my class.

private void useClass()
{
     var myClass = new MyClass();
}

Well, they have just opened a file and it wasn't disposed of properly.

Modify the code and it can be used like so...

public sealed class MyClass : IDisposable
{
     private File txtFile = new File.Create(...)

     public void Dispose()
     {
          txtFile.Dispose();
          GC.SuppressFinalize(this);
     }

     ~MyClass() => Dispose();
}

And when the use it they can use it like so...

private void useClass()
{
     using (var myClass = new MyClass())
     {
         //some code
     }
}

Hopefully this answers your questions. Just remember, is you declare a disposable object local to a method then you don't have to implement IDisposable in your class because you're going to dispose of it in that method. But if you implement it at class level scope, whether you have a method disposing it or not, you should implement IDisposable and check to make sure it's disposed when that containing class calls dispose. Make sense?
Hope this helps.

Proper Use of the Idisposable Interface