I've been reading about IDisposable interface lately (this topic was quite useful Proper use of the IDisposable interface) and also about usage of using statement (topic Uses of "using" in C#). Then I started wondering what if I should somehow free my own collection class from memory.
class User{
private string username {set;get;}
private string password {set;get;}
...
}
Should it implement IDisposable interface?
class User : IDisposable
{
...
public void Dispose()
{
this.Dispose();
}
}
so it could be freed from memory? Or does GC do it automaticly and I shouldn't even bother about it.
As far as I understand it's important to free unmanaged resources like DB connections and such but what about those collection classes. Since I use them quite frequently It really started to bug me.
tl;dr;
should I implement IDisposable on User class?
Kind Regards.
edit: thanks everyone for the replies!
Or does GC do it automaticly and I shouldn't even bother about it.
This. Unless you have unmanaged resources (either directly or by way of a reference to something else which is disposable), you almost certainly shouldn't implement IDisposable.
Your current implementation would just call itself:
public void Dispose()
{
this.Dispose();
}
... so assuming you don't really want to call this.Dispose(), what would you want to do when Dispose() is called? It's not like disposal causes garbage collection - so what action do you want to take? If the answer is "nothing" then you probably shouldn't be implementing IDisposable. (The exception here is if this is designed to be a base class and you're expecting some derived classes to require disposal... that's a more complex scenario.)
All unused resources will be Garbage collected eventually when the program unloads or after separate interval if the resources are no longer used.
Its a better practice to clean/dispose used resources(variables, objects) when you no longer wish to use them to free memory.
As said in the other answers, the only times you have to implement IDisposable is when you deal with unmanaged resources or class members that are IDisposable themselves (in this case you should dispose them in your own Dispose method).
Another scenario you might see is when people want to use the using { } syntactic sugar to automatically call some method at the end of the variable scope without using the try { } finally { } form.
I.e.:
public class MyObject : IDisposable
{
public void Foo()
{
}
public void Dispose()
{
// Something they want to call after the use of an instance of MyObject
}
}
...
using (var myObj = new MyObject())
{
myObj.Foo();
}
instead of
public class MyObject
{
public void Foo()
{
}
public void MethodToCallAfterUse()
{
// Something they want to call after the use of an instance of MyObject
}
}
var myObj = new MyObject();
try
{
myObj.Foo();
}
finally
{
myObj.MethodToCallAfterUse();
}
If an object asks an outside entity to "do something" (perform an action, reserve a resource, etc.) on its behalf until further notice, and that outside entity might continue to exist after the object requesting its services has ceased to be useful, then the object should ensure that the outside entity receives notice when its services are no longer required. The IDisposable resource exists as a standard means by which an object can say "I may be obligated to let outside entities know when I don't need their services; let me know when my services will no longer be needed, so I can satisfy my obligation to tell any outside entities that I no longer need their services."
In .NET, once no references to an object exist anywhere in the universe, the object itself will likewise cease to exist. If the only object which knows of an obligation to do perform some action ceases to exist without having performed the action, the action will not get performed. If an object has no obligations, however, having it simply cease to exist once there are no references is just fine.
Related
I have been doing research on Interfaces in C# for some time now,According to MSDN
"Interfaces are better suited to situations in which your applications require many possibly unrelated object types to provide certain functionality."
https://msdn.microsoft.com/en-in/library/3b5b8ezk(v=vs.90).aspx
When implementing Dispose() instead of using Interface IDisposable I can simply define 3
methods of Dispose() & give it to the user.My Question here is "Why has Microsoft created IDisposable interface and what is the purpose of using Interface to implement Dispose()".
This is what I meant
//This method is used to release Managed Resources.
public void Dispose()
{
this.Dispose();
}
//This method is used to release both managed & unmanaged Resources.
public void DisposeAll()
{
this.Dispose();
GC.SuppressFinalize(this);
ReleaseUnmangedResources();
}
//This method is used to release only unmanaged Resources.
public void DisposeUnmanaged()
{
ReleaseUnmangedResources();
}
I am sorry if this question is too stupid or simple.Please help me in understanding interfaces.
IDisposable has special language support. Any object that implements IDisposable can be used as the subject of a using statement.
So,
using(var myDisposable = new ClassThatImplementsIDisposable())
{
//do some stuff/ even throw an exception
}//myDisposable.Dispose() is automatically called, even if an exception happened.
using statements are a very (very very) useful way to ensure that stuff gets cleaned up without having to write a whole bunch of boilerplate to ensure that it happens (even in the case of exceptions).
By providing the IDisposable interface, you are advertising that the object needs disposing. Without it, disposal might be overlooked, and tools (such as FXCop) will not pick this up.
By implementing IDisposable interface, you are telling the user of your class, that he should call Dispose() method when he is done with the class.
So the user is going to do something like this:
DisposableClass c = new DisposableClass();
//doing something
if (c is IDisposable)
c.Dispose();
Also, IDisposable objects are automatically disposed when created with using statement.
using(var c = new DisposableClass())
{
//doing something
} //c.Dispose() is called
In this case, Dispose() is called even if an exception was thrown inside the using block.
In C++, I'm used to using the sentry pattern to ensure that resources acquired are properly released when the block or function exits (see here for example). I used this, for example, for grabbing the graphics state, and then I can do whatever I want to that state, and it gets put back when the sentry object (reference) goes out of scope.
Now I'm using C#, and the same trick doesn't work, because the destructor doesn't get called until who-knows-when later.
Is there some OTHER method that is guaranteed to fire when an object's last reference is released? Or do I just have to remember to call some Restore() or Apply() or DoYourThing() method before returning from any method where I would otherwise use a sentry?
C# has finalizers, like C++, that are called when an object is destroyed. They are in the same form as C++, that is:
~ClassName()
{
}
As you know, though, C# does not have deterministic memory management, and so this isn't really a great guarantee. AS a direct result, you should not use finalizers for releasing unmanaged resources in C#. Instead, we use the IDisposable interface. This exposes a single method on the implementor, Dispose, which is intended to be called when you want to release unmanaged resources.
public class MyDisposable : IDisposable
{
public void Dispose()
{
// get rid of some expensive unmanaged resource like a connection
}
}
We can also use the using sugar to allow semantic invocation of Dispose() when the using block terminates (gracefully or not).
using(var disposable = new MyDisposable)
{
// Do something with the disposable
} // Dispose is invoked here
Should you find yourself using finalizers and Dispose, you can consider using the GC.SuppressFinalize method, although that's a bit out of my scope. There was a really good discussion elsewhere on StackOverflow about this here
This can be used to perform RAII-esque trickery, of course, such as
using(var guard = new Guard(resource))
{
}
C# provides the using block for this case, which works on any object that implements IDisposable.
For example, if you have a type class Foo : IDisposable then you can do this:
using (new Foo(/* ... */)) {
// Your code that depends on this resource.
}
This is equivalent to:
Foo x = new Foo(/* ... */);
try {
// Your code
} finally {
if (x != null) {
((IDisposable)x).Dispose();
}
}
Except, of course, that you don't have access to x. You can, however, gain such access if you need it by creating a variable in the using block:
using (Foo foo = new Foo(/* ... */)) {
// Your code that uses "foo"
}
The standard way of creating a class that can be disposed of this way or by the garbage collector is:
class Foo : IDisposable {
protected virtual void Dispose(bool disposing) {
// Your cleanup code goes here.
}
public void Dispose() {
GC.SuppressFinalize(this);
Dispose(true);
}
~Foo() {
Dispose(false);
}
}
To implement deterministic deallocation of unmanaged resources, you would use a Dispose pattern. For many cases you can combine this with a using block if you want this to occur within a specific scope.
File handles or handles to a graphics device would usually be considered a unmanaged resource. These are resources that the .NET framework does not track references for and does not automatically deallocate.
If it is managed resource, just references to C# objects/lists, then usually deterministic deallocation is not necessary, and the garbage collector can usually handle these more efficiently than you can. Don't worry so much about when managed resources are deallocated. If you no longer have a reference to them, then they should be of no concern. The concern comes when you are done with something but somehow have a lingering reference to it that is preventing the GC from deallocating it.
It sounds like what you're describing is objects that implement the IDisposable interface for C#. the IDisposable has a Dispose() method, that when defined properly on your own objects, is used to release resources.
Additionally C# does have destructors defined as such:
public class Foo
{
public Foo()
{
// constructor
}
~Foo() // destructor
}
when wrapping IDisposable objects in a using block, the Dispose() method is automatically called, the other option is to throw it in a finally block (when not using... using)
The easiest way to do this is enclosing the sentry object in a using statement. So something like this.
using (sentry = new Sentry())
{
//do your stuff here
}
After this block, sentry will have been released and out of scope.
Let's say I have a class with a object field. When Dispose() is called I would like to clear the reference to that object. The private field can only be set once, so ideally I would like it to be readonly, but if it is readonly there is a compile time error when I try to release the reference to the object during Dispose(). Ideally I would like to have a safe dispose AND mark the _value field as readonly. Is this possible or even necessary?
public class Foo : IDisposable
{
public Foo(object value)
{
_value = value;
}
public object Value { get { return _value; } }
private readonly object _value;
public void Dispose()
{
//Cleanup here
_value = null // causes compile time error
}
}
That's not necessary, even if it were possible.
Dispose is typically intended to clean up unmanaged resources, although there can be exceptions to that rule (note the comments). However, in this situation, you should allow the garbage collector to do its job. It will run indeterminately once the objects are considered to have no active roots. Under normal circumstances, you do not need to take action to force it. Just write your code with objects in properly limited scope and you will be fine.
Setting the reference to null does not in fact do anything. The garbage collector will clean up the object when there are no longer any references to it, which in this case since you are doing this in the Dispose method presumably the instance of Foo is about to no longer have any references to it and the difference in timing is probably not meaningful. Typically you implement the Dispose pattern because your type has as a member a class that itself implements IDisposable (in this case the type is Object, which does not implement IDisposable), or you have unmanaged resources that you would like to release deterministically. You can find a description of the Dispose pattern here. Note that if you create a readonly member variable of a type that implements IDisposable you can call the Dispose method on that object inside your Dispose method:
public class SomeClass : IDisposable
{
private Boolean mDisposed;
private readonly MemoryStream mStream = new MemoryStream(); // Could be any class that implements IDisposable
public void Dispose() {
Dispose(true);
GC.SuppressFinalize(this);
}
protected void Dispose(Boolean disposing) {
if (disposing & !mDisposed) {
mStream.Dispose(); // Could and should call Dispose
mDisposed = true;
}
return;
}
}
This works because the readonly nature is on the reference to the object, not the object itself.
That is not necessary nor correct. Wanting to do what you ask in your question seems to indicate you are somewhere accessing a disposed object, and that is wrong.
Rather than try to do what you have asked, you should perhaps implement IsDisposed (which btw is part of the standard Dispose pattern) and check it first.
As others have noted, the Dispose pattern is intended for releasing unmanaged resources.
This is an older post, but I face the same question regularly because I unsubscribe from objects that I subscribed to.
I do this unsubscription during dispose because those objects live longer than this object, and the way delegates are implemented means that the publisher keeps the subscriber alive, not the other way around like we would think.
BUT, when you unsubscribe from the events, you have the natural tendency to want to clear the reference to the publisher too, which really isn't such a big deal. I suggest you keep the 'readonly' constraint because it makes the rules clearer and the code more robust while the object is still alive. If it means you sit with the reference after dispose, that is okay becuase the object is now collectible.
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.
This question already has answers here:
Proper use of the IDisposable interface
(20 answers)
Closed 5 years ago.
What is the best practice for when to implement IDisposable?
Is the best rule of thumb to implement it if you have one managed object in the class, or does it depend if the object was created in the class or just passed in? Should I also do it for classes with no managed objects at all?
If you mean unmanaged objects then yes, you should be implementing it whenever you have one or more unmanaged resource you are handling in your class. You should also be using the pattern when you are possibly holding on to objects that are IDisposable themselves, and make sure to dispose of them when your class is disposed.
(agreed that this question has already been asked enough times as to run a small printer out of ink should they be printed...)
While everyone has mentioned (unmanaged) resources, I have another thing to add: I use it when I need to eliminate event handler hookups that would otherwise prevent a class from going out of scope and being garbage collected.
As an example, I have a service which gets injected in to a child view, that child view will subscribe to various async finished type events on the service. The owner of that child view has no idea what the concrete type is, it simply has it as an interface. The service may go out of scope at some arbitrary point in the future, and I don't want it hanging around not being GC'ed. Upon getting rid of that child view, the owner will call Dispose on it to give it the chance to unhook any event handlers. Here is a slightly contrived (and very pseudo code) example, note how the interface for the child view also implements IDisposable.
public class OwnerView {
public OwnerView() {
_childView = new ChildView(myServiceReference);
}
public void CloseChildView() {
if (childView != null) {
_childView.Close();
_childView.Dispose();
}
_childView = null;
}
private IChildView _childView;
}
public class ChildView : IChildView {
public ChildView(MyService serviceRef) {
_serviceRef = serviceRef;
_serviceRef.GetSettingsAsyncFinished += new EventHandler(someEventHandler);
}
public void IDisposable.Dispose() {
_serviceRef -= someEventHandler;
}
private MyService _serviceRef;
}
public interface IChildView : IDisposable {
void DoSomething();
... etc ...
}
There are far more authoritative comments about this from others on SO, like Do event handlers stop garbage collection from occuring? and Garbage collection when using anonymous delegates for event handling. You may also want to check out this codeproject article.
I think the docs are pretty clear about what IDisposable is good for.
The primary use of this interface is
to release unmanaged resources. The
garbage collector automatically
releases the memory allocated to a
managed object when that object is no
longer used. However, it is not
possible to predict when garbage
collection will occur. Furthermore,
the garbage collector has no knowledge
of unmanaged resources such as window
handles, or open files and streams.
It even has an example. In that example the class that implements IDisposable contains a handle. A handle needs to be freed when it no longer used. This is done in the Dispose() method. So the user of that class sees that it implements IDisposable and knows that the class needs to be explictily disposed when it is no longer needed (so the handle can be freed). It is a best practise (i.e. rule) that you should always call Dispose() on IDisosable instances when the instance is no longer needed.
You should implement IDisposable when your class holds resources that you want to release when you are finished using them.
When your class contains unmanaged objects, resources, opened files or database objects, you need to implement IDisposable.
If it has properties which also need to be disposed.