I tried to further narrow down the problem in Flush StreamWriter at the end of its lifetime implementing a singleton-like self-closing StreamWriter:
class Foo : System.IO.StreamWriter
{
private static readonly Foo instance = new Foo( "D:/tmp/test" );
private Foo( string path )
: base( path )
{
}
~Foo()
{
this.Close();
}
public static Foo Instance
{
get
{
return instance;
}
}
}
The intended effect is that in a sample program like
class Program
{
private static void Main( string[] args )
{
Foo.Instance.Write( "asdf\n" );
}
}
the garbage collector causes the instance of Foo to be closed.
This does not work. Apparently, the StreamWriter is already gone when Foo's destructor runs, and I get a System.ObjectDisposedException.
How do I call Close() on the stream in an appropiate way and prevent loss of data?
Apparently, the StreamWriter is already gone when Foo's destructor runs
Yes. At program termination, assuming your object gets GC'ed at all (there's no guarantee it will be), the runtime doesn't provide any guarantees about the order in which it executes finalizers. Both objects are unreachable and eligible for finalization at the same time, and so their finalizers could run in any order.
Finalizers are there only as a backstop for buggy code. And they are not 100% reliable for that purpose either. It's why you should work hard to avoid buggy code.
You'll need a deterministic way to dispose the singleton object on process exit. You can put this into the control of your program's main logic, which is controlling the process lifetime, and have it dispose the singleton (e.g. via a public method you write for that purpose). Another alternative is to subscribe to the AppDomain.ProcessExit event in your singleton's constructor, and have the handler close the singleton there.
See these related questions for additional information:
How can I ensure that I dispose of an object in my singleton before the application closes?
Disposable singleton in C#
Related
C# has static constructor which do some initialization (likely do some unmanaged resource initialization).
I am wondering if there is static destructor?
Not exactly a destructor, but here is how you would do it:
class StaticClass
{
static StaticClass() {
AppDomain.CurrentDomain.ProcessExit +=
StaticClass_Dtor;
}
static void StaticClass_Dtor(object sender, EventArgs e) {
// clean it up
}
}
This is the best way (ref: https://stackoverflow.com/a/256278/372666)
public static class Foo
{
private static readonly Destructor Finalise = new Destructor();
static Foo()
{
// One time only constructor.
}
private sealed class Destructor
{
~Destructor()
{
// One time only destructor.
}
}
}
No, there isn't.
A static destructor supposedly would run at the end of execution of a process. When a process dies, all memory/handles associated with it will get released by the operating system.
If your program should do a specific action at the end of execution (like a transactional database engine, flushing its cache), it's going to be far more difficult to correctly handle than just a piece of code that runs at the end of normal execution of the process. You have to manually handle crashes and unexpected termination of the process and try recovering at next run anyway. The "static destructor" concept wouldn't help that much.
No, there isn't. The closest thing you can do is set an event handler
to the DomainUnload event on the AppDomain and perform your cleanup there.
Initializing and cleaning up unmanaged resources from a Static implementation is quite problematic and prone to issues.
Why not use a singleton, and implement a Finalizer for the instance (an ideally inherit from SafeHandle)
No there is nothing like destructor for static classes but you can use Appdomain.Unloaded event if you really need to do something
I have a base class that when any class inherited from will add a GUID to a list in another singleton class. I want to have a method in the base class that will remove the GUID from the list when the inherited class has completed its execution. I was looking at putting a method call in a Dispose method but wasn't sure if this was the right way to go as I want the method call to remove the GUID to happen as soon as possible so I did not want to wait for the .NET GC to begin it's work. Also as I have never implemented Dispose before can I just add the IDisposable interface to my base class and create a Dispose method that includes the GUID removal logic or do you have to explicitly include other logic in a Dispose method?
There is no magic in writing the Dispose method. It is no different from any other method. If you do not call it the system will not do it for you.
EXCEPT:
If you really want to make sure that those precious resources are cleaned up properly, you also need to build some safeguards for the situations when the GC gets to your object even though the Dispose method was not called in a usual way.
The way to do it is to also call it from the object destructor and as you do it there are a few things to watch out. First of all when the Dispose runs from the destructor it happens on a thread different from the main thread of your app. Also you cannot use any of the objects your object references by this time they might'of been collected.
You are mixing things, actually.
The garbage collector will never call Dispose on your class. You should implement IDisposable so that a client can ensure that the cleanup method is always called:
using System;
class BaseClass : IDisposable {
bool _guidremoved = false;
public void RemoveGuid() {
if (!_guidremoved) {
_guidremoved = true;
// your logic here
}
}
public void Dispose() {
RemoveGuid();
}
}
class Derived : BaseClass {
}
static class Program {
static void Main(string[] args) {
using (Derived d = new Derived()) {
// do stuff here...
// call RemoveGuid explicitly, if needed
d.RemoveGuid();
} // when we exit this block, Dispose is called
}
}
In this example, the using block ensures that the RemoveGuid method is called by Dispose() if the line with d.RemoveGuid() is not reached.
The garbage collector would, instead, call the destructor (or better, the Finalize method) on your class when it runs, but that would probably not be the proper place to remove the GUID in your case.
Check Implementing Finalize and Dispose to Clean Up Unmanaged Resources for more details.
Is it legal to call a method on disposed object? If yes, why?
In the following demo program, I've a disposable class A (which implements IDisposable interface).As far as I know, if I pass disposable object to using() construct, then Dispose() method gets called automatically at the closing bracket:
A a = new A();
using (a)
{
//...
}//<--------- a.Dispose() gets called here!
//here the object is supposed to be disposed,
//and shouldn't be used, as far as I understand.
If that is correct, then please explain the output of this program:
public class A : IDisposable
{
int i = 100;
public void Dispose()
{
Console.WriteLine("Dispose() called");
}
public void f()
{
Console.WriteLine("{0}", i); i *= 2;
}
}
public class Test
{
public static void Main()
{
A a = new A();
Console.WriteLine("Before using()");
a.f();
using ( a)
{
Console.WriteLine("Inside using()");
a.f();
}
Console.WriteLine("After using()");
a.f();
}
}
Output (ideone):
Before using()
100
Inside using()
200
Dispose() called
After using()
400
How can I call f() on the disposed object a? Is this allowed? If yes, then why? If no, then why the above program doesn't give exception at runtime?
I know that the popular construct of using using is this:
using (A a = new A())
{
//working with a
}
But I'm just experimenting, that is why I wrote it differently.
Disposed doesn't mean gone. Disposed only means that any unmanaged resource (like a file, connection of any kind, ...) has been released. While this usually means that the object doesn't provide any useful functionality, there might still be methods that don't depend on that unmanaged resource and still work as usual.
The Disposing mechanism exist as .net (and inheritly, C#.net) is a garbage-collected environment, meaning you aren't responsable for memory management. However, the garbage collector can't decide if an unmanaged resource has been finished using, thus you need to do this yourself.
If you want methods to throw an exception after the object has been diposed, you'll need a boolean to capture the dispose status, and once the object is disposed, you throw the exception:
public class A : IDisposable
{
int i = 100;
bool disposed = false;
public void Dispose()
{
disposed = true;
Console.WriteLine("Dispose() called");
}
public void f()
{
if(disposed)
throw new ObjectDisposedException();
Console.WriteLine("{0}", i); i *= 2;
}
}
The exception is not thrown because you have not designed the methods to throw ObjectDisposedException after Dispose has been called.
The clr does not automagically know that it should throw ObjectDisposedException once Dispose is called. It's your responsibility to throw an exception if Dispose has released any resources needed for successful execution of your methods.
A typical Dispose() implementation only calls Dispose() on any objects that it stores in its fields that are disposable. Which in turn release unmanaged resources. If you implement IDisposable and not actually do anything, like you did in your snippet, then the object state doesn't change at all. Nothing can go wrong. Don't mix up disposal with finalization.
The purpose of IDisposable is to allow an object to fix the state of any outside entities which have, for its benefit, been put into a state that is less than ideal for other purposes. For example, an Io.Ports.SerialPort object might have changed the state of a serial port from "available for any application that wants it" to "only usable by one particular Io.Ports.SerialPort object"; the primary purpose of SerialPort.Dispose is to restore the state of the serial port to "available for any application".
Of course, once an object that implements IDisposable has reset entities that had been maintaining a certain state for its benefit, it will no longer have the benefit of those entities' maintained state. For example, once the state of the serial port has been set to "available for any application", the data streams with which it had been associated can no longer be used to send and receive data. If an object could function normally without outside entities being put into a special state for its benefit, there would be no reason to leave outside entities in a special state in the first place.
Generally, after IDisposable.Dispose has been called on an object, the object should not be expected to be capable of doing much. Attempting to use most methods on such an object would indicate a bug; if a method can't reasonably be expected to work, the proper way to indicate that is via ObjectDisposedException.
Microsoft suggests that nearly all methods on an object which implements IDisposable should throw ObjectDisposedException if they are used on an object which has been disposed. I would suggest that such advice is overbroad. It is often very useful for devices to expose methods or properties to find out what happened while the object was alive. Although one could give a communications class a Close method as well as a Dispose method, and only allow one to query things like NumberOfPacketsExchanged after a close but not after a Dispose, but that seems excessively complicated. Reading properties related to things that happened before an object was Disposed seems a perfectly reasonable pattern.
Calling Dispose() doesn't set the object reference to null, and your custom disposable class doesn't contain any logic to throw an exception if its functions are accessed after Dispose() has been called so it is of course legal.
In the real world, Dispose() releases unmanaged resources and those resources will be unavailable thereafter, and/or the class author has it throw ObjectDisposedException if you try to use the object after calling Dispose(). Typically a class-level boolean would be set to true within the body of Dispose() and that value checked in the other members of the class before they do any work, with the exception being thrown if the bool is true.
A disposer in C# is not the same as a destructor in C++. A disposer is used to release managed (or unmanaged) resources while the object remains valid.
Exceptions are thrown depending on the implementation of the class. If f() does not require the use of your already disposed objects, then it doesn't necessarily need to throw an exception.
Are there some advices about how I should deal with the IDisposable object sequences?
For example, I have a method that builds a IEnumerable<System.Drawing.Image> sequence and
at some point I would need to dispose that objects manually, because otherwise this might lead to some leaks.
Now, is there a way to bind the Dispose() call to garbage collector actions, because I want these objects disposed right in the moment they are no longer accessible from other code parts?
**Or maybe you could advice me some other approach? **
Generally, this seems to be the same problem as it comes, for example, in unmanaged C++ without shared pointers, where you can have a method:
SomeObject* AllocateAndConstruct();
and then you can't be sure when to dispose it, if you don't use code contracts or don't state something in the comments.
I guess the situation with disposable objects is pretty the same, but I hope there is an appropriate solution for this.
(from the question)
Now, is there a way to bind the
Dispose() call to garbage collector
actions, because I want these objects
disposed right in the moment they are
no longer accessible from other code
parts?
GC doesn't happen immediately when your object goes out of scope / reach; it is non-deterministic. By the time GC sees it, there is no point doing anything else (that isn't already handled by the finalizer), as it is too late.
The trick, then, is to know when you are done with it, and call Dispose() yourself. In many cases using achieves this. For example you could write a class that implements IDisposable and encapsulates a set of Images - and wrap your use of that encapsulating object with using. The Dispose() on the wrapper could Dispose() all the images held.
i.e.
using(var imageWrapper = GetImages()) {
foreach(var image in imageWrapper) {
...
}
// etc
} // assume imageWrapper is something you write, which disposes the child items
however, this is a bit trickier if you are displaying the data on the UI. There is no shortcut there; you will have to track when you are done with each image, or accept non-deterministic finalization.
If you want to determiniscally dispose of the objects in the collection, you should call Dispose on each:
myImages.ToList().ForEach(image => image.Dispose());
If you don't do this, and if your objects become unreachable, the GC will eventually run and release them.
Now, if you don't want to manually code the Dispose calls, you can create a wrapper class that implements IDisposable and use it through a using statement:
using (myImages.AsDisposable()) {
// ... process the images
}
This is the needed "infrastructure":
public class DisposableCollectionWrapper<D> : IDisposable
where D : IDisposable {
private readonly IEnumerable<D> _disposables;
public DisposableCollectionWrapper(IEnumerable<D> disposables) {
_disposables = disposables;
}
public void Dispose() {
if (_disposables == null) return;
foreach (var disposable in _disposables) {
disposable.Dispose();
}
}
}
public static class CollectionExtensions {
public static IDisposable AsDisposable<D>(this IEnumerable<D> self)
where D : IDisposable {
return new DisposableCollectionWrapper<D>(self);
}
}
Also notice that this is not the same as the situation you described with C++. In C++, if you don't delete your object, you have a genuine memory leak. In C#, if you don't dispose of your object, the garbage collector will eventually run and clean it up.
You should design your system in a way that you know when the resources are no longer needed. In the worst case, they'll be eventually disposed when the garbage collector gets to it, but the point of IDisposable is that you can release important resources earlier.
This "earlier" is up to you to define, for example, you can release them when the window that's using them closes, or when your unit of work finishes doing whatever operations on them. But at some point, some object should "own" these resources, and therefore should know when they're no longer needed.
You can use the 'using' block, to make sure, the IDisposable is disposed as soon the block is left. The compiler does encapsulate such blocks into try - finally statements in order to make sure, Dispose is called in any case when leaving the block.
By using a finalizer, one can make the GC call the Dispose method for those objects which where "missed" somehow. However, implementing a finalizer is more expensive and decreases the garbage collection efficiency - and possibly the overall performance of your application. So if any possible, you should try to make sure to dispose your IDisposables on your own; deterministically:
public class Context : IDisposable {
List<IDisposable> m_disposable = new List<IDisposable>();
public void AddDisposable(IDisposable disposable) {
m_disposable.Add(disposable);
}
public void Dispose() {
foreach (IDisposable disp in m_disposable)
disp.Dispose();
}
// the Context class is used that way:
static void Main(string[] args) {
using (Context context = new Context()) {
// create your images here, add each to the context
context.AddDisposable(image);
// add more objects here
} // <- leaving the scope will dispose the context
}
}
By using some clever design, the process of adding objects to the context may can get even more easier. One might give the context to the creation method or publish it via a static singleton. That way, it would be available for child method scopes as well - without having to pass a reference to the contex around. By utilizing that scheme it is even possible, to simulate an artificial destructor functionality like f.e. known from C++.
The neat method would be to create your own generic collection class that implements IDisposable. When this collection class is Disposed() ask for each element if it implements IDisposed, and if so Dispose it.
Example (look elsewhere if you don't know about the IDisposable pattern)
public class MyDisposableList<T> : List<T> : IDisposable
{
private bool disposed = false;
~MyDisposableList()
{
Dispose(false);
}
public void Dispose()
{
Dispose(true);
}
protected void Dispose(bool disposing)
{
if (!disposed)
{
foreach (T myT in this)
{
IDisposable myDisposableT = myT as IDisposable;
if (myDisposableT != null)
{
myDisposableT.Dispose();
}
myT = null;
}
this.Clear();
this.TrimExcess();
disposed = true;
}
}
...
}
usage:
using (MyDisposableList<System.Drawing.Bitmap> myList = new ...)
{
// add bitmaps to the list (bitmaps are IDisposable)
// use the elements in the list
}
The end of the using statement automatically Disposes myList, and thus all bitMaps in myList
By the way: if you loaded the bitmap from a file and you forgot to Dispose() the bitmap you don't know when you can delete that file.
You can call GC.Collect() if you really was to dispose those objects right away but to my understanding it is up to the GC to decide whether to collect the memory.
This in turn will call the Finalize() method for each object that should be released.
Note that if the collection goes out of scope the GC will eventually collect the memory used by the images.
You can also use the using construct if you use a collection that implements IDisposeable. That will guarantee that the objects will be disposed exactly when the collection goes out of scope (or nearly after the end of the scope).
C# has static constructor which do some initialization (likely do some unmanaged resource initialization).
I am wondering if there is static destructor?
Not exactly a destructor, but here is how you would do it:
class StaticClass
{
static StaticClass() {
AppDomain.CurrentDomain.ProcessExit +=
StaticClass_Dtor;
}
static void StaticClass_Dtor(object sender, EventArgs e) {
// clean it up
}
}
This is the best way (ref: https://stackoverflow.com/a/256278/372666)
public static class Foo
{
private static readonly Destructor Finalise = new Destructor();
static Foo()
{
// One time only constructor.
}
private sealed class Destructor
{
~Destructor()
{
// One time only destructor.
}
}
}
No, there isn't.
A static destructor supposedly would run at the end of execution of a process. When a process dies, all memory/handles associated with it will get released by the operating system.
If your program should do a specific action at the end of execution (like a transactional database engine, flushing its cache), it's going to be far more difficult to correctly handle than just a piece of code that runs at the end of normal execution of the process. You have to manually handle crashes and unexpected termination of the process and try recovering at next run anyway. The "static destructor" concept wouldn't help that much.
No, there isn't. The closest thing you can do is set an event handler
to the DomainUnload event on the AppDomain and perform your cleanup there.
Initializing and cleaning up unmanaged resources from a Static implementation is quite problematic and prone to issues.
Why not use a singleton, and implement a Finalizer for the instance (an ideally inherit from SafeHandle)
No there is nothing like destructor for static classes but you can use Appdomain.Unloaded event if you really need to do something