Related
I was having a discussion with a colleague recently about the value of Dispose and types that implement IDisposable.
I think there is value in implementing IDisposable for types that should clean up as soon as possible, even if there are no unmanaged resources to clean up.
My colleague thinks differently; implementing IDisposable if you don't have any unmanaged resources isn't necessary as your type will eventually be garbage collected.
My argument was that if you had an ADO.NET connection that you wanted to close as soon as possible, then implementing IDisposable and using new MyThingWithAConnection() would make sense. My colleage replied that, under the covers, an ADO.NET connection is an unmanaged resource. My reply to his reply was that everything ultimately is an unmanaged resource.
I am aware of the recommended disposable pattern where you free managed and unmanaged resources if Dispose is called but only free unmanaged resources if called via the finalizer/destructor (and blogged a while ago about how to alert consumers of improper use of your IDisposable types)
So, my question is, if you've got a type that doesn't contain unmanaged resources, is it worth implementing IDisposable?
There are different valid uses for IDisposable. A simple example is holding an open file, which you need to be closed at certain moment, as soon as you don't need it any more. Of course, you could provide a method Close, but having it in Dispose and using pattern like using (var f = new MyFile(path)) { /*process it*/ } would be more exception-safe.
A more popular example would be holding some other IDisposable resources, which usually means that you need to provide your own Dispose in order to dispose them as well.
In general, as soon as you want to have deterministic destruction of anything, you need to implement IDisposable.
The difference between my opinion and yours is that I implement IDisposable as soon as some resource needs deterministic destruction/freeing, not necessary as soon as possible. Relying on garbage collection is not an option in this case (contrary to your colleague's claim), because it happens at unpredictable moment of time, and actually may not happen at all!
The fact that any resource is unmanaged under the cover really doesn't mean anything: the developer should think in terms of "when and how is it right to dispose of this object" rather than "how does it work under the cover". The underlying implementation may change with the time anyway.
In fact, one of the main differences between C# and C++ is the absence of default deterministic destruction. The IDisposable comes to close the gap: you can order the deterministic destruction (although you cannot ensure the clients are calling it; the same way in C++ you cannot be sure that the clients call delete on the object).
Small addition: what is actually the difference between the deterministic freeing the resources and freeing them as soon as possible? Actually, those are different (though not completely orthogonal) notions.
If the resources are to be freed deterministically, this means that the client code should have a possibility to say "Now, I want this resource freed". This may be actually not the earliest possible moment when the resource may be freed: the object holding the resource might have got everything it needs from the resource, so potentially it could free the resource already. On the other hand, the object might choose to keep the (usually unmanaged) resource even after the object's Dispose ran through, cleaning it up only in finalizer (if holding the resource for too long time doesn't make any problem).
So, for freeing the resource as soon as possible, strictly speaking, Dispose is not necessary: the object may free the resource as soon as it realizes itself that the resource is not needed any more. Dispose however serves as a useful hint that the object itself is not needed any more, so perhaps the resources may be freed at that point if appropriate.
One more necessary addition: it's not only unmanaged resources that need deterministic deallocation! This seems to be one of key points of the difference in opinions among the answers to this question. One can have purely imaginative construct, which may need to be freed deterministically.
Examples are: a right to access some shared structure (think RW-lock), a huge memory chunk (imagine that you are managing some of the program's memory manually), a license for using some other program (imagine that you are not allowed to run more than X copies of some program simultaneously), etc. Here the object to be freed is not an unmanaged resource, but a right to do/to use something, which is a purely inner construct to your program logic.
Small addition: here is a small list of neat examples of [ab]using IDisposable: http://www.introtorx.com/Content/v1.0.10621.0/03_LifetimeManagement.html#IDisposable.
I think it's most helpful to think of IDisposable in terms of responsibilities. An object should implement IDisposable if it knows of something that will need to be done between the time it's no longer needed and the end of the universe (and preferably as soon as possible), and if it's the only object with both the information and impetus to do it. An object which opens a file, for example, would have a responsibility to see that the file gets closed. If the object were to simply disappear without closing the file, the file might not get closed in any reasonable timeframe.
It's important to note that even objects which only interact with 100% managed objects can do things that need to be cleaned up (and should use IDisposable). For example, an IEnumerator which attaches to a collection's "modified" event will need to detach itself when it is no longer needed. Otherwise, unless the enumerator uses some complex trickery, the enumerator will never be garbage-collected as long as the collection is in scope. If the collection is enumerated a million times, a million enumerators would get attached to its event handler.
Note that it's sometimes possible to use finalizers for cleanup in cases where, for whatever reason, an object gets abandoned without Dispose having been called first. Sometimes this works well; sometimes it works very badly. For example, even though Microsoft.VisualBasic.Collection uses a finalizer to detach enumerators from "modified" events, attempting to enumerate such an object thousands of times without an intervening Dispose or garbage-collection will cause it to get very slow--many orders of magnitude slower than the performance that would result if one used Dispose correctly.
So, my question is, if you've got a type that doesn't contain
unmanaged resources, is it worth implementing IDisposable?
When someone places an IDisposable interface on an object, this tells me that the creator intends on this either doing something in that method or, in the future they may intend to. I always call dispose in this instance just to be sure. Even if it doesn't do anything right now, it might in the future, and it sucks to get a memory leak because they updated an object, and you didn't call Dispose when you were writing code the first time.
In truth it's a judgement call. You don't want to over implement it, because at that point why bother having a garbage collector at all. Why not just manually dispose every object. If there is a possibility that you'll need to dispose unmanaged resources, then it might not be a bad idea. It all depends, if the only people using your object are the people on your team, you can always follow up with them later and say, "Hey this needs to use an unmanaged resource now. We have to go through the code and make sure we've tidied up." If you are publishing this for other organizations to use that's different. There is no easy way to tell everyone who might have implemented that object, "Hey you need to be sure this is now disposed." Let me tell you there are few things that make people madder than upgrading a third party assembly to find out that they are the ones who changed their code and made your application have run away memory problems.
My colleage replied that, under the covers, an ADO.NET connection is a
managed resource. My reply to his reply was that everything ultimately
is an unmanaged resource.
He's right, it's a managed resource right now. Will they ever change it? Who knows, but it doesn't hurt to call it. I don't try and make guesses as to what the ADO.NET team does, so if they put it in and it does nothing, that's fine. I'll still call it, because one line of code isn't going to affect my productivity.
You also run into another scenario. Let's say you return an ADO.NET connection from a method. You don't know that ADO connection is the base object or a derived type off the bat. You don't know if that IDisposable implementation has suddenly become necessary. I always call it no matter what, because tracking down memory leaks on a production server sucks when it's crashing every 4 hours.
While there are good answers to this already, I just wanted to make something explicit.
There are three cases for implementing IDisposable:
You are using unmanaged resources directly. This typically involves retrieving an IntPrt or some other form of handle from a P/Invoke call that has to be released by a different P/Invoke call
You are using other IDisposable objects and need to be responsible for their disposition
You have some other need of or use for it, including the convenience of the using block.
While I might be a bit biased, you should really read (and show your colleague) the StackOverflow Wiki on IDisposable.
Dispose should be used for any resource with a limited lifetime. A finalizer should be used for any unmanaged resource. Any unmanaged resource should have a limited lifetime, but there are plenty of managed resources (like locks) that also have limited lifetimes.
Note that unmanaged resources may well include standard CLR objects, for instance held in some static fields, all ran in safe mode with no unmanaged imports at all.
There is no simple way to tell if a given class implementing IDiposable actually needs to clean something. My rule of thumb is to always call Dispose on objects I don't know too well, like some 3rd party library.
No, it's not only for unmanaged resources.
It's suggested like a basic cleanup built-in mechanism called by framework, that enables you possibility to cleanup whatever resource you want, but it's best fit is naturally unmanaged resources management.
If you aggregate IDisposables then you should implement the interface in order that those members get cleaned up in a timely way. How else is myConn.Dispose() going to get called in the ADO.Net connection example you cite?
I don't think it's correct to say that everything is an unmanaged resource in this context though. Nor do I agree with your colleague.
You are right. Managed database connections, files, registry keys, sockets etc. all hold on to unmanaged objects. That is why they implement IDisposable. If your type owns disposable objects you should implement IDisposable and dispose them in your Dispose method. Otherwise they may stay alive until garbage collected resulting in locked files and other unexpected behavior.
everything ultimately is an unmanaged resource.
Not true. Everything except memory used by CLR objects which is managed (allocated and freed) only by the framework.
Implementing IDisposable and calling Dispose on an object that does not hold on to any unmanaged resources (directly or indirectly via dependent objects) is pointless. It does not make freeing that object deterministic because you can't directly free object's CLR memory on your own as it is always only GC that does that. Object being a reference type because value types, when used directly at a method level, are allocated/freed by stack operations.
Now, everyone claims to be right in their answers. Let me prove mine. According to documentation:
Object.Finalize Method allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.
In other words object's CLR memory is released just after Object.Finalize() is called. [note: it is possible to explicitly skip this call if needed]
Here is a disposable class with no unmanaged resources:
internal class Class1 : IDisposable
{
public Class1()
{
Console.WriteLine("Construct");
}
public void Dispose()
{
Console.WriteLine("Dispose");
}
~Class1()
{
Console.WriteLine("Destruct");
}
}
Note that destructor implicitly calls every Finalize in the inheritance chain down to Object.Finalize()
And here is the Main method of a console app:
static void Main(string[] args)
{
for (int i = 0; i < 10; i++)
{
Class1 obj = new Class1();
obj.Dispose();
}
Console.ReadKey();
}
If calling Dispose was a way to free a managed object in a deterministic way, every "Dispose" would be immediately followed by a "Destruct", right? See for yourself what happens. It is most interesting to run this app from a command line window.
Note: There is a way to force GC to collect all objects which are pending finalization in the current app domain but no for a single specific object. Nevertheless you do not need to call Dispose to have an object in the finalization queue. It is strongly discouraged to force collection as it will likely hurt overall application performance.
EDIT
There is one exception - state management. Dispose can handle state change if your object happens to manage an outside state. Even if state is not an unmanaged object it is very convenient to use it like one because of special treatment IDisposable has. Example would be a security context or impersonation context.
using (WindowsImpersonationContext context = SomeUserIdentity.Impersonate()))
{
// do something as SomeUser
}
// back to your user
It is not the best example because WindowsImpersonationContext uses system handle internally but you get the picture.
Bottom line is that when implementing IDisposable you need to have (or plan to have) something meaningful to do in the Dispose method. Otherwise it's just a waste of time. IDisposable does not change how your object is managed by GC.
Your Type should implement IDisposable if it references unmanaged resources or if it holds references to objects that implement IDisposable.
In one of my projects I had a class with managed threads inside it, we'll call them thread A, and thread B, and an IDisposable object, we'll call it C.
A used to dispose of C on exiting.
B used to use C to save exceptions.
My class had to implement IDisposable and a descrtuctor to ensure things are disposed of in the correct order.
Yes the GC could clean up my items, but my experience was there was a race condition unless I managed the clean up of my class.
Short Answer: Absolutely NOT. If your type has members that are managed or unmanaged, you should implement IDisposable.
Now details:
I've answered this question and provided much more detail on the internals of memory management and the GC on questions here on StackOverflow. Here are just a few:
Is it bad practice to depend on the .NET automated garbage collector?
What happens if I don't call Dispose on the pen object?
Dispose, when is it called?
As far as best practices on the implementation of IDisposable, please refer to my blog post:
How do you properly implement the IDisposable pattern?
Implement IDisposable if the object owns any unmanaged objects or any managed disposable objects
If an object uses unmanaged resources, it should implement IDisposable. The object that owns a disposable object should implement IDisposable to ensure that the underlying unmanaged resources are released. If the rule/convention is followed, it is therefore logical to conclude that not disposing managed disposable objects equals not freeing unmanaged resources.
Not necessary resources at all (either managed or unmanaged). Often, IDisposable is just a convenient way to elimnate combersome try {..} finally {..}, just compare:
Cursor savedCursor = Cursor.Current;
try {
Cursor.Current = Cursors.WaitCursor;
SomeLongOperation();
}
finally {
Cursor.Current = savedCursor;
}
with
using (new WaitCursor()) {
SomeLongOperation();
}
where WaitCursor is IDisposable to be suitable for using:
public sealed class WaitCursor: IDisposable {
private Cursor m_Saved;
public Boolean Disposed {
get;
private set;
}
public WaitCursor() {
Cursor m_Saved = Cursor.Current;
Cursor.Current = Cursors.WaitCursor;
}
public void Dispose() {
if (!Disposed) {
Disposed = true;
Cursor.Current = m_Saved;
}
}
}
You can easily combine such classes:
using (new WaitCursor()) {
using (new RegisterServerLongOperation("My Long DB Operation")) {
SomeLongRdbmsOperation();
}
SomeLongOperation();
}
I was having a discussion with a colleague recently about the value of Dispose and types that implement IDisposable.
I think there is value in implementing IDisposable for types that should clean up as soon as possible, even if there are no unmanaged resources to clean up.
My colleague thinks differently; implementing IDisposable if you don't have any unmanaged resources isn't necessary as your type will eventually be garbage collected.
My argument was that if you had an ADO.NET connection that you wanted to close as soon as possible, then implementing IDisposable and using new MyThingWithAConnection() would make sense. My colleage replied that, under the covers, an ADO.NET connection is an unmanaged resource. My reply to his reply was that everything ultimately is an unmanaged resource.
I am aware of the recommended disposable pattern where you free managed and unmanaged resources if Dispose is called but only free unmanaged resources if called via the finalizer/destructor (and blogged a while ago about how to alert consumers of improper use of your IDisposable types)
So, my question is, if you've got a type that doesn't contain unmanaged resources, is it worth implementing IDisposable?
There are different valid uses for IDisposable. A simple example is holding an open file, which you need to be closed at certain moment, as soon as you don't need it any more. Of course, you could provide a method Close, but having it in Dispose and using pattern like using (var f = new MyFile(path)) { /*process it*/ } would be more exception-safe.
A more popular example would be holding some other IDisposable resources, which usually means that you need to provide your own Dispose in order to dispose them as well.
In general, as soon as you want to have deterministic destruction of anything, you need to implement IDisposable.
The difference between my opinion and yours is that I implement IDisposable as soon as some resource needs deterministic destruction/freeing, not necessary as soon as possible. Relying on garbage collection is not an option in this case (contrary to your colleague's claim), because it happens at unpredictable moment of time, and actually may not happen at all!
The fact that any resource is unmanaged under the cover really doesn't mean anything: the developer should think in terms of "when and how is it right to dispose of this object" rather than "how does it work under the cover". The underlying implementation may change with the time anyway.
In fact, one of the main differences between C# and C++ is the absence of default deterministic destruction. The IDisposable comes to close the gap: you can order the deterministic destruction (although you cannot ensure the clients are calling it; the same way in C++ you cannot be sure that the clients call delete on the object).
Small addition: what is actually the difference between the deterministic freeing the resources and freeing them as soon as possible? Actually, those are different (though not completely orthogonal) notions.
If the resources are to be freed deterministically, this means that the client code should have a possibility to say "Now, I want this resource freed". This may be actually not the earliest possible moment when the resource may be freed: the object holding the resource might have got everything it needs from the resource, so potentially it could free the resource already. On the other hand, the object might choose to keep the (usually unmanaged) resource even after the object's Dispose ran through, cleaning it up only in finalizer (if holding the resource for too long time doesn't make any problem).
So, for freeing the resource as soon as possible, strictly speaking, Dispose is not necessary: the object may free the resource as soon as it realizes itself that the resource is not needed any more. Dispose however serves as a useful hint that the object itself is not needed any more, so perhaps the resources may be freed at that point if appropriate.
One more necessary addition: it's not only unmanaged resources that need deterministic deallocation! This seems to be one of key points of the difference in opinions among the answers to this question. One can have purely imaginative construct, which may need to be freed deterministically.
Examples are: a right to access some shared structure (think RW-lock), a huge memory chunk (imagine that you are managing some of the program's memory manually), a license for using some other program (imagine that you are not allowed to run more than X copies of some program simultaneously), etc. Here the object to be freed is not an unmanaged resource, but a right to do/to use something, which is a purely inner construct to your program logic.
Small addition: here is a small list of neat examples of [ab]using IDisposable: http://www.introtorx.com/Content/v1.0.10621.0/03_LifetimeManagement.html#IDisposable.
I think it's most helpful to think of IDisposable in terms of responsibilities. An object should implement IDisposable if it knows of something that will need to be done between the time it's no longer needed and the end of the universe (and preferably as soon as possible), and if it's the only object with both the information and impetus to do it. An object which opens a file, for example, would have a responsibility to see that the file gets closed. If the object were to simply disappear without closing the file, the file might not get closed in any reasonable timeframe.
It's important to note that even objects which only interact with 100% managed objects can do things that need to be cleaned up (and should use IDisposable). For example, an IEnumerator which attaches to a collection's "modified" event will need to detach itself when it is no longer needed. Otherwise, unless the enumerator uses some complex trickery, the enumerator will never be garbage-collected as long as the collection is in scope. If the collection is enumerated a million times, a million enumerators would get attached to its event handler.
Note that it's sometimes possible to use finalizers for cleanup in cases where, for whatever reason, an object gets abandoned without Dispose having been called first. Sometimes this works well; sometimes it works very badly. For example, even though Microsoft.VisualBasic.Collection uses a finalizer to detach enumerators from "modified" events, attempting to enumerate such an object thousands of times without an intervening Dispose or garbage-collection will cause it to get very slow--many orders of magnitude slower than the performance that would result if one used Dispose correctly.
So, my question is, if you've got a type that doesn't contain
unmanaged resources, is it worth implementing IDisposable?
When someone places an IDisposable interface on an object, this tells me that the creator intends on this either doing something in that method or, in the future they may intend to. I always call dispose in this instance just to be sure. Even if it doesn't do anything right now, it might in the future, and it sucks to get a memory leak because they updated an object, and you didn't call Dispose when you were writing code the first time.
In truth it's a judgement call. You don't want to over implement it, because at that point why bother having a garbage collector at all. Why not just manually dispose every object. If there is a possibility that you'll need to dispose unmanaged resources, then it might not be a bad idea. It all depends, if the only people using your object are the people on your team, you can always follow up with them later and say, "Hey this needs to use an unmanaged resource now. We have to go through the code and make sure we've tidied up." If you are publishing this for other organizations to use that's different. There is no easy way to tell everyone who might have implemented that object, "Hey you need to be sure this is now disposed." Let me tell you there are few things that make people madder than upgrading a third party assembly to find out that they are the ones who changed their code and made your application have run away memory problems.
My colleage replied that, under the covers, an ADO.NET connection is a
managed resource. My reply to his reply was that everything ultimately
is an unmanaged resource.
He's right, it's a managed resource right now. Will they ever change it? Who knows, but it doesn't hurt to call it. I don't try and make guesses as to what the ADO.NET team does, so if they put it in and it does nothing, that's fine. I'll still call it, because one line of code isn't going to affect my productivity.
You also run into another scenario. Let's say you return an ADO.NET connection from a method. You don't know that ADO connection is the base object or a derived type off the bat. You don't know if that IDisposable implementation has suddenly become necessary. I always call it no matter what, because tracking down memory leaks on a production server sucks when it's crashing every 4 hours.
While there are good answers to this already, I just wanted to make something explicit.
There are three cases for implementing IDisposable:
You are using unmanaged resources directly. This typically involves retrieving an IntPrt or some other form of handle from a P/Invoke call that has to be released by a different P/Invoke call
You are using other IDisposable objects and need to be responsible for their disposition
You have some other need of or use for it, including the convenience of the using block.
While I might be a bit biased, you should really read (and show your colleague) the StackOverflow Wiki on IDisposable.
Dispose should be used for any resource with a limited lifetime. A finalizer should be used for any unmanaged resource. Any unmanaged resource should have a limited lifetime, but there are plenty of managed resources (like locks) that also have limited lifetimes.
Note that unmanaged resources may well include standard CLR objects, for instance held in some static fields, all ran in safe mode with no unmanaged imports at all.
There is no simple way to tell if a given class implementing IDiposable actually needs to clean something. My rule of thumb is to always call Dispose on objects I don't know too well, like some 3rd party library.
No, it's not only for unmanaged resources.
It's suggested like a basic cleanup built-in mechanism called by framework, that enables you possibility to cleanup whatever resource you want, but it's best fit is naturally unmanaged resources management.
If you aggregate IDisposables then you should implement the interface in order that those members get cleaned up in a timely way. How else is myConn.Dispose() going to get called in the ADO.Net connection example you cite?
I don't think it's correct to say that everything is an unmanaged resource in this context though. Nor do I agree with your colleague.
You are right. Managed database connections, files, registry keys, sockets etc. all hold on to unmanaged objects. That is why they implement IDisposable. If your type owns disposable objects you should implement IDisposable and dispose them in your Dispose method. Otherwise they may stay alive until garbage collected resulting in locked files and other unexpected behavior.
everything ultimately is an unmanaged resource.
Not true. Everything except memory used by CLR objects which is managed (allocated and freed) only by the framework.
Implementing IDisposable and calling Dispose on an object that does not hold on to any unmanaged resources (directly or indirectly via dependent objects) is pointless. It does not make freeing that object deterministic because you can't directly free object's CLR memory on your own as it is always only GC that does that. Object being a reference type because value types, when used directly at a method level, are allocated/freed by stack operations.
Now, everyone claims to be right in their answers. Let me prove mine. According to documentation:
Object.Finalize Method allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.
In other words object's CLR memory is released just after Object.Finalize() is called. [note: it is possible to explicitly skip this call if needed]
Here is a disposable class with no unmanaged resources:
internal class Class1 : IDisposable
{
public Class1()
{
Console.WriteLine("Construct");
}
public void Dispose()
{
Console.WriteLine("Dispose");
}
~Class1()
{
Console.WriteLine("Destruct");
}
}
Note that destructor implicitly calls every Finalize in the inheritance chain down to Object.Finalize()
And here is the Main method of a console app:
static void Main(string[] args)
{
for (int i = 0; i < 10; i++)
{
Class1 obj = new Class1();
obj.Dispose();
}
Console.ReadKey();
}
If calling Dispose was a way to free a managed object in a deterministic way, every "Dispose" would be immediately followed by a "Destruct", right? See for yourself what happens. It is most interesting to run this app from a command line window.
Note: There is a way to force GC to collect all objects which are pending finalization in the current app domain but no for a single specific object. Nevertheless you do not need to call Dispose to have an object in the finalization queue. It is strongly discouraged to force collection as it will likely hurt overall application performance.
EDIT
There is one exception - state management. Dispose can handle state change if your object happens to manage an outside state. Even if state is not an unmanaged object it is very convenient to use it like one because of special treatment IDisposable has. Example would be a security context or impersonation context.
using (WindowsImpersonationContext context = SomeUserIdentity.Impersonate()))
{
// do something as SomeUser
}
// back to your user
It is not the best example because WindowsImpersonationContext uses system handle internally but you get the picture.
Bottom line is that when implementing IDisposable you need to have (or plan to have) something meaningful to do in the Dispose method. Otherwise it's just a waste of time. IDisposable does not change how your object is managed by GC.
Your Type should implement IDisposable if it references unmanaged resources or if it holds references to objects that implement IDisposable.
In one of my projects I had a class with managed threads inside it, we'll call them thread A, and thread B, and an IDisposable object, we'll call it C.
A used to dispose of C on exiting.
B used to use C to save exceptions.
My class had to implement IDisposable and a descrtuctor to ensure things are disposed of in the correct order.
Yes the GC could clean up my items, but my experience was there was a race condition unless I managed the clean up of my class.
Short Answer: Absolutely NOT. If your type has members that are managed or unmanaged, you should implement IDisposable.
Now details:
I've answered this question and provided much more detail on the internals of memory management and the GC on questions here on StackOverflow. Here are just a few:
Is it bad practice to depend on the .NET automated garbage collector?
What happens if I don't call Dispose on the pen object?
Dispose, when is it called?
As far as best practices on the implementation of IDisposable, please refer to my blog post:
How do you properly implement the IDisposable pattern?
Implement IDisposable if the object owns any unmanaged objects or any managed disposable objects
If an object uses unmanaged resources, it should implement IDisposable. The object that owns a disposable object should implement IDisposable to ensure that the underlying unmanaged resources are released. If the rule/convention is followed, it is therefore logical to conclude that not disposing managed disposable objects equals not freeing unmanaged resources.
Not necessary resources at all (either managed or unmanaged). Often, IDisposable is just a convenient way to elimnate combersome try {..} finally {..}, just compare:
Cursor savedCursor = Cursor.Current;
try {
Cursor.Current = Cursors.WaitCursor;
SomeLongOperation();
}
finally {
Cursor.Current = savedCursor;
}
with
using (new WaitCursor()) {
SomeLongOperation();
}
where WaitCursor is IDisposable to be suitable for using:
public sealed class WaitCursor: IDisposable {
private Cursor m_Saved;
public Boolean Disposed {
get;
private set;
}
public WaitCursor() {
Cursor m_Saved = Cursor.Current;
Cursor.Current = Cursors.WaitCursor;
}
public void Dispose() {
if (!Disposed) {
Disposed = true;
Cursor.Current = m_Saved;
}
}
}
You can easily combine such classes:
using (new WaitCursor()) {
using (new RegisterServerLongOperation("My Long DB Operation")) {
SomeLongRdbmsOperation();
}
SomeLongOperation();
}
I'm having issues with finalizers seemingly being called early in a C++/CLI (and C#) project I'm working on. This seems to be a very complex problem and I'm going to be mentioning a lot of different classes and types from the code. Fortunately it's open source, and you can follow along here: Pstsdk.Net (mercurial repository) I've also tried linking directly to the file browser where appropriate, so you can view the code as you read. Most of the code we deal with is in the pstsdk.mcpp folder of the repository.
The code right now is in a fairly hideous state (I'm working on that), and the current version of the code I'm working on is in the Finalization fixes (UNSTABLE!) branch. There are two changesets in that branch, and to understand my long-winded question, we'll need to deal with both. (changesets: ee6a002df36f and a12e9f5ea9fe)
For some background, this project is a C++/CLI wrapper of an unmanaged library written in C++. I am not the coordinator of the project, and there are several design decisions that I disagree with, as I'm sure many of you who look at the code will, but I digress. We wrap much of the layers of original library in the C++/CLI dll, but expose the easy-to-use API in the C# dll. This is done because the intention of the project is to convert the entire library to managed C# code.
If you're able to get the code to compile, you can use this test code to reproduce the problem.
The problem
The latest changeset, entitled moved resource management code to finalizers, to show bug, shows the original problem I was having. Every class in this code is uses the same pattern to free the unmanaged resources. Here is an example (C++/CLI):
DBContext::~DBContext()
{
this->!DBContext();
GC::SuppressFinalize(this);
}
DBContext::!DBContext()
{
if(_pst.get() != nullptr)
_pst.reset(); // _pst is a clr_scoped_ptr (managed type)
// that wraps a shared_ptr<T>.
}
This code has two benefits. First, when a class such as this is in a using statement, the resources are properly freed immediately. Secondly, if a dispose is forgotten by the user, when the GC finally decides to finalize the class, the unmanaged resources will be freed.
Here is the problem with this approach, that I simply cannot get my head around, is that occasionally, the GC will decide to finalize some of the classes that are used to enumerate over data in the file. This happens with many different PST files, and I've been able to determine it has something to do with the Finalize method being called, even though the class is still in use.
I can consistently get it to happen with this file (download)1. The finalizer that gets called early is in the NodeIdCollection class that is in DBAccessor.cpp file. If you are able to run the code that was linked to above (this project can be difficult to setup because of the dependencies on the boost library), the application would fail with an exception, because the _nodes list is set to null and the _db_ pointer was reset as a result of the finalizer running.
1) Are there any glaring problems with the enumeration code in the NodeIdCollection class that would cause the GC to finalize this class while it's still in use?
I've only been able to get the code to run properly with the workaround I've described below.
An unsightly workaround
Now, I was able to work around this problem by moving all of the resource management code from the each of the finalizers (!classname) to the destructors (~classname). This has solved the problem, though it hasn't solved my curiosity of why the classes are finalized early.
However, there is a problem with the approach, and I'll admit that it's more a problem with the design. Due to the heavy use of pointers in the code, nearly every class handles its own resources, and requires each class be disposed. This makes using the enumerations quite ugly (C#):
foreach (var msg in pst.Messages)
{
// If this using statement were removed, we would have
// memory leaks
using (msg)
{
// code here
}
}
The using statement acting on the item in the collection just screams wrong to me, however, with the approach it's very necessary to prevent any memory leaks. Without it, the dispose never gets called and the memory is never freed, even if the dispose method on the pst class is called.
I have every intention trying to change this design. The fundamental problem when this code was first being written, besides the fact that I knew little to nothing about C++/CLI, was that I couldn't put a native class inside of a managed one. I feel it might be possible to use scoped pointers that will free the memory automatically when the class is no longer in use, but I can't be sure if that's a valid way to go about this or if it would even work. So, my second question is:
2) What would be the best way to handle the unmanaged resources in the managed classes in a painless way?
To elaborate, could I replace a native pointer with the clr_scoped_ptr wrapper that was just recently added to the code (clr_scoped_ptr.h from this stackexchange question). Or would I need to wrap the native pointer in something like a scoped_ptr<T> or smart_ptr<T>?
Thank you for reading all of this, I know it was a lot. I hope I've been clear enough so that I might get some insight from people a little more experienced than I am. It's such a large question, I intend on adding a bounty when it allows me too. Hopefully, someone can help.
Thanks!
1This file is part of the freely available enron dataset of PST files
The clr_scoped_ptr is mine, and comes from here.
If it has any errors, please let me know.
Even if my code isn't perfect, using a smart pointer is the correct way to deal with this issue, even in managed code.
You do not need to (and should not) reset a clr_scoped_ptr in your finalizer. Each clr_scoped_ptr will itself be finalized by the runtime.
When using smart pointers, you do not need to write your own destructor or finalizer. The compiler-generated destructor will automatically call destructors on all subobjects, and every subobject finalizer will run when it is collected.
Looking closer at your code, there is indeed an error in NodeIdCollection. GetEnumerator() must return a different enumerator object each time it is called, so that each enumeration would begin at the start of the sequence. You're reusing a single enumerator, meaning that position is shared between successive calls to GetEnumerator(). That's bad.
Refreshing my memory of destructors/finalalisers, from some Microsoft documentation, you could at least simplify your code a little, I think.
Here's my version of your sequence:
DBContext::~DBContext()
{
this->!DBContext();
}
DBContext::!DBContext()
{
delete _pst;
_pst = NULL;
}
The "GC::SupressFinalize" is automatically done by C++/CLI, so no need for that. Since the _pst variable is initialised in the constructor (and deleting a null variable causes no problems anyway), I can't see any reason to complicate the code by using smart pointers.
On a debugging note, I wonder if you can help make the problem more apparent by sprinkling in a few calls to "GC::Collect". That should force finalization on dangling objects for you.
Hope this helps a little,
I'm not quite understanding why there are finalizers in languages such as java and c#. AFAIK, they:
are not guaranteed to run (in java)
if they do run, they may run an arbitrary amount of time after the object in question becomes a candidate for finalization
and (at least in java), they incur an amazingly huge performance hit to even stick on a class.
So why were they added at all? I asked a friend, and he mumbled something about "you want to have every possible chance to clean up things like DB connections", but this strikes me as a bad practice. Why should you rely on something with the above described properties for anything, even as a last line of defense? Especially when, if something similar was designed into any API, said API would get laughed out of existence.
Well, they are incredibly useful, in certain situations.
In the .NET CLR, for example:
are not guaranteed to run
The finalizer will always, eventually, run, if the program isn't killed. It's just not deterministic as to when it will run.
if they do run, they may run an arbitrary amount of time after the object in question becomes a candidate for finalization
This is true, however, they still run.
In .NET, this is very, very useful. It's quite common in .NET to wrap native, non-.NET resources into a .NET class. By implementing a finalizer, you can guarantee that the native resources are cleaned up correctly. Without this, the user would be forced to call a method to perform the cleanup, which dramatically reduces the effectiveness of the garbage collector.
It's not always easy to know exactly when to release your (native) resources- by implementing a finalizer, you can guarantee that they will get cleaned up correctly, even if your class is used in a less-than-perfect manner.
and (at least in java), they incur an amazingly huge performance hit to even stick on a class
Again, the .NET CLR's GC has an advantage here. If you implement the proper interface (IDisposable), AND if the developer implements it correctly, you can prevent the expensive portion of finalization from occuring. The way this is done is that the user-defined method to do the cleanup can call GC.SuppressFinalize, which bypasses the finalizer.
This gives you the best of both worlds - you can implement a finalizer, and IDisposable. If your user disposes of your object correctly, the finalizer has no impact. If they don't, the finalizer (eventually) runs and cleans up your unmanaged resources, but you run into a (small) performance loss as it runs.
Hmya, you are getting a picture painted here that's a bit too rosy. Finalizers are not guaranteed to run in .NET either. Typical mishaps are a finalizer that throws an exception or a time-out on the finalizer thread (2 seconds).
That was a problem when Microsoft decided to provide .NET hosting support in SQL Server. The kind of application where restarting the app to solve resource leaks isn't considered a viable workaround. .NET 2.0 acquired critical finalizers, enabled by deriving from the CriticalFinalizerObject class. The finalizer of such a class must adhere to the rulez of constrained execution regions (CERs), essentially a region of code where exceptions are suppressed. The kind of things you can do in a CER are very limited.
Back to your original question, finalizers are necessary to release operating system resources other than memory. The garbage collector manages memory very well but doesn't do anything to release pens, brushes, files, sockets, windows, pipes, etc. When an object uses such a resource, it must make sure to release the resource after it is done with it. Finalizers ensure that happens, even when the program forgot to do so. You almost never write a class with a finalizer yourself, operating resources are wrapped by classes in the framework.
The .NET framework also has a programming pattern to ensure such a resource is released early so the resource doesn't linger around until the finalizer runs. All classes that have finalizers also implement the IDisposable.Dispose() method, allowing your code to release a resource explicitly. This is often forgotten by a .NET programmer but that doesn't typically cause problems because the finalizer ensures it will eventually be done. Many .NET programmers have lost hours of sleep worrying whether or not all Dispose() calls are taken care of and massive numbers of threads have been started about it on forums. Java folks must be a happier lot.
Following up on your comment: exceptions and timeouts in the finalizer thread is something that you don't have to worry about. Firstly, if you find yourself writing a finalizer, take a deep breath and ask yourself if you're on the Right Path. Finalizers are for framework classes, you should be using such a class to use an operating resource, you'll get the finalizer built into that class for free. All the way down to the SafeHandle classes, they have a critical finalizer.
Secondly, finalizer thread failures are gross program failures. Similar to getting an OutOfMemory exception or tripping over the power cord and unplugging the machine. There isn't anything you can do about them, other than fixing the bug in your code or re-route the cable. It was important for Microsoft to design critical finalizers, they can't rely on all programmers that write .NET code for SQL Server to get that code right. If you fumble a finalizer yourself then there is no such liability, it will be you that gets the call from the customer, not Microsoft.
In java finalizers exist to allow for the clean up of external resources (things that exist outside of the JVM and can't be garbage collected when the 'parent' java object is). This has always been rare. On example might be if you are interfacing with some custom hardware.
I think the reason that finalizers in java aren't guaranteed to run is that they might not have a chance to do so at program termination.
One thing you might do with a finalizer in 'pure' java is use it to test termination conditions- for example to check that all connections are closed and report an error if they are not. You aren't guaranteed that the error will be always caught but it will likely be caught at least some of the time which is enough to reveal a bug.
Most java code has no call for finalizers.
If you read the JavaDoc for finalize() it says it is "Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. A subclass overrides the finalize method to dispose of system resources or to perform other cleanup."
http://java.sun.com/javase/6/docs/api/java/lang/Object.html#finalize
So that's the "why". I guess you can argue whether their implementation is effective.
The best use I've found for finalize() is to detect bugs with freeing pooled resources. Most leaked objects will get garbage collected eventually and you can generate debug information.
class MyResource {
private Throwable allocatorStack;
public MyResource() {
allocatorStack = new RuntimeException("trace to allocator");
}
#Override
protected void finalize() throws Throwable {
try {
System.out.println("Bug!");
allocatorStack.printStackTrace();
} finally {
super.finalize();
}
}
}
They're meant for freeing up native resources (e.g. sockets, open files, devices) that can't be released until all references to the object have been broken, which is something that a particular caller would (in general) have no way of knowing. The alternative would be subtle, impossible-to-trace resource leaks...
Of course, in many cases as the application author you'll know that there's only one reference to the DB connection (for example); in which case finalizers are no substitute for closing it properly when you know you're finished with it.
In .Net land, t is not guaranteed when they run. But they will run.
Are you refering to Object.Finalize?
According to msdn, "In C# code, Object.Finalize cannot be called or overridden". In fact, they recommend using the Dispose method because it is more controllable.
There's an additional complication with finalizers in .NET. If the class has a finalizer and does not get Dispose()'d, or Dispose() does not suppress the finalizer, the garbage collector will defer collecting until after compacting generation 2 memory (the last generation), so the object is "sort of" but not quite a memory leak. (Yes, it will get cleaned up eventually, but quite possibly not until application termination.)
As others have mentioned, if an object holds non-managed resources, it should implement the IDisposable pattern. Developers should be aware that if an object implements IDisposable, then it's Dispose() method should always be called. C# provides a way to automate this with the using statement:
using (myDataContext myDC = new myDataContext())
{
// code using the data context here
}
The using block automatically calls Dispose() on block exit, even exits by return or exceptions being thrown. The using statement only works with objects that implement IDisposable.
And beware another confusion point; Dispose() is an opportunity for an object to release resources, but it does not actually release the Dispose()'d object. .NET objects are elligible for garbage collection when there are no active references to them. Technically, they can't be reached by any chain of object references, starting from the AppDomain.
The equivalent ofdestructor() in C++ is finalizer() in Java.
They are invoked when the life cycle of an object is about to end.
I'm trying to make sure that my understanding of IDisposable is correct and there's something I'm still not quite sure on.
IDisposable seems to serve two purpose.
To provide a convention to "shut down" a managed object on demand.
To provide a convention to free "unmanaged resources" held by a managed object.
My confusion comes from identifying which scenarios have "unmanaged resources" in play.
Say you are using a Microsoft-supplied IDisposable-implementing (managed) class (say, database or socket-related).
How do you know whether it is implementing IDisposable for just 1 or 1&2 above?
Are you responsible for making sure that unmanaged resources it may or may not hold internally are freed? Should you be adding a finalizer (would that be the right mechanism?) to your own class that calls instanceOfMsSuppliedClass.Dispose()?
How do you know whether it is implementing IDisposable for just 1 or
1&2 above?
The answer to your first question is "you shouldn't need to know". If you're using third party code, then you are its mercy to some point - you'd have to trust that it's disposing of itself properly when you call Dispose on it. If you're unsure or you think there's a bug, you could always try using Reflector() to disassemble it (if possible) and check out what it's doing.
Am I responsible for making sure that unmanaged resources it may or may
not hold internally are freed? Should
I be adding a finalizer (would that be
the right mechanism?) to my own class
that calls
instanceOfMsSuppliedClass.Dispose()?
You should rarely, if ever, need to implement a finalizer for your classes if you're using .Net 2.0 or above. Finalizers add overhead to your class, and usually provide no more functionality then you'd need with just implementing Dispose. I would highly recommend visiting this article for a good overview on disposing properly. In your case, you would want to call instanceofMSSuppliedClass.Dispose() in your own Dispose() method.
Ultimately, calling Dispose() on an object is good practice, because it explictly lets the GC know that you're done with the resource and allows the user the ability to clean it up immediately, and indirectly documents the code by letting other programmers know that the object is done with the resources at that point. But even if you forget to call it explicitly, it will happen eventually when the object is unrooted (.Net is a managed platform after all). Finalizers should only be implemented if your object has unmanaged resources that will need implicit cleanup (i.e. there is a chance the consumer can forget to clean it up, and that this will be problematic).
You should always call Dispose on objects that implement IDisposable (unless they specifically tell you' it's a helpful convention, like ASP.NET MVC's HtmlHelper.BeginForm). You can use the "using" statement to make this easy. If you hang on to a reference of an IDisposable in your class as a member field then you should implement IDisposable using the Disposable Pattern to clean up those members. If you run a static-analysis tool like FxCop it will tell you the same.
You shouldn't be trying to second-guess the interface. Today that class might not use an unmanaged resource but what about the next version?
You're not responsible for the contents of an object. Dispose() should be transparent, and free what it needs to free. After that, you're not responsible for it.
Unmanaged resources are resources like you would create in (managed) C++, where you allocate memory through pointers and "new" statements, rather than "gcnew" statements. When you're creating a class in C++, you're responsible for deleting this memory, since it is native memory, or unmanaged, and the garbage collector does not about it. You can also create this unmanaged memory through Marshal allocations, and, i'd assume, unsafe code.
When using Managed C++, you don't have to manually implement the IDisposable class either. When you write your deconstructor, it will be compiled to a Dispose() function.
If the class in question is Microsoft-supplied (ie.. database, etc..) then the handling of Dispose (from IDisposable) will most likely already be taken care of, it is just up to you to call it. For instance, standard practice using a database would look like:
//...
using (IDataReader dataRead = new DataReaderObject())
{
//Call database
}
This is essentially the same as writing:
IDataReader dataRead = null;
try
{
dataRead = new DataReaderObject()
//Call database
}
finally
{
if(dataRead != null)
{
dataRead.Dispose();
}
}
From what I understand, it is generally good practice for you use the former on objects that inherit from IDisposable since it will ensure proper freeing of resources.
As for using IDisposable yourself, the implementation is up to you. Once you inherit from it you should ensure the method contains the code needed to dispose of any DB connections you may have manually created, freeing of resources that may remain or prevent the object from being destroyed, or just cleaning up large resource pools (like images). This also includes unmanaged resources, for instance, code marked inside an "unsafe" block is essentially unmanaged code that can allow for direct memory manipulation, something that definitely needs cleaning up.
The term "unmanaged resource" is something of a misnomer. The essential concept is that of a paired action--performing one action creates a need to perform some cleanup action. Opening a file creates a need to close it. Dialing a modem creates a need to hang up. A system may survive a failure to perform a cleanup operation, but the consequences may be severe.
When an object is said to "hold unmanaged resources", what is really meant is that the object has the information and impetus necessary to perform some required cleanup operation on some other entity, and there's no particular reason to believe that information and impetus exists anywhere else. If the only object with such information and impetus is completely abandoned, the required cleanup operation will never occur. The purpose of .Dispose is to force an object to perform any required cleanup, so that it may be safely abandoned.
To provide some protection against code which abandons objects without first calling Dispose, the system allows classes to register for "finalization". If an object of a registered class is abandoned, the system will give the object a chance to perform cleanup operation on other entities before it is abandoned for good. There's no guarantee as to how quickly the system will notice that an object has been abandoned, however, and various circumstances may prevent an object from being offered its chance to clean up. The term "managed resource" is sometimes used to refer to an object which will have to perform some cleanup before it's abandoned, but which will automatically register and attempt to perform such cleanup if someone fails to call Dispose.
Why should it matter to you?
When possible I wrap the scope of a disposable object in a using. This calls dispose at the end of the using. When not I explicitly call dispose whenever I no longer need the object.
Whether for reason 1 or reason 2 is not necessary.
Yes, you're responsible to call the Dispose method - or better use using statement. If an object is implementing IDisposable, you should always dispose it, no matter what.
using (var myObj = new Whatever())
{
// ..
}
is similar to
{
var myObj;
try
{
myObj = new Whatever();
// ..
}
finally
{
if (myObj != null)
{
((IDisposable)myObj).Dispose();
}
}
} // object scope ends here
EDIT: Added try/finally thanks to Talljoe - wow, that's complicated to get it right :)
EDIT2: I am not saying that you should use the second variant. I just wanted to show that "using" is nice syntactic sugar for a bunch of code that can get quite messy and hard to get right.
One piece that is missing here is the finalizer - my habit has been if I implement IDisposable I also have a finalizer to call Dispose() just in case my client doesn't. Yes, it adds overhead but if Dispose() IS called than the GC.SuppressFinalize(this) call eliminates it.