I was reading about this scenario where making use of the C# using statement can cause problems. Exceptions thrown within the scope of the using block can be lost if the Dispose function called at the end of the using statement was to throw an exception also. This highlights that care should be taken in certain cases when deciding whether to add the using statement.
I only tend to make use of using statements when using streams and the classes derived from DbConnection. If I need to clean up unmanaged resources, I would normally prefer to use a finally block.
This is another use of the IDisposable interface to create a performance timer that will stop the timer and log the time to the registry within the Dispose function.
http://thebuildingcoder.typepad.com/blog/2010/03/performance-profiling.html
Is this good use of the IDisposable interface? It is not cleaning up resources or disposing of any further objects. However, I can see how it could clean up the calling code by wrapping the code it is profiling neatly in a using statement.
Are there times when the using statement and the IDisposable interface should never be used? Has implementing IDisposable or wrapping code in a using statement caused problems for you before?
Thanks
I would say, always use using unless the documentation tells you not to (as in your example).
Having a Dispose method throw exceptions rather defeats the point of using it (pun intended). Whenever I implement it, I always try to ensure that no exceptions will be thrown out of it regardless of what state the object is in.
PS: Here's a simple utility method to compensate for WCF's behaviour. This ensures that Abort is called in every execution path other than when Close is called, and that errors are propogated up to the caller.
public static void CallSafely<T>(ChannelFactory<T> factory, Action<T> action) where T : class {
var client = (IClientChannel) factory.CreateChannel();
bool success = false;
try {
action((T) client);
client.Close();
success = true;
} finally {
if(!success) {
client.Abort();
}
}
}
If you find any other funny behaviour cases elsewhere in the framework, you can come up with a similar strategy for handling them.
The general rule of thumb is simple: when a class implements IDisposable, use using. When you need to catch errors, use try/catch/finally, to be able to catch the errors.
A few observations, however.
You ask whether situations exist where IDisposable should not be used. Well: in most situations you shouldn't need to implement it. Use it when you want to free up resources timely, as opposed to waiting until the finalizer kicks in.
When IDisposable is implemented, it should mean that the corresponding Dispose method clears its own resources and loops through any referenced or owned objects and calls Dispose on them. It should also flag whether Dispose is called already, to prevent multiple cleanups or referenced objects to do the same, resulting in an endless loop. However, all this is no guarantee that all references to the current object are gone, which means it will remain in memory until all references are gone and the finalizer kicks in.
Throwing exceptions in Dispose is frowned upon and when it happens, state is possibly not guaranteed anymore. A nasty situation to be in. You can fix it by using try/catch/finally and in the finally block, add another try/catch. But like I said: this gets ugly pretty quickly.
Using using is one thing, but don't confuse it with using try/finally. Both are equal, but the using-statement makes life easier by adding scoping and null-checks, which is a pain to do by hand each time. The using-statement translates to this (from C# standard):
{
SomeType withDispose = new SomeType();
try
{
// use withDispose
}
finally
{
if (withDispose != null)
{
((IDisposable)withDispose).Dispose();
}
}
}
There are occasions where wrapping an object into a using-block is not necessary. These occasions are rare. They happen when you find yourself inheriting from an interface that inherits from IDisposable just in case a child would require disposing. An often-used example is IComponent, which is used with every Control (Form, EditBox, UserControl, you name it). And I rarely see people wrapping all these controls in using-statements. Another famous example is IEnumerator<T>. When using its descendants, one rarely sees using-blocks either.
Conclusion
Use the using-statement ubiquitously, and be judicious about alternatives or leaving it out. Make sure you know the implications of (not) using it, and be aware of the equality of using and try/finally. Need to catch anything? Use try/catch/finally.
I think the bigger problem is throwing exceptions in Dispose. RAII patterns usually explicitly state that such should not be done, as it can create situations just like this one. I mean, what is the recovery path for something not disposing correctly, other than simply ending execution?
Also, it seems like this can be avoided with two try-catch statements:
try
{
using(...)
{
try
{
// Do stuff
}
catch(NonDisposeException e)
{
}
}
}
catch(DisposeException e)
{
}
The only problem that can occur here is if DisposeException is the same or a supertype of NonDisposeException, and you are trying to rethrow out of the NonDisposeException catch. In that case, the DisposeException block will catch it. So you might need some additional boolean marker to check for this.
The only case I know about is WCF clients. That's due to a design bug in WCF - Dispose should never throw exceptions. They missed that one.
One example is the IAsyncResult.AsyncWaitHandle property. The astute programmer will recognize that WaitHandle classes implement IDisposable and naturally try to greedily dispose them. Except that most of the implementations of the APM in the BCL actually do lazy initialization of the WaitHandle inside the property. Obviously the result is that the programmer did more work than was necessary.
So where is the breakdown? Well, Microsoft screwed up the IAsyncResult inteface. Had they followed their own advice IAsyncResult would have derived from IDisposable since the implication is that it holds disposable resources. The astute programmer would then just call Dispose on the IAsyncResult and let it decide how best to dispose its constituents.
This is one of the classic fringe cases where disposing of an IDisposable could be problematic. Jeffrey Richter actually uses this example to argue (incorrectly in my opinion) that calling Dispose is not mandatory. You can read the debate here.
Related
In refactoring some code, I added a "using" statement like so:
using (SerialPort serialPort = new SerialPort())
{
serialPort.BaudRate = 19200;
serialPort.Handshake = Handshake.XOnXOff;
serialPort.Open();
serialPort.Write(cmd);
serialPort.Close();
}
...but now wonder whether I can or should do away with the call to Close. I reckon so, but is it just a nicety (style points) or a veritable necessity?
It really depends on the particular class implementing IDisposable. It's perfectly possible for a badly written class that implements IDisposable NOT to properly release resources and close connections. In the specific case of the SerialPort class, the documentation states that Close() calls Dispose(). I think you should in this case be fine to put it in a using block and not call Close() manually.
It really depends on the class and if the author implemented it as recommended - implemented IDisposable correctly, and have Close do nothing but call Dispose(). In such a case, the Close() call is redundant and can be removed when wrapping the class in a using block.
This is generally speaking - in this specific case with SerialPort, they did so, so the Close() call is redundant and can be removed.
As per Richter:
Types that offer the capability to be deterministically disposed of or
closed implement the dispose pattern. The dispose pattern defines
conventions a developer should adhere to when defining a type that
wants to offer explicit cleanup to a user of the type.
Being that SerialPort defines an open and close it implies that it can be opened and closed multiple times during it's lifetime, which is mutually exclusive from it being disposed (e.g. never used again)
But back to your original question, yes - in this case it is redundant, decompiling the SerialPort object reveals that dispose closes the port for you when called:
protected override void Dispose(bool disposing)
{
if (disposing && this.IsOpen)
{
this.internalSerialStream.Flush();
this.internalSerialStream.Close();
this.internalSerialStream = (SerialStream) null;
}
base.Dispose(disposing);
}
If an exceptional condition arises in Dispose, it is usually better to have Dispose throw the exception than complete silently (whether it's better to stifle the exception in any particular situation will depend upon whether an exception is already pending and there is as yet alas no mechanism via which Dispose can know when that's the case). Because there is no good way to handle an exception within Dispose, it's often a good idea to ensure when possible that any actions which could go wrong if done within a Dispose will be done before the Dispose is invoked.
If Dispose were the only method of cleanup in the normal case, then having it stifle exceptions would pose a significant risk that problems could occur but go undetected. Having a class support both Close and using, and having clients call Close in the main-line case, will allow such risk to be reduced (if an exception is pending, Dispose will get called without Close, and thus any exceptions on cleanup will get stifled but code will know that something went wrong somewhere because of the pending exception; if no exception occurs before Close, the fact that it's not being called in a Dispose cleanup context will mean that it can assume no exception is pending, and it may thus safely throw one of its own).
There's not much consistency in the exception-handling practices of Dispose and Close, but I would recommend calling Close on principle. Depending upon how Dispose and Close are implemented, explicitly calling Close before the implicit Dispose may or may not be helpful, but it should be at worst harmless. Given the possibility of its being helpful (if not in the present version of a class, perhaps in a future version) I would suggest it as a general habit.
TL;DR -- Is it ever appropriate to execute business logic in IDisposable.Dispose?
In my search for an answer, I read through the question: Is it abusive to use IDisposable and "using" as a means for getting "scoped behavior" for exception safety? It came very close to addressing this issue, but I'd like to attack it dead on. I recently encountered some code that looked like this:
class Foo : IDisposable
{
public void Dispose()
{
ExecuteSomeBusinessBehavior();
NormalCleanup();
}
}
and is used in a context such as:
try
{
using (var myFoo = new Foo())
{
DoStuff();
foo.DoSomethingFooey();
...
DoSomethingElse();
Etc();
}
}
catch (Exception ex)
{
// Handle stuff
}
Upon seeing this code I immediately began to itch. Here's what I see when I look at this code:
First, looking at just the usage context, it's not remotely apparent that actual business logic, not just cleanup code, will be executed when the code leaves the using scope.
Second, if any of the code within the "using" scope throws an exception, the business logic in the Dispose method will still execute and does so before the Try/Catch can handle the exception.
My questions to the StackOverflow community are these: Does it ever make sense to put business logic in the IDisposable.Dispose method? Is there a pattern that achieves similar results without making me itch?
(Sorry, this is more of a comment, but it exceeds the comment length limit.)
Actually, there is an example in the .NET framework where IDisposable is used to create a scope and do useful work when disposing: TransactionScope.
To quote from TransactionScope.Dispose:
Calling this method marks the end of the transaction scope. If the TransactionScope object created the transaction and Complete was called on the scope, the TransactionScope object attempts to commit the transaction when this method is called.
If you decide to take that route, I would suggest that
you make it blatantly obvious that your object creates a scope, e.g., by calling it FooScope instead of Foo and
you think very hard about what should happen when an exception causes the code to leave your scope. In TransactionScope, the pattern of calling Complete at the end of the block ensures that Dispose can distinguish between the two cases.
The real meaning of IDisposable is that an object knows of something, somewhere which has been put into a state that should be cleaned up, and it has the information and impetus necessary to perform such cleanup. Although the most common "states" associated with IDisposable are things like files being open, unmanaged graphic objects being allocated, etc. those are only examples of uses, and not a definition of "proper" use.
The biggest issue to consider when using IDisposable and using for scoped behavior is that there is no way for the Dispose method to distinguish scenarios where an exception is thrown from a using block from those where it exits normally. This is unfortunate, since there are many situations where it would be useful to have scoped behavior which was guaranteed to have one of two exit paths depending upon whether the exit was normal or abnormal.
Consider, for example, a reader-writer lock object with a method that returns an IDisposable "token" when the lock is acquired. It would be nice to say:
using (writeToken = myLock.AcquireForWrite())
{
... Code to execute while holding write lock
}
If one were to manually code the acquisition and release of the lock without a try/catch or try/finally lock, an exception thrown while the lock was held would cause any code that was waiting on the lock to wait forever. That is a bad thing. Employing a using block as shown above will cause the lock to be released when the block exits, whether normally or via exception. Unfortunately, that may also be a bad thing.
If an unexpected exception is thrown while a write-lock is held, the safest course of behavior would be to invalidate the lock so that any present or future attempt to acquire the lock will throw an immediate exception. If the program cannot usefully proceed without the locked resource being usable, such behavior would cause it to shut down quickly. If it can proceed e.g. by switching to some alternate resource, invalidating the resource will allow it to get on with that much more effectively than would leaving the lock uselessly acquired. Unfortunately, I don't know of any nice pattern to accomplish that. One could do something like:
using (writeToken = myLock.AcquireForWrite())
{
... Code to execute while holding write lock
writeToken.SignalSuccess();
}
and have the Dispose method invalidate the token if it's called before success has been signaled, but an accidental failure to signal the success could cause the resource to become invalid without offering indication as to where or why that happened. Having the Dispose method throw an exception if code exits a using block normally without calling SignalSuccess might be good, except that throwing an exception when it exits because of some other exception would destroy all information about that other exception, and there's no way Dispose can tell which method applies.
Given those considerations, I think the best bet is probably to use something like:
using (lockToken = myLock.CreateToken())
{
lockToken.AcquireWrite(Describe how object may be invalid if this code fails");
... Code to execute while holding write lock
lockToken.ReleaseWrite();
}
If code exits without calling ReleaseWrite, other threads that try to acquire the lock will receive exceptions that include the indicated message. Failure to properly manually pair the AcquireWrite and ReleaseWrite will leave the locked object unusable, but not leave other code waiting for it to become usable. Note that an unbalanced AcquireRead would not have to invalidate the lock object, since code inside the read would never put the object into an invalid state.
Business logic code should never be written in any circumstances to Dispose method reason is, you are relying on a unreliable path. What if user does not call your dispose method? You missed to call a complete functionality ? What if there was an exception thrown in the method call of your dispose method? And why would you perform a business operation when user is asking to dispose the object itself. So logically, technically it should not be done.
I'm currently reading Introduction to Rx, by Lee Campbell, and it has a chapter called IDisposable, where he explicitly advocates taking advantage of the integration with the using construct, in order to "create transient scope".
Some key quotations from that chapter:
"If we consider that we can use the IDisposable interface to effectively create a scope, you can create some fun little classes to leverage this."
(...see examples below...)
"So we can see that you can use the IDisposable interface for more than just common use of deterministically releasing unmanaged resources. It is a useful tool for managing lifetime or scope of anything; from a stopwatch timer, to the current color of the console text, to the subscription to a sequence of notifications.
The Rx library itself adopts this liberal usage of the IDisposable interface and introduces several of its own custom implementations:
BooleanDisposable
CancellationDisposable
CompositeDisposable
ContextDisposable
MultipleAssignmentDisposable
RefCountDisposable
ScheduledDisposable
SerialDisposable
SingleAssignmentDisposable"
He gives two fun little examples, indeed:
Example 1 - Timing code execution. "This handy little class allows you to create scope and measure the time certain sections of your code base take to run."
public class TimeIt : IDisposable
{
private readonly string _name;
private readonly Stopwatch _watch;
public TimeIt(string name)
{
_name = name;
_watch = Stopwatch.StartNew();
}
public void Dispose()
{
_watch.Stop();
Console.WriteLine("{0} took {1}", _name, _watch.Elapsed);
}
}
using (new TimeIt("Outer scope"))
{
using (new TimeIt("Inner scope A"))
{
DoSomeWork("A");
}
using (new TimeIt("Inner scope B"))
{
DoSomeWork("B");
}
Cleanup();
}
Output:
Inner scope A took 00:00:01.0000000
Inner scope B took 00:00:01.5000000
Outer scope took 00:00:02.8000000
Example 2 - Temporarily changing console text color
//Creates a scope for a console foreground color. When disposed, will return to
// the previous Console.ForegroundColor
public class ConsoleColor : IDisposable
{
private readonly System.ConsoleColor _previousColor;
public ConsoleColor(System.ConsoleColor color)
{
_previousColor = Console.ForegroundColor;
Console.ForegroundColor = color;
}
public void Dispose()
{
Console.ForegroundColor = _previousColor;
}
}
Console.WriteLine("Normal color");
using (new ConsoleColor(System.ConsoleColor.Red))
{
Console.WriteLine("Now I am Red");
using (new ConsoleColor(System.ConsoleColor.Green))
{
Console.WriteLine("Now I am Green");
}
Console.WriteLine("and back to Red");
}
Output:
Normal color
Now I am Red
Now I am Green
and back to Red
Regarding the Microsoft built classes that inherit IDisposable, do I explicitly have to call Dispose to prevent memory leaks?
I understand that it is best practice to call Dispose (or better yet use a using block), however when programming, typically I don't always immediately realise that a class inherits from IDisposable.
I also understand that Microsoft implementation of IDisposable is a bit borked, which is why they created the article explaining the correct usage of IDisposable.
Long story short, in which instances is it okay to forget to call Dispose?
There are a couple of issues in the primary question
Do I explicitly have to call Dispose to prevent memory leaks?
Calling Dispose on any type which implements IDisposable is highly recomended and may even be a fundamental part of the types contract. There is almost no good reason to not call Dispose when you are done with the object. An IDisposable object is meant to be disposed.
But will failing to call Dispose create a memory leak? Possibly. It's very dependent on what exactly that object does in it's Dispose method. Many free memory, some unhook from events, others free handles, etc ... It may not leak memory but it will almost certainly have a negative effect on your program
In which instances is it okay to forget to call Dispose?
I'd start with none. The vast majority of objects out there implement IDisposable for good reason. Failing to call Dispose will hurt your program.
It depends on two things:
What happens in the Dispose method
Does the finalizer call Dispose
Dispose functionlity
Dispose can do several type of actions, like closing a handle to a resource (like file stream), change the class state and release other components the class itself uses.
In case of resource being released (like file) there's a functionality difference between calling it explicitly and waiting for it to be called during garbage collection (assuming the finalizer calls dispose).
In case there's no state change and only components are released there'll be no memory leak since the object will be freed by the GC later.
Finalizer
In most cases, disposable types call the Dispose method from the finalizer. If this is the case, and assuming the context in which the dispose is called doesn't matter, then there's a high chance that you'll notice no difference if the object will not be disposed explicitly. But, if the Dispose is not called from the finalizer then your code will behave differently.
Bottom line - in most cases, it's better to dispose the object explicitly when you're done with it.
A simple example to where it's better to call Dispose explicitly: Assuming you're using a FileStream to write some content and enable no sharing, then the file is locked by the process until the GC will get the object. The file may also not flush all the content to the file so if the process crashes in some point after the write was over it's not guaranteed that it will actually be saved.
It can be safe to not call Dispose, but the problem is knowing when this is the case.
A good 95% of IEnumerator<T> implementations have a Dispose that's safe to ignore, but the 5% is not just 5% that'll cause a bug, but 5% that'll cause a nasty hard to trace bug. More to the point, code that gets passed an IEnumerator<T> will see both the 95% and the 5% and won't be able to dynamically tell them apart (it's possible to implement the non-generic IEnumerable without implementing IDisposable, and how well that turned out can be guessed at by MS deciding to make IEnumerator<T> inherit from IDisposable!).
Of the rest, maybe there's 3 or 4% of the time it's safe. For now. You don't know which 3% without looking at the code, and even then the contract says you have to call it, so the developer can depend on you doing so if they release a new version where it is important.
In summary, always call Dispose(). (I can think of an exception, but it's frankly too weird to even go into the details of, and it's still safe to call it in that case, just not vital).
On the question of implementing IDisposable yourself, avoid the pattern in that accursed document.
I consider that pattern an anti-pattern. It is a good pattern for implementing both IDisposable.Dispose and a finaliser in a class that holds both managed and unmanaged resources. However holding both managed IDisposable and unmanaged resources is a bad idea in the first place.
Instead:
If you have an unmanaged resource, then don't have any unmanaged resources that implement IDisposable. Now the Dispose(true) and Dispose(false) code paths are the same, so really they can become:
public class HasUnmanaged : IDisposable
{
IntPtr unmanagedGoo;
private void CleanUp()
{
if(unmanagedGoo != IntPtr.Zero)
{
SomeReleasingMethod(unmanagedGoo);
unmanagedGoo = IntPtr.Zero;
}
}
public void Dispose()
{
CleanUp();
GC.SuppressFinalize(this);
}
~HasUnmanaged()
{
CleanUp();
}
}
If you have managed resources that need to be disposed, then just do that:
public class HasUnmanaged : IDisposable
{
IDisposable managedGoo;
public void Dispose()
{
if(managedGoo != null)
managedGoo.Dispose();
}
}
There, no cryptic "disposing" bool (how can something be called Dispose and take false for something called disposing?) No worrying about finalisers for the 99.99% of the time you won't need them (the second pattern is way more common than the first). All good.
Really need something that has both a managed and an unmanaged resource? No, you don't really, wrap the unmanaged resource in a class of your own that works as a handle to it, and then that handle fits the first pattern above and the main class fits the second.
Only implement the CA10634 pattern when you're forced to because you inherited from a class that did so. Thankfully, most people aren't creating new ones like that any more.
It is never OK to forget to call Dispose (or, as you say, better yet use using).
I guess if the goal of your program is to cause unmanaged resource leaks. Then maybe it would be OK.
The implementation of IDisposable indicates that a class uses un-managed resources. You should always call Dispose() (or use a using block when possible) when you're sure you're done with the class. Otherwise you are unnecessarily keeping un-managed resources allocated.
In other words, never forget to call Dispose().
Yes, always call dispose. Either explicitly or implicitly (via using). Take, for example, the Timer class. If you do not explicitly stop a timer, and do not dispose it, then it will keep firing until the garbage collector gets around to collecting it. This could actually cause crashes or unexpected behavior.
It's always best to make sure Dispose is called as soon as you are done with it.
Microsoft (probably not officially) says it is ok to not call Dispose in some cases.
Stephen Toub from Microsoft writes (about calling Dispose on Task):
In short, as is typically the case in .NET, dispose aggressively if
it's easy and correct to do based on the structure of your code. If
you start having to do strange gyrations in order to Dispose (or in
the case of Tasks, use additional synchronization to ensure it's safe
to dispose, since Dispose may only be used once a task has completed),
it's likely better to rely on finalization to take care of things. In
the end, it's best to measure, measure, measure to see if you actually
have a problem before you go out of your way to make the code less
sightly in order to implement clean-up functionality.
[bold emphasize is mine]
Another case is base streams
var inner = new FileStrem(...);
var outer = new StreamReader(inner, Encoding.GetEncoding(1252));
...
outer.Dispose();
inner.Dispose(); -- this will trigger a FxCop performance warning about calling Dispose twice.
(I have turned off this rule)
I've been debugging some code recently that was a bit memory leaky. It's a long running program that runs as a Windows service.
If you find a class wearing an IDisposable interface, it is telling you that some of the resources it uses are outside the abilities of the garbage collector to clean up for you.
The reason it is telling you this is that you, the user of this object, are now responsible for when these resources are cleaned up. Congratulations!
As a conscientious developer, you are nudged towards calling the .Dispose() method when you've finished with the object in order to release those unmanaged resources.
There is the nice using() pattern to help clean up these resources once they are finished with. Which just leaves finding which exact objects are causing the leakyness?
In order to aid tracking down these rogue unmanaged resources, is there any way to query what objects are loitering around waiting to be Disposed at any given point in time?
There shouldn't be any cases where you don't want to call Dispose, but the compiler cannot tell you where you should call dispose.
Suppose you write a factory class which creates and returns disposable objects. Should the compiler bug you for not calling Dispose when the cleanup should be the responsibility of your callers?
IDisposable is more for making use of the using keyword. It's not there to force you to call Dispose() - it's there to enable you to call it in a slick, non-obtrusive way:
class A : IDisposable {}
/// stuff
using(var a = new A()) {
a.method1();
}
after you leave the using block, Dispose() is called for you.
"Is there any way to detect at the end of the program which objects are loitering around waiting to be Disposed?"
Well, if all goes well, at the end of the program the CLR will call all object's finalizers, which, if the IDisposable pattern was implemented properly, will call the Dispose() methods. So at the end, everything will be cleared up properly.
The problem is that if you have a long running program, chances are some of your IDiposable instances are locking some resources that shouldn't be locked. For cases like this, user code should use the using block or call Dispose() as soon as it is done with an object, but there's really no way for a anyone except the code author to know that.
You are not required to call the Dispose method. Implementing the IDisposable interface is a reminder that your class probably is using resources such as a database connection, a file handle, that need to be closed, so GC is not enough.
The best practice AFAIK is to call Dispose or even better, put the object in a using statement.
A good example is the .NET 2.0 Ping class, which runs asynchronously. Unless it throws an exception, you don't actually call Dispose until the callback method. Note that this example has some slightly weird casting due to the way Ping implements the IDisposable interface, but also inherits Dispose() (and only the former works as intended).
private void Refresh( Object sender, EventArgs args )
{
Ping ping = null;
try
{
ping = new Ping();
ping.PingCompleted += PingComplete;
ping.SendAsync( defaultHost, null );
}
catch ( Exception )
{
( (IDisposable)ping ).Dispose();
this.isAlive = false;
}
}
private void PingComplete( Object sender, PingCompletedEventArgs args )
{
this.isAlive = ( args.Error == null && args.Reply.Status == IPStatus.Success );
( (IDisposable)sender ).Dispose();
}
Can I ask how you're certain that it's specifically objects which implement IDisposable? In my experience the most-likely zombie objects are objects which have not properly had all their event handlers removed (thereby leaving a reference to them from another 'live' object and not qualifying them as unreachable during garbage collection).
There are tools which can help track these down by taking a snapshot of the managed heap and stacks and allowing you to see what objects are considered in-use at a given point in time. A freebie is windbg using sos.dll; it'll take some googling for tutorials to show you the commands you need--but it works and it's free. A more user-friendly (don't confused that with "simple") option is Red Gate's ANTS Profiler running in Memory Profiling mode--it's a slick tool.
Edit: Regarding the usefulness of calling Dispose--it provides a deterministic way to cleanup objects. Garbage Collection only runs when your app has ran out of its allocated memory--it's an expensive task which basically stops your application from executing and looks at all objects in existance and builds a tree of "reachable" (in-use) objects, then cleans up the unreachable objects. Manually cleaning up an object frees it before GC ever has to run.
Because the method creating the disposable object may be legitimately returning it as a value, that is, the compiler can't tell how the programming is intending to use it.
What if the disposable object is created in one class/module (say a factory) and is handed off to a different class/module to be used for a while before being disposed of? That use case should be OK, and the compiler shouldn't badger you about it. I suspect that's why there's no compile-time warning---the compiler assumes the Dispose call is in another file.
Determining when and where to call Dispose() is a very subjective thing, dependent on the nature of the program and how it uses disposable objects. Subjective problems are not something compilers are very good at. Instead, this is more a job for static analysis, which is the arena of tools like FxCop and StyleCop, or perhaps more advanced compilers like Spec#/Sing#. Static analysis uses rules to determine if subjective requirements, such as "Always ensure .Dispose() is called at some point.", are met.
I am honestly not sure if any static analyzers exist that are capable of checking whether .Dispose() is called. Even for static analysis as it exists today, that might be a bit on the too-subjective side of things. If you need a place to start looking, however, "Static Analysis for C#" is probably the best place.
I've got a C# class with a Dispose function via IDisposable. It's intended to be used inside a using block so the expensive resource it handles can be released right away.
The problem is that a bug occurred when an exception was thrown before Dispose was called, and the programmer neglected to use using or finally.
In C++, I never had to worry about this. The call to a class's destructor would be automatically inserted at the end of the object's scope. The only way to avoid that happening would be to use the new operator and hold the object behind a pointer, but that required extra work for the programmer isn't something they would do by accident, like forgetting to use using.
Is there any way to for a using block to be automatically used in C#?
Many thanks.
UPDATE:
I'd like to explain why I'm not accepting the finalizer answers. Those answers are technically correct in themselves, but they are not C++ style destructors.
Here's the bug I found, reduced to the essentials...
try
{
PleaseDisposeMe a = new PleaseDisposeMe();
throw new Exception();
a.Dispose();
}
catch (Exception ex)
{
Log(ex);
}
// This next call will throw a time-out exception unless the GC
// runs a.Dispose in time.
PleaseDisposeMe b = new PleaseDisposeMe();
Using FXCop is an excellent suggestion, but if that's my only answer, my question would have to become a plea to the C# people, or use C++. Twenty nested using statements anyone?
Where I work we use the following guidelines:
Each IDisposable class must have a finalizer
Whenever using an IDisposable object, it must be used inside a "using" block. The only exception is if the object is a member of another class, in which case the containing class must be IDisposable and must call the member's 'Dispose' method in its own implementation of 'Dispose'. This means 'Dispose' should never be called by the developer except for inside another 'Dispose' method, eliminating the bug described in the question.
The code in each Finalizer must begin with a warning/error log notifying us that the finalizer has been called. This way you have an extremely good chance of spotting such bugs as described above before releasing the code, plus it might be a hint for bugs occuring in your system.
To make our lives easier, we also have a SafeDispose method in our infrastructure, which calls the the Dispose method of its argument within a try-catch block (with error logging), just in case (although Dispose methods are not supposed to throw exceptions).
See also: Chris Lyon's suggestions regarding IDisposable
Edit:
#Quarrelsome: One thing you ought to do is call GC.SuppressFinalize inside 'Dispose', so that if the object was disposed, it wouldn't be "re-disposed".
It is also usually advisable to hold a flag indicating whether the object has already been disposed or not. The follwoing pattern is usually pretty good:
class MyDisposable: IDisposable {
public void Dispose() {
lock(this) {
if (disposed) {
return;
}
disposed = true;
}
GC.SuppressFinalize(this);
// Do actual disposing here ...
}
private bool disposed = false;
}
Of course, locking is not always necessary, but if you're not sure if your class would be used in a multi-threaded environment or not, it is advisable to keep it.
Unfortunately there isn't any way to do this directly in the code. If this is an issue in house, there are various code analysis solutions that could catch these sort of problems. Have you looked into FxCop? I think that this will catch these situations and in all cases where IDisposable objects might be left hanging. If it is a component that people are using outside of your organization and you can't require FxCop, then documentation is really your only recourse :).
Edit: In the case of finalizers, this doesn't really guarantee when the finalization will happen. So this may be a solution for you but it depends on the situation.
#Quarrelsome
If will get called when the object is moved out of scope and is tidied by the garbage collector.
This statement is misleading and how I read it incorrect: There is absolutely no guarantee when the finalizer will be called. You are absolutely correct that billpg should implement a finalizer; however it will not be called automaticly when the object goes out of scope like he wants. Evidence, the first bullet point under Finalize operations have the following limitations.
In fact Microsoft gave a grant to Chris Sells to create an implementation of .NET that used reference counting instead of garbage collection Link. As it turned out there was a considerable performance hit.
~ClassName()
{
}
EDIT (bold):
If will get called when the object is moved out of scope and is tidied by the garbage collector however this is not deterministic and is not guaranteed to happen at any particular time.
This is called a Finalizer. All objects with a finaliser get put on a special finalise queue by the garbage collector where the finalise method is invoked on them (so it's technically a performance hit to declare empty finalisers).
The "accepted" dispose pattern as per the Framework Guidelines is as follows with unmanaged resources:
public class DisposableFinalisableClass : IDisposable
{
~DisposableFinalisableClass()
{
Dispose(false);
}
public void Dispose()
{
Dispose(true);
}
protected virtual void Dispose(bool disposing)
{
if (disposing)
{
// tidy managed resources
}
// tidy unmanaged resources
}
}
So the above means that if someone calls Dispose the unmanaged resources are tidied. However in the case of someone forgetting to call Dispose or an exception preventing Dispose from being called the unmanaged resources will still be tidied away, only slightly later on when the GC gets its grubby mitts on it (which includes the application closing down or unexpectedly ending).
The best practice is to use a finaliser in your class and always use using blocks.
There isn't really a direct equivalent though, finalisers look like C destructors, but behave differently.
You're supposed to nest using blocks, that's why the C# code layout defaults to putting them on the same line...
using (SqlConnection con = new SqlConnection("DB con str") )
using (SqlCommand com = new SqlCommand( con, "sql query") )
{
//now code is indented one level
//technically we're nested twice
}
When you're not using using you can just do what it does under the hood anyway:
PleaseDisposeMe a;
try
{
a = new PleaseDisposeMe();
throw new Exception();
}
catch (Exception ex) { Log(ex); }
finally {
//this always executes, even with the exception
a.Dispose();
}
With managed code C# is very very good at looking after its own memory, even when stuff is poorly disposed. If you're dealing with unmanaged resources a lot it's not so strong.
This is no different from a programmer forgetting to use delete in C++, except that at least here the garbage collector will still eventually catch up with it.
And you never need to use IDisposable if the only resource you're worried about is memory. The framework will handle that on it's own. IDisposable is only for unmanaged resources like database connections, filestreams, sockets, and the like.
A better design is to make this class release the expensive resource on its own, before its disposed.
For example, If its a database connection, only connect when needed and release immediately, long before the actual class gets disposed.