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IDisposable for convenience? [duplicate]

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();
}

are destructors necessary in C#?

I have a concern. I'm a first year student of computer science. Normally I'm very inquisitive in class but, not always my teacher has an answer, or not always knows the answer. Are destructors necessary in C#? What I mean is if I have to implement a destructor method as I normally do with constructors, is it a good practice or i can avoid it and the garbage collector will do it for me?

Destructors (or finalizers) are good to have in the language - but you should almost never use them. Basically you should only need them if you have a direct handle on an unmanaged resource, and not only is that incredibly rare, but using SafeHandle as a tiny level of indirection is a better idea anyway (which handles clean-up for you). See Joe Duffy's blog post on the topic for more details.
For what it's worth, I can't remember the last time I wrote a finalizer other than to test some odd behaviour or other.
For the vast majority of the time, life is simpler:
The garbage collector can handle memory resource cleanup
If you use an unmanaged resource (e.g. a file) locally within a method, use a using statement to make sure you release it when you're done with it
If you need a reference to an unmanaged resource (or anything else which implements IDisposable) as an instance variable within your type, your type should itself implement IDisposable. (I try to avoid this where possible. Even when it is necessary, you can make life simpler by making your class sealed, at which point you at least don't need to worry about other subclasses having even more unmanaged state to clean up.)

No destructors are not neccesary in C#. The reason why that's true is that in C# the memory is managed automatically and you haven't to do anything except from creating an object. When the garbage collector verifies that an object is not referred anywhere else in your application, then it reclaims its memory, without having you declared any destructor for this object, like we do on C++ for instance.

Nothing is unnecessary in any language. They serve their purpose.
Destructors will destruct the object and you'll end up with object resurrection.(If you try to access destructed object, you might get an error)
GC will automatically do this for you when the object has no longer any references to it. So there's is no need for you to do this explicitly..
Also, implementing IDisposable should be given preference over destructor.

Should "Dispose" only be used for types containing unmanaged resources?

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();
}

Dispose question

I have a number of classes which have private member variables that implement IDisposable (timers, brushes, etc). Do I need to do anything to ensure these variables are cleaned up properly by the .NET Framework?
The literature I've come across is referring to "managed resources" vs. "unmanaged resources". These terms are confusing to me because you can have a managed class which implements functionality using unmanaged resources. Is that considered an "unmanaged resource" or "managed resource" ?
My understanding is if you aren't calling Dispose() on an object that implements IDisposable, then the resources aren't being freed until the application exits. This situation could cause OutOfMemory exceptions when running the program for a long period of time.
How can I be sure my code is handling resource management correctly? It's important for these objects because they are custom controls and there may be a lot of drawing which consumes IDisposable resources. I use the C# using statement whenever I can, but sometimes I need to make an object implementing IDisposable a member variable, and the using statement won't help me there.

Three simple rules.
A managed resource is anything implementing IDisposable. An unmanaged resource is something like a HANDLE that you got via p/Invoke. A class like SafeHandle (or one derived from SafeHandle) owns an unmanaged resource, but it is considered a managed resource itself. So any class that owns unmanaged resource is itself a managed resource.
Since you have a class owning managed resources, follow Rule 2: implement IDisposable (but not a finalizer).
IDisposable allows for earlier cleanup. If you don't call it, the resources will be cleaned up anyway (they won't hang around until process exit); they'll just be cleaned up later, and you don't have a choice about when they get cleaned up.

Yes - if your class "contains" an IDisposable, that class should almost certainly implement IDisposable too.
"Managed" resources are basically memory. "Unmanaged" resources can be file handles, network connections, handles to graphics objects etc. In most cases types which have direct access to native handles have finalizers, so the resource will be released at some point, but it's still better to release it explicitly - in some cases (such as with HttpWebResponse) there can be a limited number of such resources available (connections in a connection pool to a single host in this case) and you can end up timing out waiting for a "dead" resource to be freed.
Where possible, it's nicer not to have such class members in the first place - have them as method parameters of local variables etc, so you can use them and then close them without tying the lifetime of the resource to the lifetime of your object. However, in some cases that's not appropriate - in which case you should implement IDisposable.

Very comprehensive IDisposable guidelines are here.
Do transitively dispose of any disposable fields defined in your type
from your Dispose method.
You should call Dispose() on any fields
whose lifecycle your object controls. For example, consider a case
where your object owns a private TextReader field. In your type's
Dispose, you should call the TextReader object's Dispose, which will
in turn dispose of its disposable fields (Stream and Encoding, for
example), and so on. If implemented inside a Dispose(bool disposing)
method, this should only occur if the disposing parameter is
true—touching other managed objects is not allowed during
finalization. Additionally, if your object doesn’t own a given
disposable object, it should not attempt to dispose of it, as other
code could still rely on it being active. Both of these could lead to
subtle-to-detect bugs.
Do implement the dispose pattern when your type is unsealed and
contains resources that explicitly need to be or can be freed, for
example raw handles, or other unmanaged resources.
This pattern
provides a standardized means for developers to deterministically
destroy or free resources owned by an object. It also aids subclasses
to correctly release base class resources.
'Unmanaged resource' usually refers to cases where your code refernces native handles directly (file handles, connections, sockets etc). In this case you would also have to implement finalizer or use SafeHandle. Most of the time you reference native handles indirectly, through .NET classes like TextReader. In this case you can simply use 'using' or, if you are writing library, implement IDisposable transitively.

If your class has member variables that implement IDisposable, then your class should implement it as well. You clean up what you own.

You have some good information and some misinformation in your understanding.
The long and short of it is that you need to Dispose() anything that implements IDisposable.
Being as these are private member variables of your class, and if these are supposed to be available for the lifetime of instances of that class, your class should also implement IDisposable and Dispose() of those types in its own Dispose() method.
If those private member variables have a limited lifetime (i.e. only within one method), just wrap them in a using block.

1) You can use a Memory Profiler Tool, there are plenty around the web, the best i know being Reg Gate's ANTS Profiler.
2) My rule of thumb is that events must always be unsubscribed, and disposable objects (Streams etc) will be disposed automatically if they're member variables and the object holding them gets destroyed.
If you create a local disposable object in a method for example, you must dispose it, or just put it in a using statement and forget about it ;)

I think it's most helpful to describe a managed resource is a class-type object that implements IDisposable and requires cleanup, but can perform such cleanup (typically using Finalize) if it's abandoned without being properly Dispose'd. An unmanaged resource generally refers to an entity which requires cleanup that simply won't happen if it's abandoned without being Dispose'd first. It's important to note that the while the term "managed resource" refers essentially exclusively to class-type objects (which generally override Finalize, but which in some cases may be the targets of WeakReference objects), unmanaged resources may be not only be anything, they may also be anywhere, including on another computer.
I would suggest that instead of using the term "resource" it's more helpful to think in terms of "responsibilities". Opening a file or socket connection creates a responsibility to close it. Acquiring a lock creates a responsibility to release it. Sending a remote system a "grant me exclusive access to this record" message creates a responsibility to send it an "I'm done with this record" message. If an object's cleanup responsibilites can get carried out even if it's abandoned, it's a "managed resource". Otherwise, it's an "unmanaged resource".
Merely categorizing things as "managed resources" or "unmanaged resources" is not quite sufficient for deciding how they should be cleaned up. Some unmanaged responsibilities can be conveniently handled by a class-type wrapper which can perform any necessary cleanup in case of improper abandonment. Such wrappers should generally contain a minimal amount of information necessary to perform the cleanup responsibility. Other responsibilities cannot very well be handled automatically. It's often better to ensure Dispose is called than try to handle everything that can happen if it isn't.

IDisposable, Finalizers and the definition of an unmanaged resource

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.