I have an object that has a disposable object as a member.
public class MyClass
{
private MyDisposableMember member;
public DoSomething
{
using (member = new MyDisposableMember())
{
// Blah...
}
}
}
There can be many methods in MyClass, all requiring a using statement. But what if I did this instead?
public class MyClass
{
private MyDisposableMember member = new MyDisposableMember();
public DoSomething
{
// Do things with member :)
}
~MyClass()
{
member.Dispose();
}
}
As you can see, member is being disposed in the destructor. Would this work? Are there any problems with this approach?
Ideally, Dispose() should have already been called prior to finalization. It would be better to follow the typical dispose pattern, and allow the user to Dispose() the object properly, and have the finalizer Dispose of it if dispose has not already been called.
In this case, since you're encapsulating an IDisposable, you really don't need to implement the finalizer at all, though. (At the the point of finalization, your encapsulated member will get finalized, so there's no need to finalize your object - it just adds overhead.) For details, read this blog article I wrote on encapsulating an IDisposable.
You should probably make MyClass implement IDisposable. Inside the Dispose() method, call member.Dispose();. That way the programmer can have control over when the member gets disposed.
DO NOT DO THAT!
The GC will do that for you (indirectly as the object to dispose or another one will contain a destructor)
MyDisposableMember might even be disposed by the GC even before you dispose it - what happens then might not be what you intended to do.
Even worse: Adding a destructor (or finalizer) to a class costs additional time when disposing of the object (much more time as the object will stay in memory for at least one collection cyclus and maybe even promoted to the next generation).
Therfore, it would be completely useless and even backfire.
In your first example the member is not really part of the object's state since you're instantiating it every time it's used and disposing it right after. Since it's not part of the state don't model it as such just use a local variable when needed.
In more general you should put all disposal logic in Dispose() and implement IDisposable then use you class together with using or try-finally
The only thing I see wrong (and it isn't an error) is the fact that in a using statement you explicitly dispose of the object at that point in time (when your function / method is called). The destructor cannot be called they are invoked automatically. So at this point it may take some time for member to be disposed of. Better to implement the IDisposeable interface for MyClass.
Following the Microsoft pattern is your best bet so the users of your class have full control over when it is disposed.
public class MyClass : IDisposable
{
private MyDisposableMember member = new MyDisposableMember();
public DoSomething
{
// Do things with member :)
}
~MyClass()
{
Dispose(false);
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
{
if (disposing) // Release managed resources
{
member.Dispose();
}
// Release unmanaged resources
}
}
When a finalizer runs, one of the following will be true about almost any IDisposable object to which it holds a reference:
The object will have already had its finalizer run, in which case calling Dispose on the object will be at best useless.
The object will not have had its finalizer run, but its finalizer will be scheduled to run, so calling Dispose on the object will be useless.
The object will still be in use by something other than the object being finalized, so calling Dispose on it would be bad.
There are a few situations where calling Dispose in a finalizer might be useful, but most situations fit the cases listed above, which all have a common feature: the finalizer shouldn't call Dispose.
Related
I have a class with a destructor that I get a null reference exception from because the variable I destroy is sometimes null.
Is this an appropriate use of the null conditional operator to be in the destructor?
I'm not even sure if this is an appropriate use of a destructor itself since it is not used to dispose of the actual object it's called on but rather a variable of it.
~clsSAPSettings()
{
mtbTemp?.Close();
}
This code was converted from VB6 so I'm trying to figure out how to handle this issue. Any information welcome.
Edit: The class mtbTemp belongs to implements IDisposable but doesn't have a finaliser/desctructor. It simply closes a connection used in an ORM model.
For anyone after a detailed explanation I found a great answer, Proper use of the IDisposable interface, it goes into detail about the use of finalizer and how garbage collection actually works.
Please do not use any fields of reference types in the finalizer: the order in which GC (Garbage Collector) collects them is unpredictable and that's why
~clsSAPSettings()
{
mtbTemp?.Close();
}
code can well be performed by GC as followes:
Collect mtbTemp instance
Start collecting this instance:
Call ~clsSAPSettings()
Call mtbTemp?.Close(); i.e. call a method of the collected (destroyed) instance
and you'll have an unstable hard to find error. It seems that you are looking for a IDisposable interface:
public class clsSAPSettings: IDisposable {
private MyTemp mtbTemp;
...
protected virtual void Dispose(bool disposing) {
if (disposing) {
mtbTemp?.Close();
GC.SuppressFinalize(this);
}
}
public void Dispose() {
Dispose(true);
}
//TODO: do you really want finalizer?
~clsSAPSettings() {
Dispose(false);
}
}
Consider:
~clsSAPSettings()
{
mtbTemp?.Close();
}
The problem here isn't the null conditional usage. That doesn't by itself present any issues.
The biggest problem is that in a finalizer you should not touch any other object. When the finalizer fires, your object is toast. You no longer have any guarantees about the life of mtbTemp, including whether or not it has already been garbage collected, so you should not touch it. It might work; it might cause a temporary resurrection, or it might crash horribly.
The correct place to do something like this is in IDisposable.Dispose. In the Dispose method, that would be absolutely fine:
public void Dispose() // where your class : IDisposable
{
mtbTemp?.Close();
mtbTemp = null;
}
You probably don't need a finalizer at all. They are incredibly rare.
When you are closing streams or other unmanaged objects, you should use the Dispose pattern, rather than a destructor. You never know when that destructor will fire.
Regarding the use of the null conditional operator in a destructor: I see no problem with the operator itself. I do with referencing other objects that might already been destructed or in destruction.
Trying to figure it out. Someone told me
Dispose is not just a method - it's equivalent to a destructor in other languages.
Ok. Msdn is also very loud about this.
But then
class Test : IDisposable
{
public string Property { get; set; } = "Test";
public void Dispose() => Console.WriteLine("Disposed, very scary");
}
class Program
{
static void Main(string[] args)
{
var test = new Test();
test.Dispose();
test.Property = "123"; // but it's disposed OMG! do not do this!
test.Dispose();
using (var another = new Test())
for (int i = 0; i < 10; i++)
{
another.Dispose();
GC.Collect(); // or what should I call to make it crash?
}
Console.ReadKey();
}
}
And there are no problems.
What I think about Dispose:
it's a normal public method;
IDisposable is useful in conjunction with using to automatically call Dispose, nothing more;
it's totally fine to put into dispose any code an call it internally at any time if object state properly maintained.
Correct me if I am wrong.
P.S: Downvote means "question is bad/not useful/has problem". If you simply disagree with my thoughts - post comment or answer. It will be useful for people who think as I do right now (because I am wrong? then prove it)
Dispose is just a method, you can call it just like any other method. It is always exposed through the IDisposable interface (yes, obviously you can name a method Dispose without implementing IDisposable, don't do that!).
However, calling it manually is sometimes a code smell, smell of code that should probably be using using instead. By "manually" here I mean calling Dispose outside of the implementation of another Dispose.
Calling it twice should also be safe and is documented:
If an object's Dispose method is called more than once, the object must ignore all calls after the first one. The object must not throw an exception if its Dispose method is called multiple times. Instance methods other than Dispose can throw an ObjectDisposedException when resources are already disposed.
(my emphasis)
Should you be calling Dispose twice? No!. That is also a code smell of code that no longer has control of what it has done and what it has left to do and ends up doing things "just to be sure". Don't do that either!
So if you write code correctly, sure you can call Dispose manually.
You're right, Dispose is just another method, belonging to IDisposable. It just has the added benefit of being able to be automatically called when a using() scope ends - it is not equivalent to a destructor and whoever told you that doesn't truly understand what they're saying.
I'd always use a using() block where possible, but if you're careful you can manually call Dispose instead and it will have the same effect.
Visual Studio 2015 offers up (finally) a suggested pattern for implementation of IDisposable which matches the guidelines.
This is the stub that VS creates for you when selecting to implement the disposable pattern. The TODOs guide us through the implementation.
class Disposable : IDisposable
{
#region IDisposable Support
private bool disposedValue = false; // To detect redundant calls
protected virtual void Dispose(bool disposing)
{
if (!disposedValue)
{
if (disposing)
{
// TODO: dispose managed state (managed objects).
}
// TODO: free unmanaged resources (unmanaged objects) and override a finalizer below.
// TODO: set large fields to null.
disposedValue = true;
}
}
// TODO: override a finalizer only if Dispose(bool disposing) above has code to free unmanaged resources.
// ~Disposable() {
// // Do not change this code. Put cleanup code in Dispose(bool disposing) above.
// Dispose(false);
// }
// This code added to correctly implement the disposable pattern.
public void Dispose()
{
// Do not change this code. Put cleanup code in Dispose(bool disposing) above.
Dispose(true);
// TODO: uncomment the following line if the finalizer is overridden above.
// GC.SuppressFinalize(this);
}
#endregion
}
Note the use of a flag to indicate if the Dispose has already been called. This flag can be used to throw ObjectDisposedException instances on method calls and properties across the classes implementation.
The intention of Dispose has always been to clean up unmanaged resources. However, the syntactic sugar of the using keyword (requires an IDisposable) lets you create really tidy 'context' pattern classes for managing resource access, even if there are no unmanaged resources used.
As usual, document the usage and the intentions clearly and you can't go wrong but please avoid methods called 'Dispose' if you don't implement the interface.
There is a critical difference between Dispose in .NET versus destructors in other languages: if one has a pointer to an object in another language and the object is deleted, one will have an invalid pointer and there is no way to do anything with it, including determine whether it is still valid, without invoking Undefined Behavior. By contrast, in .NET if one has a reference to an object which gets Disposed, the reference will continue to be a valid reference to the object in question. The object will likely refuse to do much because it's been disposed, but such refusal is affirmatively generated by the object itself. No Undefined Behavior involved.
On the flip side, when a destructor is invoked in other languages like C++ an object can be certain that no valid pointers exist to it anymore, but that is not true of Dispose. Consequently, code should avoid pooling public-facing objects and instead have every request for a new object return a new object (which may be a wrapper to a pooled object). If no reference to the pooled object exists outside the wrapper, and Dispose invalidates that reference before returning the object to the pool, then a future object request can be satisfied by encapsulating the pooled object in a new wrapper which will again hold the only reference to it.
Please consider the following class:
public class Level : IDisposable
{
public Level() { }
public void Dispose() { }
}
My question is, if I call the Dispose method, will the class actually be disposed (garbage collected)?
ex:
Level level = new Level();
level.Dispose();
Thanks.
My question is, if I call the Dispose method, will the class actually be disposed?
If by disposed you mean garbage collected, then no, this won't happen. What will happen when you call the Dispose method is, well, the Dispose method be called and its body executed.
Also it is recommended to wrap disposable resources in a using statement to ensure that the Dispose method will always be called even in the event of an exception. So instead of manually calling it you could:
using (Level level = new Level())
{
// ... do something with the level
}
Normally the Dispose method is used when the object holds pointers to some unmanaged resources and provides a mechanism to deterministically release those resources.
I assume that what you are after is a way to know that Dispose() was called?
You can do that either in the consuming code by setting the instance to null after disposing:
Level level = new Level();
//do stuff with the instance..
level.Dispose();
level = null;
//in other place:
if (level != null)
{
//still available
}
Or in the class itself, add boolean flag and in every method check for it:
public class Level : IDisposable
{
private bool disposing = false;
public Level() { }
public void Foo()
{
if (disposing)
return;
MessageBox.Show("foo");
}
public void Dispose()
{
if (disposing)
return;
disposing = true;
}
}
Each class that implements IDisposable defines what it means to be disposed. By that line of reasoning, yes, your class will be as disposed as it wants to be.
No, the instance won't be garbage collected due to calling Dispose. The Dispose method is where you can release any resources held by the instance, it isn't about disposing the instance itself.
If you call Dispose() it will be disposed, which doesn't absolutely mean that it will be garbage collected,seems to me that is your question, if not please clarify.
In .NET Disposing and garbage collection are two different things:
The point of disposing is to release any resources that are either not managed by .NET (like manually allocated memory) or that have interests in being released as soon es they aren't needed anymore (like file handles or network connections).
The purpose of garbage collection is to free memory that is managed by .NET (like normal objects).
So, as others pointed out: your object will not necessarily be garbage collected when it gets disposed.
I have few Question for which I am not able to get a proper answer .
1) Why should we call SuppressFinalize in the Dispose function when we don't have a destructor .
2) Dispose and finalize are used for freeing resources before the object is garbage collected. Whether it is managed or unmanaged resource we need to free it , then why we need a condition inside the dispose function , saying pass 'true' when we call this overridden function from IDisposable:Dispose and pass false when called from a finalize.
See the below code I copied from net.
class Test : IDisposable
{
private bool isDisposed = false;
~Test()
{
Dispose(false);
}
protected void Dispose(bool disposing)
{
if (disposing)
{
// Code to dispose the managed resources of the class
}
// Code to dispose the un-managed resources of the class
isDisposed = true;
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}
what if I remove the boolean protected Dispose function and implement the as below.
class Test : IDisposable
{
private bool isDisposed = false;
~Test()
{
Dispose();
}
public void Dispose()
{
// Code to dispose the managed resources of the class
// Code to dispose the un-managed resources of the class
isDisposed = true;
// Call this since we have a destructor . what if , if we don't have one
GC.SuppressFinalize(this);
}
}
I'm going out on a limb here, but... most people don't need the full-blown dispose pattern. It's designed to be solid in the face of having direct access to unmanaged resources (usually via IntPtr) and in the face of inheritance. Most of the time, neither of these is actually required.
If you're just holding a reference to something else which implements IDisposable, you almost certainly don't need a finalizer - whatever holds the resource directly is responsible for dealing with that. You can make do with something like this:
public sealed class Foo : IDisposable
{
private bool disposed;
private FileStream stream;
// Other code
public void Dispose()
{
if (disposed)
{
return;
}
stream.Dispose();
disposed = true;
}
}
Note that this isn't thread-safe, but that probably won't be a problem.
By not having to worry about the possibility of subclasses holding resources directly, you don't need to suppress the finalizer (because there isn't one) - and you don't need to provide a way of subclasses customising the disposal either. Life is simpler without inheritance.
If you do need to allow uncontrolled inheritance (i.e. you're not willing to bet that subclasses will have very particular needs) then you need to go for the full pattern.
Note that with SafeHandle from .NET 2.0, it's even rarer that you need your own finalizer than it was in .NET 1.1.
To address your point about why there's a disposing flag in the first place: if you're running within a finalizer, other objects you refer to may already have been finalized. You should let them clean up themselves, and you should only clean up the resources you directly own.
Here are the main facts
1) Object.Finalize is what your class overrides when it has a Finalizer. the ~TypeName() destructor method is just shorthand for 'override Finalize()' etc
2) You call GC.SuppressFinalize if you are disposing of resources in your Dispose method before finalization (i.e. when coming out of a using block etc). If you do not have a Finalizer, then you do not need to do this. If you have a Finalizer, this ensures that the object is taken off of the Finalization queue (so we dont dispose of stuff twice as the Finalizer usually calls the Dispose method as well)
3) You implement a Finalizer as a 'fail safe' mechanism. Finalizers are guaranteed to run (as long as the CLR isnt aborted), so they allow you to make sure code gets cleaned up in the event that the Dispose method was not called (maybe the programmer forgot to create the instance within a 'using' block etc.
4) Finalizers are expensive as Types that have finalizers cant be garbage collected in a Generation-0 collection (the most efficient), and are promoted to Generation-1 with a reference to them on the F-Reachable queue, so that they represent a GC root. it's not until the GC performs a Generation-1 collection that the finalizer gets called, and the resources are released - so implement finalizers only when very important - and make sure that objects that require Finalization are as small as possible - because all objects that can be reached by your finalizable object will be promoted to Generation-1 also.
Keep the first version, it is safer and is the correct implementation of the dispose pattern.
Calling SuppressFinalize tells the GC that you have done all the destruction/disposing yourself (of resources held by your class) and that it does not need to call the destructor.
You need the test in case the code using your class has already called dispose and you shouldn't tell the GC to dispose again.
See this MSDN document (Dispose methods should call SuppressFinalize).
1. Answer for the first question
Basically, you don't have to call SuppressFinalize method if your class doesn't have a finalize method (Destructor). I believe people call SupressFinalize even when there is no finalize method because of lack of knowledge.
2. Answer for the second question
Purpose of the Finalize method is to free un-managed resources. The most important thing to understand is that, Finalize method is called when the object is in the finalization queue. Garbage collector collects all the objects that can be destroy. Garbage Collector adds objects those have got finalization to the finalization queue before destroy. There is another .net background process to call the finalize method for the objects those are in the finalization queue. By the time that background process execute the finalize method, that particular object's other managed reference may have been destroyed. Because there is no specific order when it comes to the finalization execution. So, the Dispose Pattern wants to make sure that finalize method do not try to access managed objects. That's why managed objects are going in side "if (disposing)" clause which is unreachable for the finalize method.
You should always call SuppressFinalize() because you might have (or have in the future) a derived class that implements a Finalizer - in which case you need it.
Let's say you have a base class that doesn't have a Finalizer - and you decided not to call SuppressFinalize(). Then 3 months later you add a derived class that adds a Finalizer. It is likely that you will forget to go up to the base class and add a call to SuppressFinalize(). There is no harm in calling it if there is no finalizer.
My suggested IDisposable pattern is posted here: How to properly implement the Dispose Pattern
I was reading this article the other day and was wondering why there was a Finalizer along with the Dispose method. I read here on SO as to why you might want to add Dispose to the Finalizer. My curiousity is, when would the Finalizer be called over the Dispose method itself? Is there a code example or is it based on something happening on the system the software is running? If so, what could happen to not have the Dispose method run by the GC.
The purpose of the finaliser here is simply a safety precaution against memory leaks (if you happen not to call Dispose explicitly). It also means you don't have to dispose your objects if you want them to release resources when the program shutdowns, since the GC will be forced to finalise and collect all objects anyway.
As a related point, it is important to dispose the object slightly differently when doing so from the finaliser.
~MyClass()
{
Dispose(false);
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected void Dispose(disposing)
{
if (!this.disposed)
{
if (disposing)
{
// Dispose managed resources here.
}
// Dispose unmanaged resources here.
}
this.disposed = true;
}
The reason you do not want to dispose managed resources in your finaliser is that you would actually be creating strong references to them in doing so, and this could prevent the GC from doing it's job properly and collecting them. Unmanaged resources (e.g. Win32 handles and such) should always be explicitly closed/disposed, of course, since the CLR has no knowledge of them.
This is mostly there to protect yourself. You cannot dictate what the end user of your class will do. By providing a finalizer in addition to a Dispose method, the GC will "Dispose" of your object, freeing your resources appropriately, even if the user forgets to call Dispose() or mis-uses your class.
The Finalizer is called when the object is garbage collected. Dispose needs to be explicitly called. In the following code the finalizer will be called but the Dispose method is not.
class Foo : IDisposable
{
public void Dispose()
{
Console.WriteLine("Disposed");
}
~Foo()
{
Console.WriteLine("Finalized");
}
}
...
public void Go()
{
Foo foo = new Foo();
}
The dispose method must be explicitly called, either by calling Dispose() or by having the object in a using statement. The GC will always call the finalizer, so if there is something that needs to happen before the objects are disposed of the finalizer should at least check to make sure that everything in the object is cleaned up.
You want to avoid cleaning up objects in the finalizer if at all possible, because it causes extra work compared to disposing them before hand (like calling dispose), but you should always at least check in the finalizer if there are objects lying around that need to be removed.
An important but subtle note not yet mentioned: a seldom-considered purpose of Dispose is to prevent an object from being cleaned up prematurely. Objects with finalizers must be written carefully, lest a finalizer run earlier than expected. A finalizer can't run before the start of the last method call that will be made on an object(*), but it might sometimes run during the last method call if the object will be abandoned once the method completes. Code which properly Dispose an object can't abandon the object before calling Dispose, so there's no danger of a finalizer wreaking havoc on code which properly uses Dispose. On the other hand, if the last method to use an object makes use of entities which will be cleaned up in the finalizer after its last use of the object reference itself, it's possible for the garbage-collector to call Finalize on the object and clean up entities that are still in use. The remedy is to ensure any call method which uses entities that are going to get cleaned up by a finalizer must be followed at some point by a method call which makes use of "this". GC.KeepAlive(this) is a good method to use for that.
(*) Non-virtual methods which are expanded to in-line code that doesn't do anything with the object may be exempt from this rule, but Dispose usually is, or invokes, a virtual method.