Do you have to release managed resources in a Dispose method? - c#

Following this MSDN resource, the code
using System;
class BaseClass : IDisposable
{
// Flag: Has Dispose already been called?
bool disposed = false;
// Public implementation of Dispose pattern callable by consumers.
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
// Protected implementation of Dispose pattern.
protected virtual void Dispose(bool disposing)
{
if (disposed)
return;
if (disposing) {
// Free any other managed objects here.
//
}
// Free any unmanaged objects here.
//
disposed = true;
}
~BaseClass()
{
Dispose(false);
}
}
has a comment about freeing managed resources/objects.
As far as I know, the garbage collector will do that, but is it good practice to actually write such code there, and by such I mean pointing the objects's references to null?
I believe that's the best you can do at "disposing" of a managed resource, although you don't achieve anything in the process, as the garbage collector will do the actual heavy lifting, not you setting their references to null. (The garbage collector will notice that the class itself holding these objects/resources is not used (no references), making this task meaningless)
Thanks!

Note that disposing an object does not frees the allocated memory. It just a way to indicate the end of use of unmanaged resources that your object may be owning.
For example, if your object owns an unmanaged resource like for example a TCP socket, you can just let the garbage collector end everything when it runs, but the socket will be potentially alive until that happen, leading to a situation in which you have many open connections at a given time and the server is rejecting new ones.
This happens when you have a SqlConnection. When you call Dispose, it actually does not close the socket to SqlServer, but returns the connection to a pool where it can be reused. If you do not dispose SqlConnection, on a concurrent system there will be a moment where those objects are candidates to be garbage-collected, but because they are still there holding a connection, at some point you will have too many connections to SqlServer and it may start rejecting the new ones.
Similar thing with a SqlTransaction. If an exception is thrown before you call Commit or Rollback and Dispose is never called, the connection will potentially hold locks on tables, pages, etc... causing performance degradation in other connections.
Even if you call Dispose, a SqlConnection object is still in memory, waiting to be collected, but unusable because the actual socket was returned to a pool. In fact if you Dispose and call any of its methods, it will throw an ObjectDisposedException, indicating that the object is unusable anymore.
And object that implements IDisposable, either owns unamanged resources, or owns other objects that owns unamanged resources. So if you write a repository class that owns a SqlConnection, you will need to implement a cascade Dispose call.

From the same page:
The body of the method consists of two blocks of code:
A block that frees unmanaged resources. This block executes regardless of the value of the disposing parameter.
A conditional block that frees managed resources. This block executes if the value of disposing is true. The managed resources that it frees can include:
Managed objects that implement IDisposable. The conditional block can
be used to call their Dispose implementation. If you have used a safe
handle to wrap your unmanaged resource, you should call the
SafeHandle.Dispose(Boolean) implementation here.
Managed objects that consume large amounts of memory or consume scarce
resources. Freeing these objects explicitly in the Dispose method
releases them faster than if they were reclaimed non-deterministically
by the garbage collector.
So certainly you should free any managed objects that are also disposable, but you could also release any managed objects that use lots of memory or scarce resources.

Related

What should go in this Dispose() method? [duplicate]

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)
}
}

Where to clean up unmanaged resource in dispose pattern?

I'm confused about this pattern.
If disposing is true(called from Dispose()) we free managed objects there.
If disposing is false(called from Finalizer()) it is not safe to access referenced object, these referenced objects are the unmanaged objects like filestream.
If disposing is true then we won't be able to free unmanaged objects?
If disposing is false we won't be able to free managed objects?
Searching this pattern, there has different implementation in Dispose(boolean) method. MSDN implementation
And this one I got from examples and tutorials.
protected virtual void Dispose(bool disposing)
{
if (!disposed)
{
if (disposing)
{
// Free any other managed objects here.
}
else
{
//not safe to access referenced object
}
// Free any unmanaged objects here.
}
disposed = true;
}
In properly formed IDisposable pattern true value of disposing implies that we reached the method from the explicit Dispose call or by leaving using scope which is preferable way of IDisposable consumption. However not all consumers are properly implemented and in order to ensure that we are able to reclaim all unmanaged resources in the IDisposable implementation we call Dispose(false) from the finalizer. So false in the Dispose indicates that we reached this call at the finalization phase and some references can be unavailable at this stage that's why we use this flag - to distinguish what we can do under normal flow execution and what we can do under the finalization stage conditions. Unmanaged resources we deallocate regardless of this flag.
the clean up of unmanged resources must be done inside the method Dispose(boolean disposing) regardless of the value of the parameter disposing. You can find more details here

Is it appropriate to call GC.Collect in a finalizer?

We have a BitmapImageclass that wraps a native image id and System.Drawing.Bitmap and implements IDisposable. The image id is created as part of a third-party imaging SDK we use that uses integers to identity an "image" pointing to some pixel data. Both the image id and bitmap point to the same underlying pixel data.
The SDK is very difficult to use correctly in many cases, so we have a facade layer that abstracts away the SDK and provides an easy to use and error-less API, for TIF and PDF documents, and part of this is to guarantee that memory is freed as soon as possible. 300 DPI multi-page images with hundreds of pages are common place, so memory can easily be high in the application.
We are currently calling GC.Collect in the Dispose method to free up the memory immediately. After thorough testing of the software, this was the only way to free up large amounts of memory immediately after releasing the underlying pixel data, especially during a large merge operation where we might be merging several hundreds of pages together in a document. It is also implemented this way so developers do not incorrectly try to scatter their code with GC.Collect, because they should not really be calling it anyway.
My question is two parts:
When a finalizer is called by the garbage collector, does it also free the memory immediately, or could there be a long period of time before this occurs? Should we call GC.Collect here as well? Especially in only a 32-bit process, we must ensure we are keeping as much free memory as possible.
The SDK we use is GdPicture, and even when you Dispose of their graphics object, it does not dispose of the pixel data or image references. It leaves them around until a developer frees them. We need to guaruntee that if a developer does not call Dispose manually, that the resources are freed. Is it appropriate to reference a managed class in a finalizer, such as GraphicsObject.ReleaseImage(id)? I read in some places that you should not call methods other than some of the static ones from things like SafeHandle, but unless we call this the memory will not be freed
I think there's a bit mess. Want to make it more clear in common understanding.
You can't manage time for GCollection.
GC-n occurs when: System has low physical memory + Memory that is used by allocated objects on the managed heap surpasses an acceptable threshold. This threshold is continuously adjusted as the process runs. + GC.Collect method is called. In almost all cases, you don’t have to call this method, because GC runs continuously. GC.Collect method is primarily used for unique situations and testing.
GC optimizing engine determines best time to perform a collection, based upon allocations being made.
Exact time when Finalizer executes is undefined. To ensure deterministic release of resources for instances of class, implement a Close() method or provide an IDisposable.Dispose implementation + The finalizers of two objects are not guaranteed to run in any specific order + Thread on which the finalizer runs is unspecified. For case someone forget to Dispose use Destructor
~ YourClass(){Dispose(false);}
is transformed into:
protected override void Finalize() { try { … cleanup…} finally { base.Finalize(); } }
Don't forget to
YourClass : IDisposable
It's ("Finalizer") used to perform cleanup operations on unmanaged resources held by current object before object is destroyed by GC. Method is protected and therefore is accessible only through this class or through a derived class. By default (when use ~), GC automatically calls an object's finalizer before reclaiming its memory.
GC adds an entry for each instance of the type to an internal structure finalization queue - an internal data structure (Queue) controlled by GC. Finalization queue contains entries for all objects in managed heap whose finalization code must run before GC can reclaim their memory. When GC finds objects to be garbage, it scans Finalization Queue looking for pointers to these objects. If a pointer is found, it is removed from Finalization Queue and added to Freachable Queue. Object is no longer considered garbage and its memory is not reclaimed. At this point, GC has finished identifying garbage. Special runtime thread empties the Freachable Queue, executing each object's Finalize method. Normally when Freachable Queue is empty, this thread sleeps.
You need minimum 2 GCollections for objects with destructor (That's why in Dispose pattern GC.SuppressFinilise(this) is used - you say to GC that don't move object to Generation_1 or Generation_2, that memory might be reclaimed as GC meet destructor) + (that also means that objects with finalizers are moved minimum to Generation_1, that GC checks about 10 times rarely (than Generaation_0) that means that objects you don't need stay in memory longer)
Suggest to apply or
using(...)
that transforms into
try { … } finally { XX.Dispose();}
or implement correctly IDisposable interface using IntPtr structure for unmanaged resources (it works with different architecture (32 or 64)). Do not recommend to use SafeHandle (Interopservices) because it works only with Win32 architecture (good in SafeHandle is that you don't need to implement IDisposable as it inherits from CriticalFinalizerObject)
Classic implementation of IDisposable when you don't need to care if someone forget to Dispose object (for that case you have ~ destructor):
For base class
public MyClass() { this.reader = new TextReader(); }
public class A : IDisposable
{
bool disposed = false; // Flag: Has Dispose already been called?
private Component handle;
private IntPtr umResource;
public void Dispose() {
Dispose(true); GC.SuppressFinalize(this); }
protected virtual void Dispose(bool disposing) {
if (disposed) return;
if (disposing) { // Free managed resources
if (reader != null) reader.Dispose();
if (handle != null) handle.Close(); }
if (umResource != IntPtr.Zero) { // Free unmanaged resources
umResource = IntPtr.Zero; }
disposed = true; }
~A() { Dispose(false); } } // Only if we have real unmanaged resources
For derived class
public class B : A {
private bool disposed = false;
protected override void Dispose(bool disposing) {
if (disposed) return;
if (disposing) { ... } // To free any managed objects
// Free unmanaged resources
disposed = true;
base.Dispose(disposing)} }
disposing false – call comes from Finalizer; disposing true – from Dispose (from you)
Sorry, forgot to RESUME:
** Use GC.Collect to force the system TO TRY to reclaim the maximum amount of available memory. You can also point out to GC which generations you want it to check. BUT AS I WROTE IN THE BEGINING - YOU HAVE NO CONTROL, even if you use GC.Collect - that doesn't guarantee anything (GC may override your call, etc), but you can try and it wouldn't bring any problems **
As a general rule, you should not have GC.Collect in productive code. It can help for debugging (if you want to see if a memory spike is temporary). But generally it is better to leave the GC to it's own devices. It is about as optimized as it can be. Even when it is not collecting, it is doing something effective (waiting if maybe application closure will make it's job a lot simpler). If you want to affect the major behavior, change teh GC strategy for the application instead.
The 2nd part of your question is about the Dipose/Finalize pattern. There are two cases:
You only handle stuff that implements IDisposeable. In this case all you need to do is Implement Dispose. And all it needs to do is to relay the "Dispose" order to anything Disposeable it contains.
You actually handle Unamaged resources. In this case you should first implement the Finalizer (so at least the GC can clean this up at applicatin closure/collection). Then you provide the Dispose function as additional thing. But I would always put the Finalizer first, because it will be called with guarantee.
Finalizer and Dispose are largely indentical. So identical it is customary to just write the Dispose function with a boolean switch. The only things in wich Finalizer and Dispose actually differ is the "relay" part:
- If applicable you always relay a Dispose call to contained instances. Because that is the common usecase for Dispose (relaying teh call of Dispose()). It is better if we all adhere to it.
- You never relay a fianlizer call to contained isntances. Finalization is between that instance and the GC.
About 95% of the cases you only need dispose.
Calling GC.Collect in the Finalizer makes no sense. The finalizer is about cleaning up unmanaged resources. The ones the GC does not govern.
GC Collect is just "do the collection of managed resources now". They apply pretty much to opposite cases (managed vs unamanged resources).

Why should Dispose() dispose managed resources and finalizer not?

We all know the System.IDisposable pattern. It's been described a zillion time, also here on StackOverflow:
link: Dispose() for cleaning up managed resources?
The Disposable patterns advises that I should only dispose managed resources if my object is being disposed, not during finalize
You can see that this happens because the following code is advised:
protected void Dispose(bool disposing)
{
if (disposing)
{
// Code to dispose the managed resources of the class
}
// Code to dispose the un-managed resources of the class
}
I know that my class should implement System.IDisposable whenever my class has a (private) member that implements System.IDisposable. The Dispose(bool) should call the Dispose() of the private member if the boolean disposing is true.
Why would it be a problem if the Dispose would be called during a finalize? So why would the following Dispose be a problem if it is called during finalize?
protected void Dispose(bool disposing)
{
if (myDisposableObject != null)
{
myDisposableObject.Dispose();
myDisposableObject = null;
}
}
When an object's finalizer runs, one of the following will be true of almost any IDisposable object it might hold:
It held the only reference to that other object, and that other object's finalizer has already run, so there's no need to do anything with it.
It held the only reference to that other object, and that other object's finalizer is scheduled to run, even though it hasn't yet, and there's no need to do anything with it.
Other objects are still using the IDisposable object, in which case the finalizer must not call Dispose.
The other object's Dispose method cannot be safely run from a finalizer threading context (or, more generally, any threading context other than the one where the object was created), in which case the finalizer must not call Dispose.
Even in the cases where none of the above would apply, code which knows that will probably know a lot about its cleanup requirements beyond the fact that it implements IDisposable, and should often use a more detailed cleanup protocol.
Generally speaking, you should get rid of resources ASAP. If you don't need a resource why would you keep it around to waste memory?
Also, in order for Dispose() to be called during finalization, you would have to create a finalizer for your object, that is - a destructor in C#. However, the exact time of invoking an object's finalizer is non-deterministic, meaning that your managed objects might not be around / accessible safely anymore at that moment. Even the thread that executes your finalizer is non-deterministic which could also result in problems that are difficult to foresee.
For these reasons finalizers should be created to release unmanaged resources only.
Very few programmers understand 'fully' how finalization works. For instance, it is the garbage collector that recognizes that your type has a finalizer during object instantiation and puts your instance in a special internal data structure called finalizequeue. Indeed when you debug your application with sos.dll (windbg) you can use the command !finalizeQueue to display the objects that have finalizers and are yet to be finalized at some later point in time.

Two questions about Dispose() and destructors in C#

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.

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