Assuming I have a method public static Rectangle DrawRectangle(Vector origin, Vector size) which returns an object of type Rectangle : IDisposable
If I call only the method DrawRectangle(origin, size), but do not assign the return value to a variable myRectangle = DrawRectangle(origin, size), will the compiler automatically detect this and call DrawRectangle(origin, size).Dispose(), or do I have to do it myself?
No. Point.
Eventually he finalizer does, but no. It does not automatically CALL Dispose.
If your Rectangle class implemets IDisposable, try the using( ) { ... } statment when posible.
Msdn link
The compiler builds code in to assemblies and is not responsible for any execution. The .net runtime is what runs your code and more specifically, the garbage collector deals with memory clean up. It is generally best practice to call objects that implement IDisposable inside of a using block. E.g:
using (var obj = new object)
{
... do your stuff
}
This will help the runtime understand when your object goes out of scope and clean it up earlier, but as long as you dont hold on to a reference to an object, the runtime will eventually clean it up and dispose it for you.
There are only two scenarios I can think of where the compiler automatically calls dispose; the most obvious would be:
using(var obj = ... )
{
// some code
}
which is an explicit instruction to say "at the end, whether success or failure, if obj is non-null, call obj.Dispose()". Basically it expands to:
{
var obj = ...
try {
// some code
} finally {
if(obj != null) obj.Dispose();
}
}
The other is foreach, where-by the iterator is disposed - although it gets a bit complicated, because IEnumerator doesn't specify that IDisposable is required (by contrast, IEnumerator<T> does specify that), and technically IEnumerable is not even required for foreach, but basically:
foreach(var item in sequence) {
// some code
}
could be expressed as (although the spec may say it slightly differently):
{
var iter = sequence.GetEnumerator();
using(iter as IDisposable)
{
while(iter.MoveNext())
{ // note that before C# 5, "item" is declared *outside* the while
var item = iter.Current;
// some code
}
}
}
In all other cases, disposing resources is your responsibility.
If you don't ensure that Dispose() is called, then nothing will happen until GC eventually collects the object; if there is a finalizer (there doesn't have to be, and usually isn't), the finalizer will be invoked - but that is different to Dispose(). It may (depending on the per-type implementation) end up doing the same thing as Dispose(): but it may not.
If you want the CLR to call the Dispose method, it is usually recommended to implement the so called Dispose pattern in your class. However, I would not rely on CLR and wouldn't wait for GC to collect an object and use the using clause instead.
Related
Consider the following code:
public Bar GetBar()
{
using(var foo = new Foo())
{
return foo.Bar;
}
}
Is this leaky, or is foo.Dispose() called? Is it possible that the return value is invalid as a result of foo being disposed?
The answers here are close but not complete and are inconsistent.
Is this leaky, or is foo.Dispose() called?
Dispose is called. The using block gets converted to a try/finally block in which foo is disposed of in the finally section. finally will be called after the try completes (whether by exception or naturally) and before returning to the caller. So Dispose will be called in virtually every situation (barring very serious exceptions such as out of memory, thread aborts, etc.)
Is it possible that the return value is invalid as a result of foo being disposed?
Sure, if Dispose does something to invalidate the object that Bar references, then yes, it certainly could be returning an "invalid" reference. But that would have to be explicit within Foo. Disposing of an object does not automatically dispose of all properties.
Is this leaky
No. This is good coding practice for almost any object implementing IDisposable because it ensures that the dispose method is called as soon as the variable being 'used' goes out of scope (eg. the code leaves the using block). There are a few exceptions (WCF clients, custom code that doesn't follow good IDisposable practices), but it is a good idea to wrap ANY IDisposable in a using block unless you have a specific reason not to.
Is it possible that the return value is invalid as a result of foo being disposed
This depends on what the Dispose method in Foo does. For example, if you try to invoke a SQLCommand that references a SQLConnection that has already been closed/disposed, you are going to get an exception. Lets look at an example that makes this behavior obvious.
public class Foo : IDisposable
{
public Foo()
{
ComplexType = new ComplexType();
}
public ComplexType ComplexType { get; set; }
public void Dispose()
{
ComplexType = null;
GC.Collect();
}
}
Now this piece of code that accesses our Foo:
static void Main(string[] args)
{
Foo foo;
ComplexType complexType;
using (var newFoo = new Foo())
{
foo = newFoo;
complexType = newFoo.ComplexType;
}
Console.WriteLine(complexType.SomeProperty); // This works :)
Console.WriteLine(foo.ComplexType.SomeProperty); // Throws an exception because ComplexType is NULL
Console.ReadKey();
}
Weird right? The reason this happens is because in the Foo constructor we create a new ComplexType and store it at a memory address. The Foo.ComplexType property contains a reference to the memory address. When we call dispose, we set the reference to null, but the actual object is not garbage collected because we have other references to it in our calling code, so we can't access it through the foo.ComplexType property anymore, but it is still accessible via the complexType variable. Note as well that foo is NOT null even though it has been assigned to an object that has been disposed. Because a reference still exists to our Foo instance, despite being outside of the using block, it is still alive since it can't be collected while that reference exists.
Now, if the Dispose method changed SomeProperty, it is possible (depending on how it was changed), that the change COULD propagate out and invalidate results.
I guess the moral of the story is that you could experience (or create) all sorts of weird behavior if you start playing around with objects that have been disposed, but that depends on what the disposed objects are doing during disposal. I wouldn't recommend it as a practice since most objects aren't intended to be used after disposal. Perform your "work" as an atomic unit inside the using block, and then let the object die.
I've created a class BinaryTree<T>. I wrote the following to dispose it:
public override void Dispose()
{
this.Key = null;
//delete this.Value;
if(this.LeftLeaf != null) this.LeftLeaf.Dispose();
this.LeftLeaf = null;
if(this.RightLeaf != null) this.RightLeaf.Dispose();
this.RightLeaf = null;
base.Dispose();
}
My question is how do I mark the "T Value" as ready for disposal? I can't set it to null because you can't set generics to null. T could be a giangantic class, but it may not be using IDisposable so you can't call Dispose on it. Since this is "safe" class written like an "unsafe" class I don't trust GC to figure out it's not being used anymore.
If for some reason you're writing a generic collection in which you know that the elements of your collection will implement IDisposable and the collection will be responsible for disposing of them (which, generally is something you would not expect of the collection) then you should be adding the generic constraint where T : IDisposable so that you can dispose of all of the items.
When it comes to normal collection of managed types, as opposed to unmanaged resources, you don't need to do anything. There's no need to set the fields to null, or the value to null. If the entire collection isn't referenced anywhere then the GC will collect it, and any values not referenced anywhere, even without you nulling out all fields.
I don't trust GC to figure out it's not being used anymore
That's going to make C# pretty much unusable for you as a language. By design, not only do you not have to manually release managed resources, you can't even if you wanted to. There is no way for you to force a managed object to be cleaned up immediately. You can only manually clean up unmanaged resources. If, for some reason, this is unacceptable for your application and you must have complete control over the memory management of your program you'll want to use some other language with more explicit memory management (or perhaps write some particularly sensitive parts in another language, and interop with C# for the rest).
This will work for all T's (classes or structs):
public void DisposeIfApplicable(ref T value)
{
if(value is IDisposable)
{
((IDisposable)value).Dispose();
}
}
or just inline it as
if(this.LeftLeaf is IDisposable)
{
((IDisposable)this.LeftLeaf).Dispose();
}
if(this.RightLeaf is IDisposable)
{
((IDisposable)this.RightLeaf).Dispose();
}
Edit:
I didn't have the ref for the DisposeIfApplicable function, thus any structs will stay un-disposed (the passed copy would get disposed, though).
Most generic abstract data type implementations (e.g. List, Dictionary) don't implement IDisposable nor do they dispose of the contained objects, since that's "outside of the scope" of what that type should be doing. As such, in your class I would simply do:
this.LeftLeaf = default(T);
To remove the references, or:
this.LeftLeaf = null;
If you specify in the generic constraints that where T : class
It really should be the higher level consumer of your BinaryTree class that would worry about disposing of anything contained in this tree.
If you want to be able to set a T value to null, you can specify a class constraint on your T generic. That will force your tree to only accept classes for your T generic parameter. You can also use as IDisposable with null checks to see if you can call .Dispose():
public class MyBinaryTree<T>
where T : class
{
/* ... class code here ... */
public override void Dispose()
{
this.Key = null;
//delete this.Value;
var leftLeafDisposable = this.LeftLeaf as IDisposable;
if(leftLeafDisposable != null) leftLeafDisposable.Dispose();
this.LeftLeaf = null;
var rightLeafDisposable = this.LeftLeaf as IDisposable;
if(rightLeafDisposable != null) rightLeafDisposable.Dispose();
this.RightLeaf = null;
base.Dispose();
}
}
There are very few times code should check if something implements IDisposable and dispose it if so; in nearly every case, the requirement stems from a factory method whose return type should have implemented IDisposable but didn't (e.g. IEnumerator.GetEnumerator). Unless one is calling such a factory, one should not call Dispose on objects one receives but is not particularly expecting to hold resources, even the actual object instances happen to implement IDisposable.
The compiler-generated types for C# iterators, for example, will implement IDisposable; calling Dispose on an iterator during the lifetime of the first enumerator it returns from a GetEnumerator call will invalidate that enumerator and force any finally blocks within it to run. The call won't invalidate the iterator, nor will it affect any enumerators other than the first. The fact that the type of iterator instances implements IDisposable is an implementation detail rather than part of their contract, and client code should ignore it.
Rereading your question, I think I may have misunderstood it previously. If something which holds a T (e.g. an array of type T[]) is needed, but a particular T within it is not, you should set that particular T to default(T).
Does the using statement always dispose the object, even if there is a return or an exception is thrown inside it? I.E.:
using (var myClassInstance = new MyClass())
{
// ...
return;
}
or
using (var myClassInstance = new MyClass())
{
// ...
throw new UnexplainedAndAnnoyingException();
}
Yes, that's the whole point. It compiles down to:
SomeDisposableType obj = new SomeDisposableType();
try
{
// use obj
}
finally
{
if (obj != null)
((IDisposable)obj).Dispose();
}
Be careful about your terminology here; the object itself is not deallocated. The Dispose() method is called and, typically, unmanaged resources are released.
No it doesn't.
But that's not the fault of using statement though. It's because how the finally blocks are handled by CLR. There ARE some cases that finally blocks will not execute. If you have an unhandled exception and if the CLR thinks that executing more code will lead to more errors, it will not execute Dispose method (because it will not execute the finally block which Dispose method is compiled down to..). Therefore, be very careful and don't put your life into the execution of Dispose method.
The other cases which can cause Dispose method not being executed can be listed as:
Environment.FailFast
OutOfMemoryExceptionand StackOverflowException
Killing the process
Power loss
If the object implements IDisposable, it will be called.
From using Statement (C# Reference) by MSDN
Defines a scope, outside of which an object or objects will be
disposed.
The using statement allows the programmer to specify when objects that
use resources should release them. The object provided to the using
statement must implement the IDisposable interface. This interface
provides the Dispose method, which should release the object's
resources.
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).