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?
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:
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.
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.");
}
}
}
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."
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 (or by hitching a ride inside reference types), 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).
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
}
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.
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.
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.
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.
If you want to delete right now, use unmanaged memory.
See:
Marshal.AllocHGlobal
Marshal.FreeHGlobal
Marshal.DestroyStructure
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...
Apart from its primary use as a way to control the lifetime of system resources (completely covered by the awesome answer of Ian, kudos!), the IDisposable/using combo can also be used to scope the state change of (critical) global resources: the console, the threads, the process, any global object like an application instance.
I've written an article about this pattern: http://pragmateek.com/c-scope-your-global-state-changes-with-idisposable-and-the-using-statement/
It illustrates how you can protect some often used global state in a reusable and readable manner: console colors, current thread culture, Excel application object properties...
I see a lot of answers have shifted to talk about using IDisposable for both managed and unmanaged resources. I'd suggest this article as one of the best explanations that I've found for how IDisposable should actually be used.
https://www.codeproject.com/Articles/29534/IDisposable-What-Your-Mother-Never-Told-You-About
For the actual question; should you use IDisposable to clean up managed objects that are taking up a lot of memory the short answer would be no. The reason is that once your object that is holding the memory goes out of scope it is ready for collection. At that point any referenced child objects are also out of scope and will get collected.
The only real exception to this would be if you have a lot of memory tied up in managed objects and you've blocked that thread waiting for some operation to complete. If those objects where not going to be needed after that call completed then setting those references to null might allow the garbage collector to collect them sooner. But that scenario would represent bad code that needed to be refactored - not a use case of IDisposable.
IDisposable is good for unsubscribing from events.
Your given code sample is not a good example for IDisposable usage. Dictionary clearing normally shouldn't go to the Dispose method. Dictionary items will be cleared and disposed when it goes out of scope. IDisposable implementation is required to free some memory/handlers that will not release/free even after they out of scope.
The following example shows a good example for IDisposable pattern with some code and comments.
public class DisposeExample
{
// A base class that implements IDisposable.
// By implementing IDisposable, you are announcing that
// instances of this type allocate scarce resources.
public class MyResource: IDisposable
{
// Pointer to an external unmanaged resource.
private IntPtr handle;
// Other managed resource this class uses.
private Component component = new Component();
// Track whether Dispose has been called.
private bool disposed = false;
// The class constructor.
public MyResource(IntPtr handle)
{
this.handle = handle;
}
// Implement IDisposable.
// Do not make this method virtual.
// A derived class should not be able to override this method.
public void Dispose()
{
Dispose(true);
// This object will be cleaned up by the Dispose method.
// Therefore, you should call GC.SupressFinalize to
// take this object off the finalization queue
// and prevent finalization code for this object
// from executing a second time.
GC.SuppressFinalize(this);
}
// Dispose(bool disposing) executes in two distinct scenarios.
// If disposing equals true, the method has been called directly
// or indirectly by a user's code. Managed and unmanaged resources
// can be disposed.
// If disposing equals 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.
protected virtual void Dispose(bool disposing)
{
// Check to see if Dispose has already been called.
if(!this.disposed)
{
// If disposing equals true, dispose all managed
// and unmanaged resources.
if(disposing)
{
// Dispose managed resources.
component.Dispose();
}
// Call the appropriate methods to clean up
// unmanaged resources here.
// If disposing is false,
// only the following code is executed.
CloseHandle(handle);
handle = IntPtr.Zero;
// Note disposing has been done.
disposed = true;
}
}
// Use interop to call the method necessary
// to clean up the unmanaged resource.
[System.Runtime.InteropServices.DllImport("Kernel32")]
private extern static Boolean CloseHandle(IntPtr handle);
// Use C# destructor syntax for finalization code.
// This destructor will run only if the Dispose method
// does not get called.
// It gives your base class the opportunity to finalize.
// Do not provide destructors in types derived from this class.
~MyResource()
{
// Do not re-create Dispose clean-up code here.
// Calling Dispose(false) is optimal in terms of
// readability and maintainability.
Dispose(false);
}
}
public static void Main()
{
// Insert code here to create
// and use the MyResource object.
}
}
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.
One problem with most discussions of "unmanaged resources" is that they don't really define the term, but seem to imply that it has something to do with unmanaged code. While it is true that many types of unmanaged resources do interface with unmanaged code, thinking of unmanaged resources in such terms isn't helpful.
Instead, one should recognize what all managed resources have in common: they all entail an object asking some outside 'thing' to do something on its behalf, to the detriment of some other 'things', and the other entity agreeing to do so until further notice. If the object were to be abandoned and vanish without a trace, nothing would ever tell that outside 'thing' that it no longer needed to alter its behavior on behalf of the object that no longer existed; consequently, the 'thing's usefulness would be permanently diminished.
An unmanaged resource, then, represents an agreement by some outside 'thing' to alter its behavior on behalf of an object, which would useless impair the usefulness of that outside 'thing' if the object were abandoned and ceased to exist. A managed resource is an object which is the beneficiary of such an agreement, but which has signed up to receive notification if it is abandoned, and which will use such notification to put its affairs in order before it is destroyed.
First of definition. For me unmanaged resource means some class, which implements IDisposable interface or something created with usage of calls to dll. GC doesn't know how to deal with such objects. If class has for example only value types, then I don't consider this class as class with unmanaged resources.
For my code I follow next practices:
If created by me class uses some unmanaged resources then it means that I should also implement IDisposable interface in order to clean memory.
Clean objects as soon as I finished usage of it.
In my dispose method I iterate over all IDisposable members of class and call Dispose.
In my Dispose method call GC.SuppressFinalize(this) in order to notify garbage collector that my object was already cleaned up. I do it because calling of GC is expensive operation.
As additional precaution I try to make possible calling of Dispose() multiple times.
Sometime I add private member _disposed and check in method calls did object was cleaned up. And if it was cleaned up then generate ObjectDisposedException
Following template demonstrates what I described in words as sample of code:
public class SomeClass : IDisposable
{
/// <summary>
/// As usually I don't care was object disposed or not
/// </summary>
public void SomeMethod()
{
if (_disposed)
throw new ObjectDisposedException("SomeClass instance been disposed");
}
public void Dispose()
{
Dispose(true);
}
private bool _disposed;
protected virtual void Dispose(bool disposing)
{
if (_disposed)
return;
if (disposing)//we are in the first call
{
}
_disposed = true;
}
}
The most justifiable use case for disposal of managed resources, is preparation for the GC to reclaim resources that would otherwise never be collected.
A prime example is circular references.
Whilst it's best practice to use patterns that avoid circular references, if you do end up with (for example) a 'child' object that has a reference back to its 'parent', this can stop GC collection of the parent if you just abandon the reference and rely on GC - plus if you have implemented a finalizer, it'll never be called.
The only way round this is to manually break the circular references by setting the Parent references to null on the children.
Implementing IDisposable on parent and children is the best way to do this. When Dispose is called on the Parent, call Dispose on all Children, and in the child Dispose method, set the Parent references to null.
I think people are conflating the PATTERN of IDisposable with the primary purpose of IDisposable which was meant to help clean up unmanaged resources. We all know this. Some think the pattern has some sort of magical powers that clears memory and frees resources. The PATTERN does NOT do this. But the usage of the pattern with the methods that are implemented DO clear memory and free resources.
The pattern is simply a built in try{} finally{} block. Nothing more. Nothing less. So what does that mean? You can create a block of code that lets you do something at the end without having to do extra code for it. It provides a CUSTOM block you can use to segment code and scope.
My example:
//My way
using (var _ = new Metric("My Test"))
{
DoSomething(); //You now know all work in your block is being timed.
}
//MS mockup from memory
var sw = new Stopwatch();
sw.Start();
DoSomething(); //something fails? I never get the elapsed time this way
sw.Stop();
Metric class
public class Metric : IDisposable
{
private string _identifier;
private DateTime _start;
public Metric(string identifier)
{
_identifier = identifier;
_start = DateTime.Now;
}
public void Dispose()
{
Console.WriteLine(_identifier + " - " + (DateTime.Now - _start).TotalMilliseconds)
}
}
In C++ I am aware that a virtual destructor should generally be used when one intends to inherit from a base class. However, with C# I am not sure what should be done. Consider the following code:
public abstract class Character
{
private string characterName;
public int health;
Character()
{
}
~Character(){
}
public virtual void SetCharacterName( string tempName )
{
characterName = tempName;
}
public virtual string GetCharacterName( )
{
return characterName;
}
}
(Note: I have heard that Unity3Ds implementation of C# is slightly different than standard. Perhaps ignore some minor formatting errors, the code is seemingly functional...)
My first instinct was the make the ~Character() destructor virtual, by defining it as:
virtual ~Character(){
}
yet doing so causes the IDE to return an error.
In C#, is it necessary or considered standard to use a virtual destructor for abstract classes which wish to be inherited? Or are there other means of making a virtual destructor with C#?
C# doesn't have deterministic destruction. In fact, it doesn't really have destructors, per se: it has finalizers and IDisposable.
The GC will clean things up if and when it gets around to garbage collecting the object instance. All objects will [eventually] get cleaned up one way or another when the app domain terminates, but it's possible that a given object might hang around for the duration of the app domain. In your case, you don't need to do anything as your object has no resources that need cleaning up. Unreferenced objects will get properly disposed of when the GC sweeps them up.
Mostly, one doesn't need finalizers. You might want to read up on Finalizers at http://msdn.microsoft.com/en-us/library/system.object.finalize.aspx and note that
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.
You might also want to read Understanding when to use a Finalizer in your .NET class and note:
Why is a Finalize method bad?
...
If an object has a finalizer, it will be placed in the FinalizationQueue and is subject
so some additional clean-up. Once the object is no longer referenced on a thread or
from a global reference, the next time the Garbage Collector (GC) runs, it will see this
object is ready to be collected. But it can’t collect it yet. It has to let the
Finalizer run first. So the GC will finish collecting, then the Finalizer will Finalize
the object, and then another GC collection will occur.
This can have a huge affect on performance as you should remember that all managed
threads will be stopped waiting on the GC and then the GC will be stopped waiting on the
Finalizer thread.
There is a lot more data out there about Finalization so I encourage you to read about it
as much as possible so that you use it in the best way possible.
If your object needs something deterministic, you can implement IDisposable and either explicitly invoke Dispose() or use a using block:
using ( Character foo = CharacterFactory.CreateInstance("Jane") )
{
// do something useful with Jane
}
when the enclosing using block is exited, foo.Dispose() is guaranteed to be invoked. It's identical to this code (with the exception of the scope of foo):
Character foo = ... ;
try
{
...
}
finally
{
foo.Dispose() ;
}
However, the point of IDisposable is more about making sure that unmanaged resource are released in a timely manner. DBAs tends to get all grumpy when you block 400 users when your object goes out of scope and is hanging out waiting to be GC'd, leaving an in-flight query running. Or your object goes out of scope, leaving an exclusively locked file open.
In C#, is it necessary or considered standard to use a virtual destructor for abstract classes which wish to be inherited?
Typically you will not use any destructors in C#. Those may or may not get called by the garbage collector at any time (or not).
When you need deterministic cleanup, implement the IDisposable interface, and use using blocks to clean up (unmanaged resources, for example).
Destructors cannot be inherited or overloaded. Related stackoverflow thread with more details: Inheritance and Destructors in C#
I always thought that the answer to this was no, but I cannot find any source stating this.
In my class below, can I access (managed) fields/properties of an instance of C in the finalizer, i.e. in ReleaseUnmanaged()? What restrictions, if any, are there? Will GC or finalization ever have set these members to null?
The only thing I can find is that stuff on the finalizer queue can be finalized in any order. So in that case, since the recommendation is that types should allow users to call Dispose() more than once, why does the recommended pattern bother with a disposing boolean? What bad things might happen if my finalizer called Dispose(true) instead of Dispose(false)?
public class C : IDisposable
{
private void ReleaseUnmanaged() { }
private void ReleaseOtherDisposables() { }
protected virtual void Dispose(bool disposing)
{
ReleaseUnmanaged();
if (disposing)
{
ReleaseOtherDisposables();
}
}
~ C()
{
Dispose(false);
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}
In general case - yes, you can. If a class has nonempty finalizer, first time GC would collect instance of this class, it calls finalizer instead (only if you didn't call GC.SuppressFinalize on it earlier). Object instance seen from finalizer looks just like it did last time you touched it. You can even create new (direct or indirect) link from root to your instance and thus resurrect it.
Even if you hold unmanaged pointer to unpinned object and inspect raw memory contents, you shouldn't be able to see partially-deallocated object, because .NET uses copying GC. If an instance is alive during collection, it is either promoted to next generation or moved to completely new memory block along with other instances. If it is not reachable it is either left where it was or the whole heap is released and returned to OS. Keep in mind, however, that finalizers can and will be called on instances of objects that failed to construct (i.e. when was an exception thrown during object construction).
Edit: As for Dispose(true) vs Dispose(false) in well-written classes there shouldn't be much of a difference in the long run. If your finalizer called Dispose(true), it would only remove links from your object to other objects, but since your object is already nonreachable, releasing other instances referenced by your object won't matter their reachability.
For more details on .NET GC implementation details, I recommend C# 5.0 in a Nutshell by Joseph and Ben Albahari.
The GC will never collect any object if there is any way via which any code could ever get a reference to it. If the only references that exist to an object are weak references, the GC will invalidate them so that there's no way any code could ever get a reference to that object, whereupon it will then be able to collect it.
If an object has an active finalizer, then if the GC would collect it (but for the existence of the finalizer), the GC will instead add it to a queue of objects whose finalizers should be run ASAP and, having done so, de-activate it. The reference within the queue will prevent the GC from collecting the object until the finalizer has run; once the finalizer finishes, if no other references exist to the object and it hasn't re-registered its finalizer, it will cease to exist.
The biggest problems with finalizers accessing outside objects are:
A finalizer will be run in a threading context which is unrelated to any threading context where the object was used, but should not perform any operations which cannot be guaranteed to complete quickly. This often creates contradictory requirement that code use locking when accessing other objects, but that code not block while waiting on locks.
If a finalizer has a reference to another object which also has a finalizer, there's no guarantee as to which will run first.
Both of these factors severely limit the ability of objects with finalizers tosafely access outside objects they don't own. Further, because of the way finalization is implemented, it's possible for the system to decide to run a finalizer when no strong references exist, but for external strong references to be created and used before the finalizer runs; the object with the finalizer has no say in whether that happens.
I've read through this post about disposing of datasets and I still have a question about the destructor. I know that post basically says that you don't need to dispose of Datasets, Datatables, and Dataviews, but my dataset is MASSIVE, so I want to release that memory ASAP. So, my question, should I include a destructor even though the dataset will be disposed when my objects' dispose method is called? Also, explain to me again why the "bool disposing" is needed.
public DEditUtil(DataSet dsTxData)
{
this.dsTxData = dsTxData;
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (!disposed)
{
if (disposing)
dsTxData.Dispose();
disposed = true;
}
}
~DEditUtil()
{
Dispose(false);
}
Yes, in general you should implement the full IDisposable pattern whenever either of the following is true:
You have unmanaged resources being allocated by your class, or
You have managed resources that implement IDisposable (which implies that they, in turn, have unmanaged resources)
The presence of the finalizer (the general CLR term for what C++/C# call a "destructor") is to handle cases where your Dispose method is not called for some reason. The boolean value being passed in to your protected Dispose() method indicated if you are being called from within the public Dispose, or from within your finalizer.
If your public Dispose method is being called, that call stack is deterministic: your dispose method is being called directly, so you can safely call methods (including Dispose) on your child objects.
If you are inside of the finalizer, then you have no idea what's going on with other objects that are also being garbage-collected. In general, it may not be safe to call methods on managed objects your control from within your finalizer.
So, the boolean value basically says: "if true, dispose everything; if false, only dispose my unmanaged resources and let everyone else deal with theirs."
The memory used by your DataSet object will be available for garbage collection as soon as it is not referenced anymore by the code.
The garbage collector will make that memory available to the program at a later (non determinate) time.
Both things do not depend of having or not a destructor or calls to Dispose, so the answer is no - you don't need a destructor.
No, you do not need any other method call here, it's already enough what you did.
Dispose will be called by the runtime and you will free resource allocated, the cleanup let's leave up to GC to decide how and when to do it.
If you have really huge problems with memory you can try to cal GC.Collect() to enforce the collection of the garbage, that usually works, but it's never a good practise to use it in that way, so try to avoid it as much as possible.
EDIT
According to the comments, it's important to pay attention on execution flow in your case, cause the DataSet cleanup will be done only if it's not disposed==false and disposing == true, which from the code provided, will be a case only during esplicit call from the code.
Very seldom should user-written classes ever use finalizers (or C# destructors) for any purpose other than to log failures to call Dispose. Unless one is delving deep into the particulars of how finalizers work, and exactly what is or is not guaranteed guaranteed about the context in which they run, one should never call any other object's Dispose method within a finalizer. In particular, if one's object's Finalize() method is running, any IDisposable objects to which it holds a reference will usually fall into one of the following categories:
Someone else still has a reference to that object and expects it to be usable, so calling `Dispose` would be bad.
The object cannot be safely disposed within a finalizer thread context, so calling `Dispose` would be bad.
The object would have kept the present object alive if there were anything meaningful for its `Dispose` handler to do; the fact that the present object's `Finalize` method is running implies that there's no longer any need to call `Dispose` on the other object (this scenario can occur with events).
The object has already had its `Finalize` method called, so calling `Dispose` would be at best superfluous.
The object is scheduled to have its `Finalize` method called, so calling `Dispose` would likely be superfluous.
Although there are a few cases where an object might need to clean up another IDisposable object within a Finalize method, using Finalize properly in such cases is tricky, and using it improperly is apt to be worse than not using it at all. Among other things, Finalize generally only runs when an entity requests an IDisposable and wrongfully fails to call Dispose before abandoning it. It's usually better to focus one's efforts on making sure that Dispose gets properly before an object is abandoned, than on trying to properly handle buggy consumer code.
I have a question about how to use Dispose() and destructors. Reading some articles and the MSDN documentation, this seems to be the recommended way of implementing Dispose() and destructors.
But I have two questions about this implementation, that you can read below:
class Testing : IDisposable
{
bool _disposed = false;
protected virtual void Dispose(bool disposing)
{
if (!_disposed) // only dispose once!
{
if (disposing)
{
// Not in destructor, OK to reference other objects
}
// perform cleanup for this object
}
_disposed = true;
}
public void Dispose()
{
Dispose(true);
// tell the GC not to finalize
GC.SuppressFinalize(this);
}
~Testing()
{
Dispose(false);
}
}
GC.SupressFinalize(this) on Dispose()
When the programmer uses using or calls Dispose() explicity, our class is calling to GC.SupressFinalize(this). My question here is:
What this exactly means? Will the object be collected but without calling the destructor?. I guess that the anwswer is yes since destructors are converted by the framework to a Finalize() call, but I'm not sure.
Finalizing without a Dispose() call
Suppose that the GC is going to clean our object but the programmer did not call Dispose()
Why don't we dispose resource at this point? In other words, why can't we free resources on destructor?
What code must be executed in the if inside, and what outside?
if (!_disposed) // only dispose once!
{
if (disposing)
{
//What should I do here and why?
}
// And what here and why?
}
Thanks in advance
1. What does SuppressFinalize do?
It unregisters the object from the finalizer list, ie when the GC later collects the object it will ignore the presence of the destructor. This is a big gain in performance since the destructor would otherwise require the object's collection, and that of everything it references, to be delayed.
2. Why don't we dispose [managed] resource at this point? In other words, why can't we free [managed] resources on destructor?
You could, but it is certain to be pointless: the object you're in has become unreachable so all those owned managed resources are unreachable too. They will be Finalized and collected by the GC in the same run and calling Dispose() on them is unnecessary but not totally without risk or cost.
2a What code must be executed in the if inside, and what outside?
Inside the if(disposing), call _myField.Dispose()
In other words, Dispose of managed resources (objects with a Dispose)
Outside, call code to cleanup (close) unmanaged resources, like Win32API.Close(_myHandle).
Note that when you don't have unmanaged resources, as will usually be the case (look up SafeHandle), you don't need a destructor and therefore no SuppressFinalize.
And that makes that the complete (official) implementation of this pattern is only needed because of the possibility that Test is inherited from.
Note that the Dispose(bool) is protected. When you declare your class Testing to be sealed, it is totally safe and conforming to omit the ~Testing().
First part:
When GC.SupressFinalize(this) is called, GC is informed that the object has already freed its resources and it can be garbage-collected as any other object. And yes, finalization and "destructors" are the same thing in .NET.
Second part:
Finalization is done by a separate thread and we have no control on time and order of finalization, so we don't know whether any other objects are still available or are already finalized. For this reason, you can't reference other object outside the disposing block.
The vast majority of the time when an object which owns IDisposable resources is finalized, at least one of the following statements will apply to each of those resources:
It has already been finalized, in which case no cleanup is necessary.
Its finalizer hasn't yet run, but is scheduled to do so, in which case no cleanup is necessary.
It can only be cleaned up within a particular thread (which isn't the finalizer thread), in which case the finalizer thread must not try to clean it up.
It may still be in use by someone else, in which case the finalizer thread must not try to clean it up.
In some of the rare circumstances where none of the above apply, cleanup within the finalizer might be appropriate, but one shouldn't even consider it unless one has first examined the above four possibilities.