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.
Related
Should you set all the objects to null (Nothing in VB.NET) once you have finished with them?
I understand that in .NET it is essential to dispose of any instances of objects that implement the IDisposable interface to release some resources although the object can still be something after it is disposed (hence the isDisposed property in forms), so I assume it can still reside in memory or at least in part?
I also know that when an object goes out of scope it is then marked for collection ready for the next pass of the garbage collector (although this may take time).
So with this in mind will setting it to null speed up the system releasing the memory as it does not have to work out that it is no longer in scope and are they any bad side effects?
MSDN articles never do this in examples and currently I do this as I cannot
see the harm. However I have come across a mixture of opinions so any comments are useful.
Karl is absolutely correct, there is no need to set objects to null after use. If an object implements IDisposable, just make sure you call IDisposable.Dispose() when you're done with that object (wrapped in a try..finally, or, a using() block). But even if you don't remember to call Dispose(), the finaliser method on the object should be calling Dispose() for you.
I thought this was a good treatment:
Digging into IDisposable
and this
Understanding IDisposable
There isn't any point in trying to second guess the GC and its management strategies because it's self tuning and opaque. There was a good discussion about the inner workings with Jeffrey Richter on Dot Net Rocks here: Jeffrey Richter on the Windows Memory Model and
Richters book CLR via C# chapter 20 has a great treatment:
Another reason to avoid setting objects to null when you are done with them is that it can actually keep them alive for longer.
e.g.
void foo()
{
var someType = new SomeType();
someType.DoSomething();
// someType is now eligible for garbage collection
// ... rest of method not using 'someType' ...
}
will allow the object referred by someType to be GC'd after the call to "DoSomething" but
void foo()
{
var someType = new SomeType();
someType.DoSomething();
// someType is NOT eligible for garbage collection yet
// because that variable is used at the end of the method
// ... rest of method not using 'someType' ...
someType = null;
}
may sometimes keep the object alive until the end of the method. The JIT will usually optimized away the assignment to null, so both bits of code end up being the same.
No don't null objects. You can check out https://web.archive.org/web/20160325050833/http://codebetter.com/karlseguin/2008/04/28/foundations-of-programming-pt-7-back-to-basics-memory/ for more information, but setting things to null won't do anything, except dirty your code.
Also:
using(SomeObject object = new SomeObject())
{
// do stuff with the object
}
// the object will be disposed of
In general, there's no need to null objects after use, but in some cases I find it's a good practice.
If an object implements IDisposable and is stored in a field, I think it's good to null it, just to avoid using the disposed object. The bugs of the following sort can be painful:
this.myField.Dispose();
// ... at some later time
this.myField.DoSomething();
It's good to null the field after disposing it, and get a NullPtrEx right at the line where the field is used again. Otherwise, you might run into some cryptic bug down the line (depending on exactly what DoSomething does).
Chances are that your code is not structured tightly enough if you feel the need to null variables.
There are a number of ways to limit the scope of a variable:
As mentioned by Steve Tranby
using(SomeObject object = new SomeObject())
{
// do stuff with the object
}
// the object will be disposed of
Similarly, you can simply use curly brackets:
{
// Declare the variable and use it
SomeObject object = new SomeObject()
}
// The variable is no longer available
I find that using curly brackets without any "heading" to really clean out the code and help make it more understandable.
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
}
The only time you should set a variable to null is when the variable does not go out of scope and you no longer need the data associated with it. Otherwise there is no need.
this kind of "there is no need to set objects to null after use" is not entirely accurate. There are times you need to NULL the variable after disposing it.
Yes, you should ALWAYS call .Dispose() or .Close() on anything that has it when you are done. Be it file handles, database connections or disposable objects.
Separate from that is the very practical pattern of LazyLoad.
Say I have and instantiated ObjA of class A. Class A has a public property called PropB of class B.
Internally, PropB uses the private variable of _B and defaults to null. When PropB.Get() is used, it checks to see if _PropB is null and if it is, opens the resources needed to instantiate a B into _PropB. It then returns _PropB.
To my experience, this is a really useful trick.
Where the need to null comes in is if you reset or change A in some way that the contents of _PropB were the child of the previous values of A, you will need to Dispose AND null out _PropB so LazyLoad can reset to fetch the right value IF the code requires it.
If you only do _PropB.Dispose() and shortly after expect the null check for LazyLoad to succeed, it won't be null, and you'll be looking at stale data. In effect, you must null it after Dispose() just to be sure.
I sure wish it were otherwise, but I've got code right now exhibiting this behavior after a Dispose() on a _PropB and outside of the calling function that did the Dispose (and thus almost out of scope), the private prop still isn't null, and the stale data is still there.
Eventually, the disposed property will null out, but that's been non-deterministic from my perspective.
The core reason, as dbkk alludes is that the parent container (ObjA with PropB) is keeping the instance of _PropB in scope, despite the Dispose().
Stephen Cleary explains very well in this post: Should I Set Variables to Null to Assist Garbage Collection?
Says:
The Short Answer, for the Impatient
Yes, if the variable is a static field, or if you are writing an enumerable method (using yield return) or an asynchronous method (using async and await). Otherwise, no.
This means that in regular methods (non-enumerable and non-asynchronous), you do not set local variables, method parameters, or instance fields to null.
(Even if you’re implementing IDisposable.Dispose, you still should not set variables to null).
The important thing that we should consider is Static Fields.
Static fields are always root objects, so they are always considered “alive” by the garbage collector. If a static field references an object that is no longer needed, it should be set to null so that the garbage collector will treat it as eligible for collection.
Setting static fields to null is meaningless if the entire process is shutting down. The entire heap is about to be garbage collected at that point, including all the root objects.
Conclusion:
Static fields; that’s about it. Anything else is a waste of time.
There are some cases where it makes sense to null references. For instance, when you're writing a collection--like a priority queue--and by your contract, you shouldn't be keeping those objects alive for the client after the client has removed them from the queue.
But this sort of thing only matters in long lived collections. If the queue's not going to survive the end of the function it was created in, then it matters a whole lot less.
On a whole, you really shouldn't bother. Let the compiler and GC do their jobs so you can do yours.
Take a look at this article as well: http://www.codeproject.com/KB/cs/idisposable.aspx
For the most part, setting an object to null has no effect. The only time you should be sure to do so is if you are working with a "large object", which is one larger than 84K in size (such as bitmaps).
I believe by design of the GC implementors, you can't speed up GC with nullification. I'm sure they'd prefer you not worry yourself with how/when GC runs -- treat it like this ubiquitous Being protecting and watching over and out for you...(bows head down, raises fist to the sky)...
Personally, I often explicitly set variables to null when I'm done with them as a form of self documentation. I don't declare, use, then set to null later -- I null immediately after they're no longer needed. I'm saying, explicitly, "I'm officially done with you...be gone..."
Is nullifying necessary in a GC'd language? No. Is it helpful for the GC? Maybe yes, maybe no, don't know for certain, by design I really can't control it, and regardless of today's answer with this version or that, future GC implementations could change the answer beyond my control. Plus if/when nulling is optimized out it's little more than a fancy comment if you will.
I figure if it makes my intent clearer to the next poor fool who follows in my footsteps, and if it "might" potentially help GC sometimes, then it's worth it to me. Mostly it makes me feel tidy and clear, and Mongo likes to feel tidy and clear. :)
I look at it like this: Programming languages exist to let people give other people an idea of intent and a compiler a job request of what to do -- the compiler converts that request into a different language (sometimes several) for a CPU -- the CPU(s) could give a hoot what language you used, your tab settings, comments, stylistic emphases, variable names, etc. -- a CPU's all about the bit stream that tells it what registers and opcodes and memory locations to twiddle. Many things written in code don't convert into what's consumed by the CPU in the sequence we specified. Our C, C++, C#, Lisp, Babel, assembler or whatever is theory rather than reality, written as a statement of work. What you see is not what you get, yes, even in assembler language.
I do understand the mindset of "unnecessary things" (like blank lines) "are nothing but noise and clutter up code." That was me earlier in my career; I totally get that. At this juncture I lean toward that which makes code clearer. It's not like I'm adding even 50 lines of "noise" to my programs -- it's a few lines here or there.
There are exceptions to any rule. In scenarios with volatile memory, static memory, race conditions, singletons, usage of "stale" data and all that kind of rot, that's different: you NEED to manage your own memory, locking and nullifying as apropos because the memory is not part of the GC'd Universe -- hopefully everyone understands that. The rest of the time with GC'd languages it's a matter of style rather than necessity or a guaranteed performance boost.
At the end of the day make sure you understand what is eligible for GC and what's not; lock, dispose, and nullify appropriately; wax on, wax off; breathe in, breathe out; and for everything else I say: If it feels good, do it. Your mileage may vary...as it should...
I think setting something back to null is messy. Imagine a scenario where the item being set to now is exposed say via property. Now is somehow some piece of code accidentally uses this property after the item is disposed you will get a null reference exception which requires some investigation to figure out exactly what is going on.
I believe framework disposables will allows throw ObjectDisposedException which is more meaningful. Not setting these back to null would be better then for that reason.
Some object suppose the .dispose() method which forces the resource to be removed from memory.
Lets say I have a class that associates itself with another class. It would look something like the following:
public class DisposableClassOne : IDisposable
{
private class mDisposableClassTwo;
public DisplosableClassOne(DisposableClassTwo dcTwoInjected)
{
mDisposableClassTwo = dcTwoInjected;
}
public void Dispose()
{
// Should I dispose here? or make caller dispose of dcTwoInjected
//mDisposableClassTwo.Dispose();
}
}
Should I call the Dispose method of mDisposableClassTwo or should I make the caller handle it like this?
using(DisposableClassTwo dcTwoInjected = new DisposableClassTwo())
using(DisposableClassOne dcOne = new DisposableClassOne(dcTwoInjected))
{
// do stuff with dcOne
}
I'm thinking making the caller handle it is the best way to go, but I think by putting the call in the Dispose method it guarantees that it will get called. Is there a better way to handle this?
If the class you are creating logically owns(1) the constructor injected resource then it should dispose of it internally. If it does not own the resource, then it should do nothing, and rely on the consumer to decide when it should be disposed.
If your class owns a reference to an unmanaged resource, you may also need to implement a finalizer (destructor) since there is no guarantee that anyone will ever call your Dispose method at all.
In general, you want to avoid cases where the caller must decide when they should dispose of an object passed to the constructor of a class. It's possible for the caller to dispose of the resource prematurely (or hold on to it longer than necessary). This is not always an easy thing to achieve in certain designs ... sadly, the Disposable Object pattern doesn't always compose cleanly with itself.
(1) By ownership I mean that your class controls the lifetime of the resource which it is passed and no other code holds a reference to it. If any other code (whose lifetime is not tied to the lifetime of your class) holds a reference to the resource and will use it independently, then you are not the owner of that resource.
I think it is a life-time decision you have to make in your design.
Is DisposableClassOne Disposable BECAUSE it references a DisposableClassTwo?
And,
Does a DisposableClassTwo have a lifetime independent of DisposableClassOne?
To me, the answers to these two questions varies in each class design. The StreamReader/Writer is a great example of the first question being 'yes' and the second 'no' - no one would expect to use a Stream inside a StreamReader once the Reader is done with it, so Reader disposes it.
But what if DisposableClassTwo was some other resource - maybe a file refernece you passed to several classes in turn to 'do something to'. In that case, you don't want it disposed until you're ready and DisposableClassOne might be long gone by then.
The standard I'm familiar with is to use the Disposable pattern. Have a virtual Dispose(bool disposing) method. The Dispose() method calls Dispose(true) and the finalizer calls Dispose(false).
Then, in the main disposal method:
if (disposing)
{
// Dispose of the referenced object, as well.
}
StreamWriter & StreamReader follow this pattern. If you explicitely call Dispose(), they also dispose the underlying Stream. If you let the finalizer do your cleanup, they leave it alone.
Alternatively, you could add a property: DisposeChild, if true, dispose the child object, if not, leave it alone.
I also agree with rcravens. Under almost all circumstances, an object should be disposed within the same scope where it was instantiated.
To use IDisposable objects properly, one must follow the principle "Would the last one out please turn off the lights". There are a few nice scenarios, and an icky one:
The creator of an object gives it to another object (the "recipient") but the creator of the first object controls the recipient's lifetime, and can know when the recipient will no longer need the passed object. In that case, the creator of the original object takes care of Dispose.
The creator of an object gives it to the recipient, and the recipient knows that nobody else will use the object once it's been handed over. In that case, the recipient will call Dispose when it's done with the object.
An object recipient may sometimes be used in ways that match #1 and sometimes in ways that match #2, but the supplier of the object will know when it's given which situation applies. This situation may be handled by telling the recipient whether or not the recipient should Dispose of the object it is given.
The supplier and recipient will both want to use the object, but may have no idea which is going to outlive the other. This is the toughest situation.
I tend to like the approach suggested in #3. Sometimes situation #4 applies, though. One way of handling that is for the recipient of the object to fire an event when it's done with the object. That event can Interlocked.Decrement a counter indicating how many objects are still using the passed object. Once that counter hits zero, the object may be deleted.
It is better not to dispose any external references, those would be disposed automatically by the caller itself.
See this thread, in which it is suggested that only member variable needs to be disposed:
Finalize/Dispose pattern in C#
The concept of "cleanup by owner" is far too common to be ignored. Ownership must be more than just coincidence.
If you want software that is easier to maintain, avoid side effects that may be incorrectly interpreted. The Stream vs StreamReader example is a great one... both are IDisposable, and as the caller, I'm making both, so in no way should I ever give myself a pass at disposing either of them. Doing so would violate the abstraction, tying my calling code's knowledge to the particulars of the stream class' implementation (which could change over time, academically).
The easiest answer is to have the two usings(), even if you then go and make (and document!) that the second IDisposable will dispose of the first.
It seems that in most cases the C# compiler could call Dispose() automatically. Like most cases of the using pattern look like:
public void SomeMethod()
{
...
using (var foo = new Foo())
{
...
}
// Foo isn't use after here (obviously).
...
}
Since foo isn't used (that's a very simple detection) and since its not provided as argument to another method (that's a supposition that applies to many use cases and can be extended), the compiler could automatically and immediately call Dispose() without the developper requiring to do it.
This means that in most cases the using is pretty useless if the compiler does some smart job. IDisposable seem low level enough to me to be taken in account by a compiler.
Now why isn't this done? Wouldn't that improve the performances (if the developpers are... dirty).
A couple of points:
Calling Dispose does not increase performance. IDisposable is designed for scenarios where you are using limited and/or unmanaged resources that cannot be accounted for by the runtime.
There is no clear and obvious mechanism as to how the compiler could treat IDisposable objects in the code. What makes it a candidate for being disposed of automatically and what doesn't? If the instance is (or could) be exposed outside of the method? There's nothing to say that just because I pass an object to another function or class that I want it to be usable beyond the scope of the method
Consider, for example, a factory patter that takes a Stream and deserializes an instance of a class.
public class Foo
{
public static Foo FromStream(System.IO.Stream stream) { ... }
}
And I call it:
Stream stream = new FileStream(path);
Foo foo = Foo.FromStream(stream);
Now, I may or may not want that Stream to be disposed of when the method exits. If Foo's factory reads all of the necessary data from the Stream and no longer needs it, then I would want it to be disposed of. If the Foo object has to hold on to the stream and use it over its lifetime, then I wouldn't want it to be disposed of.
Likewise, what about instances that are retrieved from something other than a constructor, like Control.CreateGraphics(). These instances could exist outside of the code, so the compiler wouldn't dispose of them automatically.
Giving the user control (and providing an idiom like the using block) makes the user's intention clear and makes it much easier to spot places where IDisposable instances are not being properly disposed of. If the compiler were to automatically dispose of some instances, then debugging would be that much more difficult as the developer had to decipher how the automatic disposal rules applied to each and every block of code that used an IDisposable object.
In the end, there are two reasons (by convention) for implementing IDisposable on a type.
You are using an unmanaged resource (meaning you're making a P/Invoke call that returns something like a handle that must be released by a different P/Invoke call)
Your type has instances of IDisposable that should be disposed of when this object's lifetime is over.
In the first case, all such types are supposed to implement a finalizer that calls Dispose and releases all unmanaged resources if the developer fails to do so (this is to prevent memory and handle leaks).
Garbage Collection (while not directly related to IDisposable, is what cleans up unused objects) isn't that simple.
Let me re-word this a little bit. Automatically calling Dispose() isn't that simple. It also won't directly increase performance. More on that a little later.
If you had the following code:
public void DoSomeWork(SqlCommand command)
{
SqlConnection conn = new SqlConnection(connString);
conn.Open();
command.Connection = conn;
// Rest of the work here
}
How would the compiler know when you were done using the conn object? Or if you passed a reference to some other method that was holding on to it?
Explicitly calling Dispose() or using a using block clearly states your intent and forces things to get cleaned up properly.
Now, back to performance. Simply calling Dispose() on an Object doesn't guarantee any performance increase. The Dispose() method is used for "cleaning up" resources when you're done with an Object.
The performance increase can come when using un-managed resources. If a managed object doesn't properly dispose of its un-managed resources, then you have a memory leak. Ugly stuff.
Leaving the determination to call Dispose() up to the compiler would take away that level of clarity and make debugging memory leaks caused by un-managed resources that much more difficult.
You're asking the compiler to perform a semantic analysis of your code. The fact that something isn't explicitly referenced after a certain point in the source does not mean that it isn't being used. If I create a chain of references and pass one out to a method, which may or may not store that reference in a property or some other persistent container, should I really expect the compiler to trace through all of that and figure out what I really meant?
Volatile entities may also be a concern.
Besides, using() {....} is more readable and intuitive, which is worth a lot in terms of maintainability.
As engineers or programmers, we strive to be efficient, but that is rarely the same thing as lazy.
Look at the MSDN Artilce for the C# Using Statement The using statement is just a short cut to keep from doing a try and finally in allot of places. Calling the dispose is not a low level functionality like Garbage Collection.
As you can see using is translated into.
{
Font font1 = new Font("Arial", 10.0f);
try
{
byte charset = font1.GdiCharSet;
}
finally
{
if (font1 != null)
((IDisposable)font1).Dispose();
}
}
How would the compiler know where to put the finally block? Does it call it on Garbage Collection?
Garabage Collection doesn't happen as soon as you leave a method. Read this article on Garbage Collection to understand it better. Only after there are no references to the object. A resource could be tied up for much longer than needed.
The thought that keeps popping into my head is that the compiler should not protect developers who do not clean up there resources. Just because a language is managed doesn't mean that it is going to protect from yourself.
C++ supports this; they call it "stack semantics for reference types". I support adding this to C#, but it will require different syntax (changing the semantics based on whether or not a local variable is passed to another method isn't a good idea).
I think that you are thinking about finalizers. Finalizers use the destructor syntax in c#, and they are called automatically by the garbage collector. Finalizers are only appropriate to use when you are cleaning up unmanaged resources.
Dispose is intended to allow for early cleanup of unmanaged resources (and it can be used to clean managed resources as well).
Detection is actually trickier than it looks. What if you have code like this:
var mydisposable = new...
AMethod(mydisposable);
// (not used again)
It's possible that some code in AMethod holds on to a reference to myDisposable.
Maybe it gets assigned to an instance variable inside of that method
Maybe myDisposable subscribes to an event inside of AMethod (then the event publisher holds a reference to myDisposable)
Maybe another thread is spawned by AMethod
Maybe mydisposable becomes "enclosed" by an anonymous method or lamba expression inside of AMethod.
All of those things make it difficult to know for absolute certain that your object is no longer in use, so Dispose is there to let a developer say "ok, I know that it's safe to run my cleanup code now);
Bear in mind also that dispose doesn't deallocate your object -- only the GC can do that. (The GC does have the magic to understand all of the scenarios that I described, and it knows when to clean up your object, and if you really need code to run when the GC detects no references, you can use a finalizer). Be careful with finalizers, though -- they are only for unmanaged allocations that your class owns.
You can read more about this stuff here:
http://msdn.microsoft.com/en-us/magazine/bb985010.aspx
and here: http://www.bluebytesoftware.com/blog/2005/04/08/DGUpdateDisposeFinalizationAndResourceManagement.aspx
If you need unmanaged handle cleanup, read about SafeHandles as well.
It's not the responsibility of the compiler to interpret the scopes in your application and do things like figure out when you no longer need memory. In fact, I'm pretty sure that's an impossible problem to solve, because there's no way for the compiler to know what your program will look like at runtime, no matter how smart it is.
This is why we have the garbage collection. The problem with garbage collection is that it runs on an indeterminate interval, and typically if an object implements IDisposable, the reason is because you want the ability to dispose of it immediately. Like, right now immediately. Constructs such as database connections aren't just disposable because they have some special work to do when they get trashed - it's also because they are scarce.
I seems difficult for the G.C. to know that you won't be using this variable anymore later in the same method. Obviously, if you leave the method, and don't keep a further reference to you variable, the G.C. will dispose it. But using using in you sample, tells the G.C. that you are sure that you will not be using this variable anymore after.
The using statement has nothing to do with performance (unless you consider avoiding resource/memory leaks as performance).
All it does for you is guarantee that the IDisposable.Dispose method is called on the object in question when it goes out of scope, even if an exception has occurred inside the using block.
The Dispose() method is then responsible for releasing any resources used by the object. These are most often unmanaged resources such as files, fonts, images etc, but could also be simple "clean-up" activities on managed objects (not garbage collection however).
Of course if the Dispose() method is implemented badly, the using statement provides zero benefit.
I think the OP is saying "why bother with 'using' when the compiler should be able to work it out magically pretty easily".
I think the OP is saying that
public void SomeMethod()
{
...
var foo = new Foo();
... do stuff with Foo ...
// Foo isn't use after here (obviously).
...
}
should be equivalent to
public void SomeMethod()
{
...
using (var foo = new Foo())
{
... do stuff with Foo ...
}
// Foo isn't use after here (obviously).
...
}
because Foo isn't used again.
The answer of course is that the compiler cannot work it out pretty easily. Garbage Collection (what magically calls "Dispose()" in .NET) is a very complicated field. Just because the symbol isn't being used below that doesn't mean that the variable isn't being used.
Take this example:
public void SomeMethod()
{
...
var foo = new Foo();
foo.DoStuffWith(someRandomObject);
someOtherClass.Method(foo);
// Foo isn't use after here (obviously).
// Or is it??
...
}
In this example, someRandomObject and someOtherClass might both have references to what Foo points out, so if we called Foo.Dispose() it would break them. You say you're just imagining the simple case, but the only 'simple case' where what you're proposing works is the case where you make no method calls from Foo and do not pass Foo or any of its members to anything else - effectively when you don't even use Foo at all in which case you probably have no need to declare it. Even then, you can never be sure that some kind of reflection or event hackery didn't get a reference to Foo just by its very creation, or that Foo didn't hook itself up with something else during its constructor.
In addition to the fine reasons listed above, since the problem can't be solved reliably for all cases, those "easy" cases are something that code analysis tools can and do detect. Let the compiler do stuff deterministically, and let your automatic code analysis tools tell you when you're doing something silly like forgetting to call Dispose.
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
}
This question already has answers here:
What are the uses of "using" in C#?
(29 answers)
Closed 8 years ago.
Are there particular instances where I should (or shouldn't?) be using "using" blocks:
using(SomeType t = new SomeType()){
...
}
Some objects need some action to be taken when you have finished with them. Usually this is because the object uses some kind of resource that needs to be disposed of. For example, if you have a file object of class File, and this object opens a file from the file system, the file in the file system will need to be closed again.
If you just left the file object, and forgot to call file.Close() it wouldn't be cleaned up until the Garbage Collector (GC) ran and worked out nothing was still using the file object. When the Garbage Collector runs should be left to the Common Language Runtime (CLR) to decide. If the GC doesn't run for quite a while after you have finished with the file, the file could remain open potentially for a long time. This can pose a big problem if there are many file objects, or if something wants to open a file, but can't because the file object you left is still hanging around.
To solve this problem, C# has the IDisposable interface. This has one method called Dispose. Classes that require some cleanup implement this Dispose method. This gives you a standard way for cleaning up any objects that use resources. There are a lot of classes that need to have Dispose called. The problem with this is that code gets covered with calls to Dispose, and they are tricky to follow because the place where you new'ed the object and call Dispose to clean it up are different. So, you had to look around the code a lot and be very careful to check there were calls to Dispose in the right place.
To solve this problem C# introduced the 'using' keyword. You can put a 'using' keyword around where you new an object, and this ensures Dispose will be called on it for you. It guarantees that Dispose will be called whatever happens... even if there is an exception thrown within the body of the using statement.
So, you should use 'using' when you want to be sure an object that allocates resources will be cleaned up.
using can only be used for objects that are declared on the stack, i.e. in a function. It doesn't work for objects that are declared as members of a class. For them, you have to call Dispose yourself. You may have to implement Dispose in your class so that in can call Dispose on any member objects it has that require it.
Common objects that need using called on them are: Files, Database connections, Graphics objects such as Pen and Brush.
Sometimes it is also used when you want two operations to happen together. For example if you want to write a log statement when a block of code is entered and when it exits you could write a log class that you could use like this:
using( Log log = new Log("Doing stuff") )
{
// Stuff
}
The constructor for the log class could be made to write out the message, and the Dispose method could also write it out. Implement the finalizer (~Log) to assert if the Dispose method doesn't get called to ensure the 'using' is remembered around the 'new Log'.
When the SomeType class implements IDisposable.
Use using whenever the type implements IDisposable, unless you're going to wrap it in a try/catch block anyway, then you might as well (depending on what look you prefer) use a finally block.
I see plenty of other answers indicated when you should have a using statement. I want to address when specifically should not have a using statement:
If you need to use your object outside of the scope of the current function, don't have a using block. Good example are a factory method that returns a database connection or a method that needs to return a datareader. In either of those cases if you create your object with a using statement it would be disposed before the method returned, and therefore not usable outside the method.
Now, you still want to be sure that those objects are disposed, so you still might want a using statement somewhere. Just don't include it in the method where the object is actually created. Instead, you can wrap the function call itself in a using statement.
When SomeType implements IDisposable.
That is a clue to you the developer that SomeType uses unmanaged resources that need to be cleaned up.
Example:
using(SqlConnection MyConnection = new SqlConnection("Connection string"))
{
MyConnection.Open();
//...
// 1. SQLConnection is a type that implements IDisposable
// 2. So you can use MyConnection in a using statement
// 3. When using block finishes, it calls Dispose method of
// SqlConnection class
// 4. In this case, it will probably close the connection to
// the database and dispose MyConnection object
}
You can create your own objects that implements IDisposable:
public class MyOwnObjectThatImplementsIDisposable : IDisposable
{
//... some code
public void Dispose()
{
// Put here the code you want to be executed when the
// using statement finish.
}
}
So you could use an object of MyOwnObjectThanImplementsIDisposable type in a using statement:
using(MyOwnObjectThatImplementsIDisposable MyObject = new MyOwnObjectThatImplementsIDisposable)
{
// When the statement finishes, it calls the
// code you´ve writed in Dispose method
// of MyOwnObjectThatImplementsIDisposable class
}
Hope this helps
In this context the using statement is handy for types that implement IDisposable. When the code block exits the scope of the using statement, Dispose() is called implicitly. It's a good habit when working with objects you want to dispose immediately after use.
One specific instance in which you should be careful using a using block is with a WCF Service Client.
As noted in this MSDN article, wrapping a WCF client (which does implement IDisposable) in a using block could mask any errors which result in the client being left in a faulted state (like a timeout or communication problem). Long story short, when Dispose() is called, the client's Close() method fires, but throws and error because it's in a faulted state. The original exception is then masked by the second exception. Not good.
There are various workarounds out there, including one in the MSDN article itself. Others can be found at IServiceOriented and blog.davidbarret.net.
I prefer the last method, myself.
If you want a summary rule. Anytime an object using IDisposable where you would not have a catch, use using. Using, essentially, is this pattern:
try
{
//instantiate and use object
}
finally
{
//dispose object
}
If you do not need a catch, using can save you typing, which is a good thing.
The primary rule is:
* Use USING statement when objects implements IDisposable interface.
This interface provides the Dispose method, which should release the object's resources. If this method is not invoked then the object will stay in memory as long, as CLR wants to perform garbage collection. If the programmer use the USING statement then on the end the object will be disposed, and all resources will be free.
It is very important that all resources that are no longer in use be free as soon as possible.
For more information about it just visit this link: microsoft
Maybe it is worth mentioning that underlying reason for adding “using” lo C# languge is following: some resources can be scarce enough that it doesn’t make sense to wait for GC to call IDisposable. For example, DB connections. If you use try/catch/finally you won’t end up with a dangling connection, but connection will be left hanging until GC doesn’t kick in and this can take a while (if you do not close it explicitly). IF you use "using" (excuse the pun) you will release the connection immediately even if you forgot to close it and even if some exception occured inside the using block.
Another reason, as previous post mentions, is that programmers do not always use finally to clean up. If not using finally in the case of exception you end up with leaking resources…
One situation is when you want to do something at the beginning of a code block, and then undo it at the end of the block, unconditionally (even if there is a throw).
The ctor for the disposable class that you build (and call within the using) would perform the action, and then the Dispose method would undo that action. This is typically how I use it.
Other people has mentioned about "IDisposable" already.
But one of the caveats when using "using" statement is that,
any exceptions thrown within "using" will not be caught
even thought "SomeType" will be disposed regardless.
So in the following snippet,
using (SomeType t = new SomeType()){
throw new Exception("thrown within using");
}
throw new Exception("thrown within using"); should not be disregarded.
I would also add that use a using() statement if something implements IDispose and also if that something you want to dispose of holds on to NON-MANAGED resources like database connections and file handles.
If it's a normal object with say a List<T>, where T is like a Customer object that holds names and address, then you don't need to. The garbage collector is smart enough to manage this for you. But the garbage collector WILL NOT return connections to the connection pool or close file handles.