Is there any sense to set custom object to null(Nothing in VB.NET) in the Dispose() method?
Could this prevent memory leaks or it's useless?!
Let's consider two examples:
public class Foo : IDisposable
{
private Bar bar; // standard custom .NET object
public Foo(Bar bar) {
this.bar = bar;
}
public void Dispose() {
bar = null; // any sense?
}
}
public class Foo : RichTextBox
{
// this could be also: GDI+, TCP socket, SQl Connection, other "heavy" object
private Bitmap backImage;
public Foo(Bitmap backImage) {
this.backImage = backImage;
}
protected override void Dispose(bool disposing) {
if (disposing) {
backImage = null; // any sense?
}
}
}
Personally I tend to; for two reasons:
it means that if somebody has forgotten to release the Foo (perhaps from an event) any downstream objects (a Bitmap in this case) can still be collected (at some point in the future - whenever the GC feels like it); it is likely that this is just a shallow wrapper around an unmanaged resource, but every little helps.
I really don't like accidentally keeping an entire object graph hanging around just because the user forgot to unhook one event; IDisposable is a handy "almost-kill" switch - why not detach everything available?
more importantly, I can cheekily now use this field to check (in methods etc) for disposal, throwing an ObjectDisposedException if it is null
The purpose of Dispose() is to allow clean up of resources that are not handled by the garbage collector. Objects are taken care of by GC, so there's really no need to set the reference to null under normal circumstances.
The exception is if you expect the caller to call Dispose and hold on to the instance after that. In that case, it can be a good idea to set the internal reference to null. However, disposable instances are typically disposed and released at the same time. In these cases it will not make a big difference.
It's just about useless. Setting to NULL back in the old COM/VB days, I believe, would decrement your reference count.
That's not true with .NET. When you set bar to null, you aren't destroying or releasing anything. You're just changing the reference that bar points to, from your object to "null". Your object still exists (though now, since nothing refers to it, it will eventually be garbage collected). With few exceptions, and in most cases, this is the same thing that would have happened had you just not made Foo IDisposable in the first place.
The big purpose of IDisposable is to allow you to release unmanaged resources, like TCP sockets or SQL connections, or whatever. This is usually done by calling whatever cleanup function the unmanaged resource provides, not by setting the reference to "null".
This can make sense if you want to somehow prevent the disposed owned instance to be re-used.
When you set references to disposable fields to null, you are guaranteed to not use the instances any more.
You will not get ObjectDisposedException or any other invalid state caused by using owned disposed instance (you may get NullReferenceException if you do not check for nulls).
This may not make sense to you as long as all IDisposable objects have a IsDisposed property and/or throw ObjectDisposedException if they are used after they are disposed - some may violate this principle and setting them to null may prevent unwanted effects from occuring.
In C# setting an object to null is just release the reference to the object.
So, it is theoretically better to release the reference on managed objects in a Dispose-Method in C#, but only for the possibility to the GC to collect the referenced object before the disposed object is collected. Since both will most likely be collected in the same run, the GC will most propably recognize, that the referenced object is only referenced by a disposed type, so both can be collected.
Also the need to release the reference is very small, since all public members of your disposable class should throw an exception if the class is alreay disposed. So no access to your referenced object would success after disposing the referenced method.
In VB.NET there is sense to set to Nothing declared Private WithEvents objects.
The handlers using Handles keyword will be removed in this way from these objects.
The purpose of the dispose() is to cleanup the resources that are unmanaged. TCP connections, database connections and other database objects and lots of such unmanaged resources are supposed to be released by the developer in the dispose method. So it really makes sense.
In general no need to set to null. But suppose you have a Reset functionality in your class.
Then you might do, because you do not want to call dispose twice, since some of the Dispose may not be implemented correctly and throw System.ObjectDisposed exception.
private void Reset()
{
if(_dataset != null)
{
_dataset.Dispose();
_dataset = null;
}
//..More such member variables like oracle connection etc. _oraConnection
}
Related
NET for a long time now and have started to learn C#. One thing I suppose I may have asked years ago, got the answer but have completely forgotten it now as it is not something I implicitly use a lot is destructors. As I am going through learning C# I read an article about how to create these in C# however it has left me wondering. Let say I instantiate a class which has an object to another class.
Class C1
{
// Do something here
}
Class A
{
C1 objObjectToClass1 = new C1();
}
Class Main
{
A objObjectToClassA = new A();
}
and I make the object objObjectToClassA to null as I have been lead to believe that is the equivalent to object = nothing in VB.NET.
objObectToClassA = null;
Does this action also destroy objObjectToClass1?
Not as such, no. An object will be reclaimed by the garbage collector some time after it has become eligible for collection. This may be after you clear the last reference to it, but it could already be before if you never need the reference anymore after a certain point. But generally, setting a field where you store the instance to null will help the object becoming no longer reachable and getting reclaimed.
Generally you have no control over when objects are reclaimed by the GC. You can write finalizers which are methods that are called prior to reclaiming an object, but I'd very much not recommend it if you can help it. If you need a predictable way of causing an object to release any resources it might hold on to (what destructors in C++ often do), then implement the IDisposable interface:
class C1 : IDisposable {
public void Dispose() {
// Do cleanup here
}
}
This also enables you to use instances of that class in a using statement, which will call Dispose at the end of its block:
using (var c1 = new C1()) {
// do stuf with c1 here
} // at this point c1.Dispose() is automatically called
The garbage collector knows when there are no references any more to objects, and as far as I know, it even destroys objects that are only referenced by another.
That means that if you dereference objObjectToClassA (set it to null), that both objects will get destroyed, if there are no more references to either of the objects. Simply letting it go out of scope is enough too.
In effect, yes it will also destroy objectToClass1, but not immediately. In this case, setting the variable to null means that your application is no longer using that object, and hence it's eligible for garbage collection. Thinking about it simplistically (I'm sure the GC is smarter than this), once objectToClassA is collected then objectToClass1 is no longer referenced and will also be collected.
Joey's comments about IDisposable are definitely worth bearing in mind; try not to think in terms of finalisers for C# as you don't have control over when they run. Using IDisposable will give you the control that you need in order to tidy up resources.
Destroy is the wrong word, C# (as far as I know) does not have destructors in the C++ sense. No longer used objects are collected/"destroyed" by the garbage collector.
If no other reference to objObjectToClass1 is kept, objObjectToClass1 can also be collected if you set objObectToClassA to null
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.
Assuming i have the following classes:
Class MainClass
{
private OtherClass1;
MainClass()
{
OtherClass1 = new OtherClass1();
}
void dispose()
{
OtherClass1 = null;
}
}
class OtherClass1
{
private OtherClass2;
OtherClass1()
{
OtherClass2 = new OtherClass2();
}
}
class OtherClass2
{
}
If i instatiate MainClass and later call dispose method, does the OtherClass1 gets garbage collected (later on)? Or do i have first to clear the reference to OtherClass2?
An object will get garbage collected if it has no references, or the references it does have are from objects that themselves don't have references (and so on).
A way of visualising it, is the garbage collector will walk the object reference graph, following all object references, making a note of ones it gets to (still referenced from somewhere). Any it doesn't get to are eligible for garbage collection as if it didn't get to them then they can't possibly be used.
See here for in-depth info (particularly "The Garbage Collection Algorithm"): http://msdn.microsoft.com/en-us/magazine/bb985010.aspx
So yes, it'll be eligible to be GC'd.
Also, if you have a dispose method you really should implement IDisposable.
In the code as provided, you don't have to null anything, you can safely remove your dispose() and all will be well.
If your OtherClass1 and/or OtherClass2 are managed resources, ie they implement the IDisposable interface then your code is not good enough. You then will have to chain the Dispose:
class MainClass : IDisposable
{
private OtherClass1;
MainClass()
{
OtherClass1 = new OtherClass1();
}
public void Dispose()
{
OtherClass1.Dispose();
// OtherClass1 = null; // not needed
}
}
If there is no other reference to it, it may at some time be garbage collected. Note that this is not deterministic, you cannot rely on it being collected in a specific timespan.
In general, you shouldn't worry too much abot this, the GC in .NET can by design handle circular references etc. without any problem. Setting fields to null is usually not required. The Dispose method is usually used to release unmanaged resources, such as database connections etc. in a deterministic fashion; it's not about freeing the memory of the object being disposed.
The best practice is to implement IDisposable interface and implementing the Dispose() method.
At Dispose(), you just release the resources used by your object such as any external resources, COM references, database connections, etc.
In terms of When the object will be garbage collected, it's up to the .NET engine to decide that as they frequently update their disposal algorithm with each release.
In general, when an object is orphan (no variable references it), it will be in the queue to be garbage collected.
You can manually call GC.Collect(); but that's not recommended since it interferes .NET garbage collection mechanism.
The term "Dispose" is a bit of a misnomer, since the Dispose method doesn't delete the targeted object but rather serves a request for the targeted object to do anything that will need to be done before it may be safely abandoned. Essentially, it's a request for the object to put its affairs in order.
The most common situation when a particular object will need to put its affairs in order is when some entities outside of it may be doing something, storing something, refraining from doing something, or otherwise temporarily altering their behavior on its behalf. Note that the entities may be .net objects, other types of OS-recognized objects (GDI handles, etc.), etc. but there's no particular requirement that the entity be any particular kind of thing, nor that they be in the same computer, or even any computer. For an object to puts its affairs in order, outside entities doing, holding, etc. anything on its behalf need to be told that they no longer need to do so. If the entities in question are .net objects that implement IDisposable, the notification would be generally performed by calling their Dispose method.
Note that .net provides a means by which objects can ask to be notified if the system notices that they've been abandoned, and use that as a cue to put their affairs in order. Such notifications may not come in timely fashion, and various factors may cause them to be delayed essentially indefinitely, but the mechanism (called "finalization") is sometimes better than nothing.
How can I make sure that the objects get disposed that I am adding into the SerializationInfo object?
For example: if I am adding a hashtable to the info object, how do I make sure that after serialization, there is no reference alive to the hastable object of my class and I can free the memory somehow?
info.AddValue("attributesHash", attributesHash, typeof(System.Collections.Hashtable));
Update (code)
[Serializable]
class A : System.Runtime.Serialization.ISerializable
{
List<int> m_Data = new List<int>();
//...
public void GetObjectData(System.Runtime.Serialization.SerializationInfo info, System.Runtime.Serialization.StreamingContext context)
{
if (m_Data != null)
{
info.AddValue("MyData", m_Data, typeof(List<int>));
m_Data.Clear();
}
m_Data = null; // <-- Will this be collected by GC?
GC.GetTotalMemory(true); //forced collection
}
};
My question is: if i make my object null after adding to the info list, will it be freed after serialization is called (when info is destroyed - I hope), or in the line when GC's function is called (which I don't think so)?
If setting 'm_Data = null' will not mark it as garbage, then how would I know that the memory occupied by m_Data has been freed or not?
"I can free the memory somehow" and "objects get disposed" do not go along very well.
Memory management is done by the GC. As soon as there is no more reference to an object, it gets flagged for garbage collection. So it deals with managed resources.
Disposing, however, is totall different animal and is calling Dispose() on types implementing IDisposable and deals with unmanaged resources such as file handles and windows resources.
You need to make it clear which you one you mean.
You need to make your containing object implement IDisposable. But just because you have become serialized does not mean that you need to also be disposed. The referencing object should then call dispose after serialization if that is what is expected.
As for the deserialized object, it should also be disposed by whatever references it when it is done being used (presumably in another appdomain?). So what this means is that both instances will need to be disposed. If the resource you share is a single instance (such as an IntPtr) then you might need to be more clever about it, such as not disposing of that unmanaged resource with this object but from a higher level.
General rule of thumb: he who creates it, disposes it.
One other common pattern, as described by the IDisposable documentation is to put a call to Dispose() into your objects destructor. This will give you non-deterministic timing for your disposal but is guaranteed to work (assuming you don't have reference leaks).
After much analysis, I don't think GC collects memory when requested... not in the case of GetTotalMemory function, at least.
Setting the object to null marks it as garbage, but it doesn't mean it will be collected immediately.
I am working on a class that deals with a lot of Sql objects - Connection, Command, DataAdapter, CommandBuilder, etc. There are multiple instances where we have code like this:
if( command != null )
{
command.Dispose();
}
if( dataAdapter != null )
{
dataAdapter.Dispose();
}
etc
I know this is fairly insufficient in terms of duplication, but it has started smelling. The reason why I think it smells is because in some instances the object is also set to null.
if( command != null )
{
command.Dispose();
command = null;
}
I would love to get rid of the duplication if possible. I have come up with this generic method to dispose of an object and set it to null.
private void DisposeObject<TDisposable>( ref TDisposable disposableObject )
where TDisposable : class, IDisposable
{
if( disposableObject != null )
{
disposableObject.Dispose();
disposableObject = null;
}
}
My questions are...
Is this generic function a bad idea?
Is it necessary to set the object to null?
EDIT:
I am aware of the using statement, however I cannot always use it because I have some member variables that need to persist longer than one call. For example the connection and transaction objects.
Thanks!
You should consider if you can use the using statement.
using (SqlCommand command = ...)
{
// ...
}
This ensures that Dispose is called on the command object when control leaves the scope of the using. This has a number of advantages over writing the clean up code yourself as you have done:
It's more concise.
The variable will never be set to null - it is declared and initialized in one statement.
The variable goes out of scope when the object is disposed so you reduce the risk of accidentally trying to access a disposed resource.
It is exception safe.
If you nest using statements then the resources are naturally disposed in the correct order (the reverse order that they were created in).
Is it necessary to set the object to null?
It is not usually necessary to set variables to null when you have finished using them. The important thing is that you call Dispose when you have finished using the resource. If you use the above pattern, it is not only unnecessary to set the variable to null - it will give a compile error:
Cannot assign to 'c' because it is a 'using variable'
One thing to note is that using only works if an object is acquired and disposed in the same method call. You cannot use this pattern is if your resources need to stay alive for the duration of more than one method call. In this case you might want to make your class implement IDisposable and make sure the resources are cleaned up when your Dispose method is called. I this case you will need code like you have written. Setting variables to null is not wrong in this case but it is not important because the garbage collector will clean up the memory correctly anyway. The important thing is to make sure all the resources you own are disposed when your dispose method is called, and you are doing that.
A couple of implementation details:
You should ensure that if your Dispose is called twice that it does not throw an exception. Your utility function handles this case correctly.
You should ensure that the relevant methods on your object raise an ObjectDisposedException if your object has already been disposed.
You should implement IDisposable in the class that owns these fields. See my blog post on the subject. If this doesn't work well, then the class is not following OOP principles, and needs to be refactored.
It is not necessary to set variables to null after disposing them.
If you're objects have a lot of cleanup to do, they might want to track what needs to be deleted in a separate list of disposables, and handle all of them at once. Then at teardown it doesn't need to remember everything that needs to be disposed (nor does it need to check for null, it just looks in the list).
This probably doesn't build, but for expanatory purposes, you could include a RecycleBin in your class. Then the class only needs to dispose the bin.
public class RecycleBin : IDisposable
{
private List<IDisposable> _toDispose = new List<IDisposable>();
public void RememberToDispose(IDisposable disposable)
{
_toDispose.Add(disposable);
}
public void Dispose()
{
foreach(var d in _toDispose)
d.Dispose();
_toDispose.Clear();
}
}
I assume these are fields and not local variables, hence why the using keyword doesn't make sense.
Is this generic function a bad idea?
I think it's a good idea, and I've used a similar function a few times; +1 for making it generic.
Is it necessary to set the object to null?
Technically an object should allow multiple calls to its Dispose method. (For instance, this happens if an object is resurrected during finalization.) In practice, it's up to you whether you trust the authors of these classes or whether you want to code defensively. Personally, I check for null, then set references to null afterwards.
Edit: If this code is inside your own object's Dispose method then failing to set references to null won't leak memory. Instead, it's handy as a defence against double disposal.
I'm going to assume that you are creating the resource in one method, disposing it in another, and using it in one or more others, making the using statement useless for you.
In which case, your method is perfectly fine.
As far the second part of your question ("Is setting it to null necessary?"), the simple answer is "No, but it's not hurting anything".
Most objects hold one resource -- memory, which the Garbage collection deals with freeing, so we don't have to worry about it. Some hold some other resource as well : a file handle, a database connection, etc. For the second category, we must implement IDisposable, to free up that other resource.
Once the Dispose method is called, both categories because the same : they are holding memory. In this case, we can just let the variable go out of scope, dropping the reference to the memory, and allowing to GC to free it eventually -- Or we can force the issue, by setting the variable to null, and explicitly dropping the reference to the memory. We still have to wait until the GC kicks in for the memory to actually be freed, and more than likely the variable will go out of scope anyway, moments after to set it to null, so in the vast majority of cases, it will have no effect at all, but in a few rare cases, it will allow the memory to be freed a few seconds earlier.
However, if your specific case, where you are checking for null to see if you should call Dispose at all, you probably should set it to null, if there is a chance you might call Dispose() twice.
Given that iDisposable does not include any standard way of determining whether an object has been disposed, I like to set things to null when I dispose of them. To be sure, disposing of an object which has already been disposed is harmless, but it's nice to be able to examine an object in a watch window and tell at a glance which fields have been disposed. It's also nice to be able to have code test to ensure that objects which should have been disposed, are (assuming the code adheres to the convention of nulling out variables when disposing of them, and at no other time).
Why don't you use using C# construct? http://msdn.microsoft.com/en-us/library/yh598w02.aspx
Setting to null if not required.
Others have recommended the using construct, which I also recommend. However, I'd like to point out that, even if you do need a utility method, it's completely unnecessary to make it generic in the way that you've done. Simply declare your method to take an IDisposable:
private static void DisposeObject( ref IDisposable disposableObject )
{
if( disposableObject != null )
{
disposableObject.Dispose();
disposableObject = null;
}
}
You never need to set variables to null. The whole point of IDisposable.Dispose is to get the object into a state whereby it can hang around in memory harmlessly until the GC finalises it, so you just "dispose and forget".
I'm curious as to why you think you can't use the using statement. Creating an object in one method and disposing it in a different method is a really big code smell in my book, because you soon lose track of what's open where. Better to refactor the code like this:
using(var xxx = whatever()) {
LotsOfProcessing(xxx);
EvenMoreProcessing(xxx);
NowUseItAgain(xxx);
}
I'm sure there is a standard pattern name for this, but I just call it "destroy everything you create, but nothing else".
I answered this in my question and answer here:
Implementing IDisposable (the Disposable Pattern) as a service (class member)
It implemented a simplistic, reusable component that works for any IDisposable member.