I'm learning to draw stuff in C# and I keep seeing recommendations to use dispose(), but I don't quite understand what it does.
When should I be using dispose() on a
code-drawn graphic?
What happens if I
don't?
Do I need to call it every
time a graphic is not visible, such
as on a GUI that has tabs and the
user switched to the other tab, and
then redraw it when they switch back?
Will I break things if I call it when I shouldn't?
Will Batman escape the evil clutches of the Joker?
When should I be using dispose() on a code-drawn graphic? Whenever you are completely done with any object that implements IDisposable, you should call Dispose just before it becomes eligible for garbage collection. As others have pointed out, it's best to use the using statement instead of calling Dispose directly.
What happens if I don't? Your program will be slightly less efficient because it will take up slightly more resources than necessary. This is the only drawback to not disposing graphics; however, not disposing other classes can actually cause errors (one famous example is StreamWriter). For this reason, it's best to always dispose any class that implements IDisposable, as a general rule.
Do I need to call it every time a graphic is not visible, such as on a GUI that has tabs and the user switched to the other tab, and then redraw it when they switch back? No. Objects should only be disposed when you are completely done with them.
Will I break things if I call it when I shouldn't? Yes. If you call Dispose before you are done with the object, and then attempt to use the disposed object, you will get an ObjectDisposedException.
Will Batman escape the evil clutches of the Joker? Tune in tomorrow, same bat-time, same bat-channel!
When you ask for a graphical object, Windows will allocate a bit of memory for you. Calling dispose will tidy up that memory for you. If you do not call dispose, all of these handles to memory wil remain open and eventually your system will run out of resources, become slower and eventually stop (closing the program may free them however).
Because you're using .NET, when you have finished using your graphics object, the garbage collector will eventually call dispose for you. The problem with the garbage collector is you never know when it will clean up the object, so it may leave these resources open for longer than necessary.
This said, you should never have to call dispose yourself. Far better will be to put your object in using scope:
using(Graphics g)
{
// do something with the resource
}
Now when you leave this using scope, the object will be destroyed and dispose will be called automatically for you. You should put all objects that have the dispose method defined inside a using scope.
In noob speak, Dispose() is about cleaning up after you're done using an unmanaged resource.
What's an unmanaged resource? It's all the stuff that CLR doesn't manage for you. They're thing like file handles, database connections, network sockets, GDI+ pens, etc. You get access to those things through a typical .NET object, but it will implement IDisposable, to allow you to clean up properly.
Why clean up? Until you've cleaned up after yourself, that resource isn't available by other parts of the program for use. In that respect, you're breaking things, because you're hogging a resource.
Why do this yourself? You should do this yourself as soon as you stop needing the resource, rather than rely on the auto-magic of the garbage collector, because it could take a long (well, unspecified) amount of time before the garbage collector gets to it. Until an object has been disposed of properly, you can't reuse the underlying resource, so you program will not function reliably.
Mitch Wheat says - Always call Dispose() on any object that implements IDisposable. GDI handles used by graphics objects are unmanaged and require disposing when you are finished with them.
Related
I have an app where I dynamically create user controls depending on the situation. When I'm done with them I remove them from the container, but they are still taking up memory.
I've implemented a "dispose" method in the code to clear the controls inside of the user control, but I still feel like there is a better way to do it.
I'm not sure what code I can post as no code is really relevant.
All I really have is what I'm doing now with the dispose method
public void Dispose()
{
ContentWebView.NavigateToString("about:blank");
YoutubeMediaPlayer.Stop();
}
The Dispose method is used for releasing unmanaged resources.
When all references to an object are removed, the garbage collector will eventually "Collect" it (freeing the memory). You can't actually control when this happens, even calling GC.Collect is technically just a suggestion. You can, however, invoke dispose on an object directly (note that this may cause odd behavior or exceptions if it is still in use!)
If you suspect a control has unmanaged resources that need to be cleaned up, you should call Dispose on it directly when you are finished with it (GDI Bitmap objects need to be cleaned up like this). From your comments, it sounds like you just need the garbage collector to actually free the memory, which is something thats hard to control (you can try calling GC.Collect though).
MSDN for Dispose: http://msdn.microsoft.com/en-us/library/system.idisposable.aspx
Do I have to use a using statement to dispose immediately even in a method? Or will the ending of the method cause an automatic Dispose of all local variables including graphics?
(I’m asking this because I’ve seen examples that have Dispose calls at the end of methods, and wanted to know if that’s really necessary.)
Thanks.
Yes, you do. Going out of scope does not do anything; it does not call Dispose(), it does not garbage-collect, it does not call a finalizer.
If the type is IDisposable, then yes. It is your job to tidy up after yourself (assuming that the object is actually "done with" at this point).
Possible side-effects of not doing this:
files get left open and cause access-exceptions (FileStream)
connections get left open and cause pool saturation (DbConnection)
unmanaged handles get saturated and cause resource starvation (any winforms/etc)
transactions get left open and cause blocking (TransactionScope / DbTransaction)
etc
basically, bad things.
Furthermore, in most cases where you see Dispose() at the bottom of the method, a using would be preferable. There are some cases where that isn't possible (fields on an object, for example), but the point remains: it sounds like those are bad examples.
You don't ever have to dispose of an object. But if you want to release unmanaged resources earlier rather than whenever the GC gets around to it (which is when it processes the fReachable queue) then yes you should since the scope of of a method does not determine when a disposable object's finalize will be called.
For more you might want to read Jeffrey Richter's Garbage Collection: Automatic Memory Management in the Microsoft .NET Framework
As delnan noted if there's a hard limit to the resources you're using you're much more likely to impacted if you let it get reclaimed by the GC rather than calling Dispose. DB connections are a good example of this kind of reasource and is certainly the reason why the NumberOfReclaimedConnections performance counter got created.
Yes, use using. It is a great statement.
Actually if you don't call Dispose (either manually or by using using), you can end up in a situation where your managed memory is stil quite empty, but unmanaged resources are depleted. Eventually it can cause a deadlock because other objects or other processes will wait for your unmanaged resources and you will wait for them. (Garbage collector won't run, because managed memory still won't be full.)
So please call Dispose as soon as you can. Once you don't need an object which has got that method defined, call it.
When would I implement IDispose on a class as opposed to a destructor? I read this article, but I'm still missing the point.
My assumption is that if I implement IDispose on an object, I can explicitly 'destruct' it as opposed to waiting for the garbage collector to do it. Is this correct?
Does that mean I should always explicitly call Dispose on an object? What are some common examples of this?
A finalizer (aka destructor) is part of garbage collection (GC) - it is indeterminate when (or even if) this happens, as GC mainly happens as a result of memory pressure (i.e. need more space). Finalizers are usually only used for cleaning up unmanaged resources, since managed resources will have their own collection/disposal.
Hence IDisposable is used to deterministically clean up objects, i.e. now. It doesn't collect the object's memory (that still belongs to GC) - but is used for example to close files, database connections, etc.
There are lots of previous topics on this:
deterministic finalization
disposing objects
using block
resources
Finally, note that it is not uncommon for an IDisposable object to also have a finalizer; in this case, Dispose() usually calls GC.SuppressFinalize(this), meaning that GC doesn't run the finalizer - it simply throws the memory away (much cheaper). The finalizer still runs if you forget to Dispose() the object.
The role of the Finalize() method is to ensure that a .NET object can clean up unmanaged resources when garbage collected. However, objects such as database connections or file handlers should be released as soon as possible, instead on relying on garbage collection. For that you should implement IDisposable interface, and release your resources in the Dispose() method.
The only thing that should be in a C# destructor is this line:
Dispose(False);
That's it. Nothing else should ever be in that method.
There is a very good description on MSDN:
The primary use of this interface is
to release unmanaged resources.
The garbage collector automatically
releases the memory allocated to a
managed object when that object is no
longer used. However, it is not
possible to predict when garbage
collection will occur. Furthermore,
the garbage collector has no
knowledge of unmanaged resources
such as window handles, or open
files and streams.
Use the Dispose method of this
interface to explicitly release
unmanaged resources in conjunction
with the garbage collector. The
consumer of an object can call this method when the object is no
longer needed.
Your question regarding whether or not you should always call Dispose is usually a heated debate. See this blog for an interesting perspective from respected individuals in the .NET community.
Personally, I think Jeffrey Richter's position that calling Dispose is not mandatory is incredibly weak. He gives two examples to justify his opinion.
In the first example he says calling Dispose on Windows Forms controls is tedious and unnecessary in mainstream scenarios. However, he fails to mention that Dispose actually is called automatically by control containers in those mainstream scenarios.
In the second example he states that a developer may incorrectly assume that the instance from IAsyncResult.WaitHandle should be aggressively disposed without realizing that the property lazily initializes the wait handle resulting in an unnecessary performance penalty. But, the problem with this example is that the IAsyncResult itself does not adhere to Microsoft's own published guidelines for dealing with IDisposable objects. That is if a class holds a reference to an IDisposable type then the class itself should implement IDisposable. If IAsyncResult followed that rule then its own Dispose method could make the decision regarding which of its constituent members needs disposing.
So unless someone has a more compelling argument I am going to stay in the "always call Dispose" camp with the understanding that there are going to be some fringe cases that arise mostly out of poor design choices.
It's pretty simple really. I know it's been answered but I'll try again but will try to keep it as simple as possible.
A destructor should generally never be used. It is only run .net wants it to run. It will only run after a garbage collectoin cycle. It may never actually be run during the lifecycle of your application. For this reason, you should not ever put any code in a destructor that 'must' be run. You also can't rely on any existing objects within the class to exist when it runs (they may have already been cleaned up as the order in which destructors run in is not garanteed).
IDisposible should be used whenever you have an object that creates resources that need cleaning up (ie, file and graphics handles). In fact, many argue that anything you put in a destructor should be putin IDisposable due to the reasons listed above.
Most classes will call dispose when the finalizer is executed but this is simply there as a safe guard and should never be relied upon. You should explicitly dispose anything that implements IDisposable when you're done with it. If you do implement IDisposable, you should call dispose in finalizer. See http://msdn.microsoft.com/en-us/library/system.idisposable.aspx for an example.
Here is another fine article which clears up some of the mist surrounding IDisposable, the GC and dispose.
Chris Lyons WebLog Demystifying Dispose
I'd like to know when i should and shouldn't be wrapping things in a USING block.
From what I understand, the compiler translates it into a try/finally, where the finally calls Dispose() on the object.
I always use a USING around database connections and file access, but its more out of habit rather than a 100% understanding. I know you should explicity (or with a using) Dispose() objects which control resources, to ensure they are released instantly rather than whenever the CLR feels like it, but thats where my understanding breaks down.
Are IDisposables not disposed of when they go out of scope?
Do I only need to use a USING when my object makes use of Dispose to tidy itself up?
Thanks
Edit: I know there are a couple of other posts on the USING keyword, but I'm more interested in answers relating the the CLR and exactly whats going on internally
Andrew
No, IDisposable items are not disposed when they go out of scope. It is for precisely this reason that we need IDisposable - for deterministic cleanup.
They will eventually get garbage collected, and if there is a finalizer it will (maybe) be called - but that could be a long time in the future (not good for connection pools etc). Garbage collection is dependent on memory pressure - if nothing wants extra memory, there is no need to run a GC cycle.
Interestingly (perhaps) there are some cases where "using" is a pain - when the offending class throws an exception on Dispose() sometimes. WCF is an offender of this. I have discussed this topic (with a simple workaround) here.
Basically - if the class implements IDisposable, and you own an instance (i.e. you created it or whatever), it is your job to ensure that it gets disposed. That might mean via "using", or it might mean passing it to another piece of code that assumes responsibility.
I've actually seen debug code of the type:
#if DEBUG
~Foo() {
// complain loudly that smoebody forgot to dispose...
}
#endif
(where the Dispose calls GC.SuppressFinalize)
"Are IDisposables not disposed of when
they go out of scope?"
No. If the IDisposable object is finalizable, which is not the same thing, then it will be finalized when it's garbage collected.
Which might be soon or might be almost never.
Jeff Richter's C#/CLR book is very good on all this stuff, and the Framework Design Guidelines book is also useful.
Do I only need to use a USING when my
object makes use of Dispose to tidy
itself up?
You can only use 'using' when the object implements IDisposable. The compiler will object if you try to do otherwise.
To add to the other answers, you should use using (or an explicit Dispose) whenever an object holds any resources other than managed memory. Examples would be things like files, sockets, database connections, or even GDI drawing handles.
The garbage collector would eventually finalise these objects, but only at some unspecified time in the future. You can't rely on it happening in a timely fashion, and you might have run out of that resource in the meantime.
What resources have to be manually cleaned up in C# and what are the consequences of not doing so?
For example, say I have the following code:
myBrush = new System.Drawing.SolidBrush(System.Drawing.Color.Black);
// Use Brush
If I don't clean up the brush using the dispose method, I'm assuming the garbage collector frees the memory used at program termination? Is this correct?
What other resources do I need to manually clean up?
If you don't dispose something, it'll be cleaned up when the garbage collector notices that there are no more references to it in your code, which may be after some time. For something like that, it doesn't really matter, but for an open file it probably does.
In general, if something has a Dispose method, you should call it when you've finished with it, or, if you can, wrap it up in a using statement:
using (SolidBrush myBrush = new System.Drawing.SolidBrush(System.Drawing.Color.Black))
{
// use myBrush
}
Handles to internal windows data structures.
Database connections.
File handles.
Network connections.
COM/OLE references.
The list goes on.
It's important to call Dispose or even better yet, use the using pattern.
using (SolidBrush myBrush = new System.Drawing.SolidBrush(System.Drawing.Color.Black))
{
// use myBrush
}
If you don't dispose something, it'll be cleaned up when the garbage collector notices that there are no more references to it, which may be after some time.
In the case of System.Drawing.Brush, Windows will keep internal windows structures for the brush loaded in memory until all programs release their handle.
The consequences of not disposing your IDisposables can vary from a negligible performance hit to crashing your app.
The Brush object in your example will be cleaned up by the GC when it feels like it. But your program won't have had the benefit of that bit of extra memory you would have gained by cleaning it up earlier. If you are using a lot of Brush objects this might become significant. The GC is also more efficient at cleaning up objects if they haven't been around very long, because it is a generational garbage collector.
On the other hand, the consequences of not disposing database connection objects could mean you run out of pooled database connections very quickly and cause your app to crash.
Either use
using (new DisposableThing...
{
...
}
Or, if you need to hold on to a reference to an IDisposable in your object for its lifetime, implement IDisposable on your object and call the IDisposable's Dispose method.
class MyClass : IDisposable
{
private IDisposable disposableThing;
public void DoStuffThatRequiresHavingAReferenceToDisposableThing() { ... }
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
protected virtual void Dispose(bool disposing)
//etc... (see IDisposable on msdn)
}
Generally, anything that implements IDisposable should cause you to pause and research the resource you're using.
GC only happens when there's memory pressure, so you cannot predict when. Although an unload of the AppDomain will certainly trigger it.
Technically anything that inherits from IDisposable should be proactively disposed. You can use the 'using' statement to make things easier.
http://msdn.microsoft.com/en-us/library/yh598w02.aspx
Sometimes you will see inconsistent use of IDisposable derived objects in documentation sample code as well as code that is generated by tools (i.e. visual studio).
What's nice about IDisposable is that it gives you the ability to proactively release the underlying unmanaged resource. Sometimes you really want to do this - think network connections and file resources for example.
As others have said, using is your friend.
I wrote this blog entry about how to implement IDisposable in a fairly straightforward way that is less error-prone by factoring out the parts that are the most important.
A trick I use when I can't remember whether a given object is a disposable resource is to type ".Dispose" (at most!) after the declaration to get Intellisense to check for me:
MemoryStream ms = new MemoryStream().Dispose
Then delete the .Dispose and use the using() directive:
using(MemoryStream ms = new MemoryStream())
{
...
}
Well, as long as you use the managed version of the resources and don't call the windows APIs by yourself, you should be OK. Only worry about having to delete/destroy a resource when what you get is an IntPtr, as "windows handles" (and a whole lot other things) are known in .NET, and not an object.
By the way, the resource (as any other .NET object) will be flagged for collection as soon as you leave the current context, so if you create the Brush inside a method, it will be flagged when you exit it.
If it's managed (i.e. part of the framework) you don't need to worry about it. If it implements IDisposable just wrap it in a using block.
If you want to use unmanaged resources then you need to read up on finalisers and implementing IDisposable yourself.
There's a lot more detail under this question
First upon program termination, you can assume that memory used by the process will be eliminated with the process itself.
While using dispose or destructor in.net, one must understand that the time of when the dispose function is called by the GC is non-deterministic. That why it is recommended to use the using or calling the dispose explicitly.
When using resources such as files, memory objects such as semaphors and resources that live outside of the managed world of .net must be freed.
The SolidBrush for example, you need to dispose because it is a GDI object and living outside of the .net world.
The garbage collector does not only free up at program termination, otherwise it would not be really useful (on any decent/recent OS, when the process exits, all its memory is cleaned up automatically by the OS anyway).
One of the big advantage of C# compared to C/C++ is that you don't have to care about freeing allocated objects (most of the time at least); the gc does it when the runtime decides (various strategies when/how to do it).
Many ressources are not taken care of by the gc: file, thread-related ressources (locks), network connections, etc...
One place to be careful is Objects that look small to GC but are not... In the SharePoint API for example, the SPWeb object has a small footprint as far as the GC is concerned and so will have low priority for collection, but it has really grabbed a bunch of memory (in the heap I believe) that the GC doesn't know about. You will run into some fun memory issues if you are foreaching a whole bunch of these for example, always remember to use using or dispose!
Rather than thinking of an object as "holding" resources that need to be released, it's better to think in terms of an object as having altered something (possibly outside the computer!) which will outlive it, in a way could be harmful if it not undone or "cleaned up", but which only the object can clean up. While this alteration commonly takes the form of some concrete object in a pool being marked "busy", its precise form doesn't matter. What matters is that the changes need to be undone, and the object holds information necessary to do that.
The garbage collector will handle any managed resources. In your example, the brush will be cleaned up when the garbage collector decides to, which will happen some time after the last reference to the brush is no longer valid.
There are certain things that need to be manually cleaned up, but those are pointers retrieved from unmanaged sources, such as DLL calls, nothing within the .NET Framework needs this treatment however.