I'm putting together a custom SynchronizedCollection<T> class so that I can have a synchronized Observable collection for my WPF application. The synchronization is provided via a ReaderWriterLockSlim, which, for the most part, has been easy to apply. The case I'm having trouble with is how to provide thread-safe enumeration of the collection. I've created a custom IEnumerator<T> nested class that looks like this:
private class SynchronizedEnumerator : IEnumerator<T>
{
private SynchronizedCollection<T> _collection;
private int _currentIndex;
internal SynchronizedEnumerator(SynchronizedCollection<T> collection)
{
_collection = collection;
_collection._lock.EnterReadLock();
_currentIndex = -1;
}
#region IEnumerator<T> Members
public T Current { get; private set;}
#endregion
#region IDisposable Members
public void Dispose()
{
var collection = _collection;
if (collection != null)
collection._lock.ExitReadLock();
_collection = null;
}
#endregion
#region IEnumerator Members
object System.Collections.IEnumerator.Current
{
get { return Current; }
}
public bool MoveNext()
{
var collection = _collection;
if (collection == null)
throw new ObjectDisposedException("SynchronizedEnumerator");
_currentIndex++;
if (_currentIndex >= collection.Count)
{
Current = default(T);
return false;
}
Current = collection[_currentIndex];
return true;
}
public void Reset()
{
if (_collection == null)
throw new ObjectDisposedException("SynchronizedEnumerator");
_currentIndex = -1;
Current = default(T);
}
#endregion
}
My concern, however, is that if the Enumerator is not Disposed, the lock will never be released. In most use cases, this is not a problem, as foreach should properly call Dispose. It could be a problem, however, if a consumer retrieves an explicit Enumerator instance. Is my only option to document the class with a caveat implementer reminding the consumer to call Dispose if using the Enumerator explicitly or is there a way to safely release the lock during finalization? I'm thinking not, since the finalizer doesn't even run on the same thread, but I was curious if there other ways to improve this.
EDIT
After thinking about this a bit and reading the responses (particular thanks to Hans), I've decided this is definitely a bad idea. The biggest issue actually isn't forgetting to Dispose, but rather a leisurely consumer creating deadlock while enumerating. I now only read-lock long enough to get a copy and return the enumerator for the copy.
You are right, that's a problem. The finalizer is useless, it will run far too late to be of any use. The code should have deadlocked heavily before that anyway. Unfortunately, there's no way for you to tell the difference between the a foreach calling your MoveNext/Current members or the client code using them explicitly.
No fix, don't do this. Microsoft didn't do it either, they had plenty of reason to back in .NET 1.x. The only real thread-safe iterator you can make is one that creates a copy of the collection object in the GetEnumerator() method. The iterator getting out of sync with the collection is no joy either though.
This seems far too error-prone to me. It encourages situations in which a lock is implicitly/silently taken out, in a way that is not clear to the reader of the code, and makes it likely that a crucial fact about the interface will be misunderstood.
Usually it's a good idea to duplicate common patterns - represent an enumerable collection with IEnumerable<T> that is disposed when you're done with it - but the added ingredient of taking out a lock makes a big difference, unfortunately.
I'd suggest the ideal approach would be to not offer enumeration on collections shared between threads at all. Try to design the whole system so it isn't needed. Obviously this is going to be a crazy pipe-dream sometimes.
So the next best thing would be to define a context within which an IEnumerable<T> is temporarily available, while the lock exists:
public class SomeCollection<T>
{
// ...
public void EnumerateInLock(Action<IEnumerable<T>> action) ...
// ...
}
That is, when the user of this collection wants to enumerate it, they do this:
someCollection.EnumerateInLock(e =>
{
foreach (var item in e)
{
// blah
}
});
This makes the lifetime of the lock explicitly stated by a scope (represented by the lambda body, working much like a lock statement), and impossible to extend accidentally by forgetting to dispose. It's impossible to abuse this interface.
The implementation of the EnumerateInLock method would be like this:
public void EnumerateInLock(Action<IEnumerable<T>> action)
{
var e = new EnumeratorImpl(this);
try
{
_lock.EnterReadLock();
action(e);
}
finally
{
e.Dispose();
_lock.ExitReadLock();
}
}
Notice how the EnumeratorImpl (which needs no particular locking code of its own) is always disposed before the lock is exited. After disposal, it throws ObjectDisposedException in response to any method call (other than Dispose, which is ignored.)
This means that even if there is an attempt to abuse the interface:
IEnumerable<C> keepForLater = null;
someCollection.EnumerateInLock(e => keepForLater = e);
foreach (var item in keepForLater)
{
// aha!
}
This will always throw, rather than failing mysteriously sometimes based on the timing.
Using a method that accepts a delegate like this is a general technique for managing resource lifetimes commonly used in Lisp and other dynamic languages, and while it is less flexible than implementing IDisposable, that reduced flexibility is often a blessing: it removes the concern over clients "forgetting to dispose".
Update
From your comment, I see that you need to be able to hand a reference to the collection to an existing UI framework, which will therefore expect to be able to use the normal interface to a collection, i.e. directly get an IEnumerable<T> from it and be trusted to clean it up quickly. In which case, why worry? Trust the UI framework to update the UI and dispose the collection rapidly.
Your only other realistic option is simply to make a copy of the collection when the enumerator is requested. That way, the lock only needs to be held when the copy is being made. As soon as it's ready, the lock is released. This may be more efficient if the collections are usually small, so the overhead of the copy is less than the performance saving due to shorter locks.
It's tempting (for about a nanosecond) to suggest that you use a simple rule: if the collection is smaller than some threshold, make the copy, otherwise do it in your original way; choose the implementation dynamically. That way you get the optimum performance - set the threshold (by experiment) such that the copy is cheaper than holding the lock. However, I'd always think twice (or a billion times) about such "clever" ideas in threaded code, because what if there is an abuse of the enumerator somewhere? If you forget to dispose it, you won't see a problem unless it's a large collection... A recipe for hidden bugs. Don't go there!
Another potential drawback with the "expose a copy" approach is that clients will undoubtedly fall under the assumption that if an item is in the collection it is exposed to the world, but as soon as it is removed from the collection it is safely hidden. This will now be wrong! The UI thread will obtain an enumerator, then my background thread will remove the last item from it, and then begin mutating it in the mistaken belief that, because it was removed, no one else can see it.
So the copying approach requires every item on the collection to effectively have its own synchronization, where most coders will assume that they can shortcut this by using the collection's synchronization instead.
I had to do this recently. The way I did it was to abstract it so that there is an inner object (reference) that contains both the actual list/array and the count (and a GetEnumerator() implementation; then I can do lock-free, thread-safe enumeration by having:
public IEnumerator<T> GetEnumerator() { return inner.GetEnumerator();}
The Add etc need to be synchronized, but they change the inner reference (since reference updates are atomic, you don't need to synchronize GetEnumerator()). This then means that any enumerator will return as many items as were there when the enumerator was created.
Of course, it helps that my scenario was simple, and my list was Add only... if you need to support mutate / remove then it is much trickier.
In the must used IDisposable implementation, you create a protected Dispose(bool managed) method which always disposes the unmanaged resources you use. By calling your protected Dispose(false) method from your finalizer, you'll dispose the lock as required. The lock is managed, do you'll only dispose it when Dispose(true) is called, where true means that managed objects need to be disposed. Otherwise, when the public Dispose() is called explicitly, it calls the protected Dispose(true) and also GC.SuppressFinalize(this) to prevent the finalizer from running (because there is nothing to dispose of anymore).
Because you never know when the user is done with the enumerator, you have no other option than documenting that the user has to dispose the object. You might want to propose that the user uses a using(){ ... } construction, which automatically disposes the object when done.
Related
I've seen many examples of the lock usage, and it's usually something like this:
private static readonly object obj = new object();
lock (obj)
{
// code here
}
Is it possible to lock based on a property of a class? I didn't want to lock globally for any calls to the method with the lock statement, I'd like to lock only if the object passed as argument had the same property value as another object which was being processed prior to that.
Is that possible? Does that make sense at all?
This is what I had in mind:
public class GmailController : Controller
{
private static readonly ConcurrentQueue<PushRequest> queue = new ConcurrentQueue<PushRequest>();
[HttpPost]
public IActionResult ProcessPushNotification(PushRequest push)
{
var existingPush = queue.FirstOrDefault(q => q.Matches(push));
if (existingPush == null)
{
queue.Enqueue(push);
existingPush = push;
}
try
{
// lock if there is an existing push in the
// queue that matches the requested one
lock (existingPush)
{
// process the push notification
}
}
finally
{
queue.TryDequeue(out existingPush);
}
}
}
Background: I have an API where I receive push notifications from Gmail's API when our users send/receive emails. However, if someone sends a message to two users at the same time, I get two push notifications. My first idea was querying the database before inserting (based on subject, sender, etc). In some rare cases, the query of the second call is made before the SaveChanges of the previous call, so I end up having duplicates.
I know that if I ever wanted to scale out, lock would become useless. I also know I could just create a job to check recent entries and eliminate duplicates, but I was trying something different. Any suggestions are welcome.
Let me first make sure I understand the proposal. The problem given is that we have some resource shared to multiple threads, call it database, and it admits two operations: Read(Context) and Write(Context). The proposal is to have lock granularity based on a property of the context. That is:
void MyRead(Context c)
{
lock(c.P) { database.Read(c); }
}
void MyWrite(Context c)
{
lock(c.P) { database.Write(c); }
}
So now if we have a call to MyRead where the context property has value X, and a call to MyWrite where the context property has value Y, and the two calls are racing on two different threads, they are not serialized. However, if we have, say, two calls to MyWrite and a call to MyRead, and in all of them the context property has value Z, those calls are serialized.
Is this possible? Yes. That doesn't make it a good idea. As implemented above, this is a bad idea and you shouldn't do it.
It is instructive to learn why it is a bad idea.
First, this simply fails if the property is a value type, like an integer. You might think, well, my context is an ID number, that's an integer, and I want to serialize all accesses to the database using ID number 123, and serialize all accesses using ID number 345, but not serialize those accesses with respect to each other. Locks only work on reference types, and boxing a value type always gives you a freshly allocated box, so the lock would never be contested even if the ids were the same. It would be completely broken.
Second, it fails badly if the property is a string. Locks are logically "compared" by reference, not by value. With boxed integers, you always get different references. With strings, you sometimes get different references! (Because of interning being applied inconsistently.) You could be in a situation where you are locking on "ABC" and sometimes another lock on "ABC" waits, and sometimes it does not!
But the fundamental rule that is broken is: you must never lock on an object unless that object has been specifically designed to be a lock object, and the same code which controls access to the locked resource controls access to the lock object.
The problem here is not "local" to the lock but rather global. Suppose your property is a Frob where Frob is a reference type. You don't know if any other code in your process is also locking on that same Frob, and therefore you don't know what lock ordering constraints are necessary to prevent deadlocks. Whether a program deadlocks or not is a global property of a program. Just like you can build a hollow house out of solid bricks, you can build a deadlocking program out of a collection of locks that are individually correct. By ensuring that every lock is only taken out on a private object that you control, you ensure that no one else is ever locking on one of your objects, and therefore the analysis of whether your program contains a deadlock becomes simpler.
Note that I said "simpler" and not "simple". It reduces it to almost impossible to get correct, from literally impossible to get correct.
So if you were hell bent on doing this, what would be the right way to do it?
The right way would be to implement a new service: a lock object provider. LockProvider<T> needs to be able to hash and compare for equality two Ts. The service it provides is: you tell it that you want a lock object for a particular value of T, and it gives you back the canonical lock object for that T. When you're done, you say you're done. The provider keeps a reference count of how many times it has handed out a lock object and how many times it got it back, and deletes it from its dictionary when the count goes to zero, so that we don't have a memory leak.
Obviously the lock provider needs to be threadsafe and needs to be extremely low contention, because it is a mechanism designed to prevent contention, so it had better not cause any! If this is the road you intend to go down, you need to get an expert on C# threading to design and implement this object. It is very easy to get this wrong. As I have noted in comments to your post, you are attempting to use a concurrent queue as a sort of poor lock provider and it is a mass of race condition bugs.
This is some of the hardest code to get correct in all of .NET programming. I have been a .NET programmer for almost 20 years and implemented parts of the compiler and I do not consider myself competent to get this stuff right. Seek the help of an actual expert.
Although I find Eric Lippert's answer fantastic and marked it as the correct one (and I won't change that), his thoughts made me think and I wanted to share an alternative solution I found to this problem (and I'd appreciate any feedbacks), even though I'm not going to use it as I ended up using Azure functions with my code (so this wouldn't make sense), and a cron job to detected and eliminate possible duplicates.
public class UserScopeLocker : IDisposable
{
private static readonly object _obj = new object();
private static ICollection<string> UserQueue = new HashSet<string>();
private readonly string _userId;
protected UserScopeLocker(string userId)
{
this._userId = userId;
}
public static UserScopeLocker Acquire(string userId)
{
while (true)
{
lock (_obj)
{
if (UserQueue.Contains(userId))
{
continue;
}
UserQueue.Add(userId);
return new UserScopeLocker(userId);
}
}
}
public void Dispose()
{
lock (_obj)
{
UserQueue.Remove(this._userId);
}
}
}
...then you would use it like this:
[HttpPost]
public IActionResult ProcessPushNotification(PushRequest push)
{
using(var scope = UserScopeLocker.Acquire(push.UserId))
{
// process the push notification
// two threads can't enter here for the same UserId
// the second one will be blocked until the first disposes
}
}
The idea is:
UserScopeLocker has a protected constructor, ensuring you call Acquire.
_obj is private static readonly, only the UserScopeLocker can lock this object.
_userId is a private readonly field, ensuring even its own class can't change its value.
lock is done when checking, adding and removing, so two threads can't compete on these actions.
Possible flaws I detected:
Since UserScopeLocker relies on IDisposable to release some UserId, I can't guarantee the caller will properly use using statement (or manually dispose the scope object).
I can't guarantee the scope won't be used in a recursive function (thus possibly causing a deadlock).
I can't guarantee the code inside the using statement won't call another function which also tries to acquire a scope to the user (this would also cause a deadlock).
I have a System.Collections.Generic.List<T> to which I only ever add items in a timer callback. The timer is restarted only after the operation completes.
I have a System.Collections.Concurrent.ConcurrentQueue<T> which stores indices of added items in the list above. This store operation is also always performed in the same timer callback described above.
Is a read operation that iterates the queue and accesses the corresponding items in the list thread safe?
Sample code:
private List<Object> items;
private ConcurrentQueue<int> queue;
private Timer timer;
private void callback(object state)
{
int index = items.Count;
items.Add(new object());
if (true)//some condition here
queue.Enqueue(index);
timer.Change(TimeSpan.FromMilliseconds(500), TimeSpan.FromMilliseconds(-1));
}
//This can be called from any thread
public IEnumerable<object> AccessItems()
{
foreach (var index in queue)
{
yield return items[index];
}
}
My understanding:
Even if the list is resized when it is being indexed, I am only accessing an item that already exists, so it does not matter whether it is read from the old array or the new array. Hence this should be thread-safe.
Is a read operation that iterates the queue and accesses the corresponding items in the list thread safe?
Is it documented as being thread safe?
If no, then it is foolish to treat it as thread safe, even if it is in this implementation by accident. Thread safety should be by design.
Sharing memory across threads is a bad idea in the first place; if you don't do it then you don't have to ask whether the operation is thread safe.
If you have to do it then use a collection designed for shared memory access.
If you can't do that then use a lock. Locks are cheap if uncontended.
If you have a performance problem because your locks are contended all the time then fix that problem by changing your threading architecture rather than trying to do dangerous and foolish things like low-lock code. No one writes low-lock code correctly except for a handful of experts. (I am not one of them; I don't write low-lock code either.)
Even if the list is resized when it is being indexed, I am only accessing an item that already exists, so it does not matter whether it is read from the old array or the new array.
That's the wrong way to think about it. The right way to think about it is:
If the list is resized then the list's internal data structures are being mutated. It is possible that the internal data structure is mutated into an inconsistent form halfway through the mutation, that will be made consistent by the time the mutation is finished. Therefore my reader can see this inconsistent state from another thread, which makes the behaviour of my entire program unpredictable. It could crash, it could go into an infinite loop, it could corrupt other data structures, I don't know, because I'm running code that assumes a consistent state in a world with inconsistent state.
Big edit
The ConcurrentQueue is only safe with regard to the Enqueue(T) and T Dequeue() operations.
You're doing a foreach on it and that doesn't get synchronized at the required level.
The biggest problem in your particular case is the fact the enumerating of the Queue (which is a Collection in it's own right) might throw the wellknown "Collection has been modified" exception. Why is that the biggest problem ? Because you are adding things to the queue after you've added the corresponding objects to the list (there's also a great need for the List to be synchronized but that + the biggest problem get solved with just one "bullet"). While enumerating a collection it is not easy to swallow the fact that another thread is modifying it (even if on a microscopic level the modification is a safe - ConcurrentQueue does just that).
Therefore you absolutely need synchronize the access to the queues (and the central List while you're at it) using another means of synchronization (and by that I mean you can also forget abount ConcurrentQueue and use a simple Queue or even a List since you never Dequeue things).
So just do something like:
public void Writer(object toWrite) {
this.rwLock.EnterWriteLock();
try {
int tailIndex = this.list.Count;
this.list.Add(toWrite);
if (..condition1..)
this.queue1.Enqueue(tailIndex);
if (..condition2..)
this.queue2.Enqueue(tailIndex);
if (..condition3..)
this.queue3.Enqueue(tailIndex);
..etc..
} finally {
this.rwLock.ExitWriteLock();
}
}
and in the AccessItems:
public IEnumerable<object> AccessItems(int queueIndex) {
Queue<object> whichQueue = null;
switch (queueIndex) {
case 1: whichQueue = this.queue1; break;
case 2: whichQueue = this.queue2; break;
case 3: whichQueue = this.queue3; break;
..etc..
default: throw new NotSupportedException("Invalid queue disambiguating params");
}
List<object> results = new List<object>();
this.rwLock.EnterReadLock();
try {
foreach (var index in whichQueue)
results.Add(this.list[index]);
} finally {
this.rwLock.ExitReadLock();
}
return results;
}
And, based on my entire understanding of the cases in which your app accesses the List and the various Queues, it should be 100% safe.
End of big edit
First of all: What is this thing you call Thread-Safe ? by Eric Lippert
In your particular case, I guess the answer is no.
It is not the case that inconsistencies might arrise in the global context (the actual list).
Instead it is possible that the actual readers (who might very well "collide" with the unique writer) end up with inconsistencies in themselves (their very own Stacks meaning: local variables of all methods, parameters and also their logically isolated portion of the heap)).
The possibility of such "per-Thread" inconsistencies (the Nth thread wants to learn the number of elements in the List and finds out that value is 39404999 although in reality you only added 3 values) is enough to declare that, generally speaking that architecture is not thread-safe ( although you don't actually change the globally accessible List, simply by reading it in a flawed manner ).
I suggest you use the ReaderWriterLockSlim class.
I think you will find it fits your needs:
private ReaderWriterLockSlim rwLock = new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion);
private List<Object> items;
private ConcurrentQueue<int> queue;
private Timer timer;
private void callback(object state)
{
int index = items.Count;
this.rwLock.EnterWriteLock();
try {
// in this place, right here, there can be only ONE writer
// and while the writer is between EnterWriteLock and ExitWriteLock
// there can exist no readers in the following method (between EnterReadLock
// and ExitReadLock)
// we add the item to the List
// AND do the enqueue "atomically" (as loose a term as thread-safe)
items.Add(new object());
if (true)//some condition here
queue.Enqueue(index);
} finally {
this.rwLock.ExitWriteLock();
}
timer.Change(TimeSpan.FromMilliseconds(500), TimeSpan.FromMilliseconds(-1));
}
//This can be called from any thread
public IEnumerable<object> AccessItems()
{
List<object> results = new List<object>();
this.rwLock.EnterReadLock();
try {
// in this place there can exist a thousand readers
// (doing these actions right here, between EnterReadLock and ExitReadLock)
// all at the same time, but NO writers
foreach (var index in queue)
{
this.results.Add ( this.items[index] );
}
} finally {
this.rwLock.ExitReadLock();
}
return results; // or foreach yield return you like that more :)
}
No because you are reading and writing to/from the same object concurrently. This is not documented to be safe so you can't be sure it is safe. Don't do it.
The fact that it is in fact unsafe as of .NET 4.0 means nothing, btw. Even if it was safe according to Reflector it could change anytime. You can't rely on the current version to predict future versions.
Don't try to get away with tricks like this. Why not just do it in an obviously safe way?
As a side note: Two timer callbacks can execute at the same time, so your code is doubly broken (multiple writers). Don't try to pull off tricks with threads.
It is thread-safish. The foreach statement uses the ConcurrentQueue.GetEnumerator() method. Which promises:
The enumeration represents a moment-in-time snapshot of the contents of the queue. It does not reflect any updates to the collection after GetEnumerator was called. The enumerator is safe to use concurrently with reads from and writes to the queue.
Which is another way of saying that your program isn't going to blow up randomly with an inscrutable exception message like the kind you'll get when you use the Queue class. Beware of the consequences though, implicit in this guarantee is that you may well be looking at a stale version of the queue. Your loop will not be able to see any elements that were added by another thread after your loop started executing. That kind of magic doesn't exist and is impossible to implement in a consistent way. Whether or not that makes your program misbehave is something you will have to think about and can't be guessed from the question. It is pretty rare that you can completely ignore it.
Your usage of the List<> is however utterly unsafe.
In C#, if a class, such as a manager class, does not have resources, is there any benefit to having it : IDisposable?
Simple example:
public interface IBoxManager
{
int addBox(Box b);
}
public class BoxManager : IBoxManager
{
public int addBox(Box b)
{
using(dataContext db = new dataContext()){
db.Boxes.add(b);
db.SaveChanges();
}
return b.id;
}
}
Will there be any benefit in memory use when using BoxManager if it also implements IDisposable? public class BoxManager : IBoxManager , IDisposable
For example:
BoxManager bm = new BoxManager();
bm.add(myBox);
bm.dispose();//is there benefit to doing this?
There are only 2 reasons for implementing IDisposable on a type
The type contains native resources which must be freed when the type is no longer used
The type contains fields of type IDisposable
If neither of these are true then don't implement IDisposable
EDIT
Several people have mentioned that IDisposable is a nice way to implement begin / end or bookended operations. While that's not the original intent of IDisposable it does provide for a very nice pattern.
class Operation {
class Helper : IDisposable {
internal Operation Operation;
public void Dispose() {
Operation.EndOperation();
}
}
public IDisposable BeginOperation() {
...
return new Helper() { Operation = this };
}
private void EndOperation() {
...
}
}
Note: Another interesting way to implement this pattern is with lambdas. Instead of giving an IDisposable back to the user and hoping they don't forget to call Dispose have them give you a lambda in which they can execute the operation and you close out the operation
public void BeginOperation(Action action) {
BeginOperationCore();
try {
action();
} finally {
EndOperation();
}
}
There won't be a scrap of difference between the disposable and non-disposable version if you don't explicitly make use of the Dispose() method.
While your code wouldn't benefit from implementing IDisposable, I can't agree with other opinions here that state that IDisposable is only meant to (directly or indirectly) free native resources. IDisposable can be used whenever the object needs to perform clean up task at the end of it's lifetime span. It's extremely useful together with using.
A very popular example: in ASP.NET MVC Html.BeginForm returns an IDisposable. On creation, the object opens the tag, when Dispose is called it closes it. No native resources involved, yet still a good use of IDisposable.
No, there will be no benefit if you don't do something useful like releasing unmanaged resources that your class might hold in the Dispose method.
One major point of confusion, which may not be applicable in your case but arises often, is what exactly constitutes a "resource". From the perspective of an object, an unmanaged resource is something which an outside entity () is "doing"(*) on its behalf, which that outside entity will keep doing--to the detriment of other entitites--until told to stop. For example, if an object opens a file, the machine which hosts the file may grant that object exclusive access, denying everyone else in the universe a chance to use it unless or until it gets notified that the exclusive access isn't needed anymore.
(*) which could be anything, anywhere; possibly not even on the same computer.
(**) or some way in which the the behavior or state of an outside entity is altered
If an outside entity is doing something on behalf of an object which is abandoned and disappears without first letting the entity know its services are no longer required, the outside entity will have no way of knowing that it should stop acting on behalf of the object which no longer exists. IDisposable provides one way of avoiding this problem by providing a standard means of notifying objects when their services are not required. An object whose services are no longer required will generally not need to ask any further favors from any other entities, and will thus be able to request that any entities that had been acting on its behalf should stop doing so.
To allow for the case where an object gets abandoned without IDisposable.Dispose() having been called first, the system allows objects to register a "failsafe" cleanup method called Finalize(). Because for whatever reason, the creators of C# don't like the term Finalize(), the language requires the use of a construct called a "destructor" which does much the same thing. Note that in general, Finalize() will mask rather than solve problems, and can create problems of its own, so it should be used with extreme caution if at all.
A "managed resource" is typically a name given to an object which implements IDisposable and usually, though not always, implements a finalizer.
No, if there are no (managed or unmanaged) resources there is no need for IDisposable either.
Small caveat: some people use IDisposable to clean up eventhandlers or large memory buffers but
you don't seem to use those
it's a questionable pattern anyway.
From my personal experience (confirmed with discussion and other posts here) I would say, that there could be a situations where your object use massive amount of events, or not massive amount but frequent subscriptions and unsubscription from the event which sometimes leads to that the object is not garbage collected. In this case I in Dispose unsubscribe from all events my object subscribed before.
Hope this helps.
IDisposable is also great if you want to benefit the using () {} syntax.
In a WPF project with ViewModels, I wanted to be able to temporarily disable NotifyPropertyChange events from raising. To be sure other developers will re-enable notifications, I wrote a bit of code to be able to write something like:
using (this.PreventNotifyPropertyChanges()) {
// in this block NotifyPropertyChanged won't be called when changing a property value
}
The syntax looks okay and is easily readable. For it to work, there's a bit of code to write. You will need a simple Disposable object and counter.
public class MyViewModel {
private volatile int notifyPropertylocks = 0; // number of locks
protected void NotifyPropertyChanged(string propertyName) {
if (this.notifyPropertylocks == 0) { // check the counter
this.NotifyPropertyChanged(...);
}
}
protected IDisposable PreventNotifyPropertyChanges() {
return new PropertyChangeLock(this);
}
public class PropertyChangeLock : IDisposable {
MyViewModel vm;
// creating this object will increment the lock counter
public PropertyChangeLock(MyViewModel vm) {
this.vm = vm;
this.vm.notifyPropertylocks += 1;
}
// disposing this object will decrement the lock counter
public void Dispose() {
if (this.vm != null) {
this.vm.notifyPropertylocks -= 1;
this.vm = null;
}
}
}
}
There are no resources to dispose here. I wanted a clean code with a kind of try/finally syntax. The using keyword looks better.
is there any benefit to having it : IDisposable?
It doesn't look so in your specific example, however: there is one good reason to implement IDisposable even if you don’t have any IDisposable fields: your descendants might.
This is one of the big architectural problems of IDisposable highlighted in IDisposable: What your mother never told you about resource deallocation. Basically, unless your class is sealed you need to decide whether your descendants are likely to have IDisposable members. And this isn't something you can realistically predict.
So, if your descendants are likely to have IDisposable members, that might be a good reason to implement IDisposable in the base class.
Short answer would be no. However, you can smartly use the nature of the Dispose() executement at the end of the object lifecycle. One have already gave a nice MVC example (Html.BeginForm)
I would like to point out one important aspect of IDisposable and using() {} statement. At the end of the Using statement Dispose() method is automatically called on the using context object (it must have implemented IDisposable interface, of course).
There one more reason that no one mentioned (though it's debateful if it really worth it):
The convension says that if someone uses a class that implement IDisposable, it must call its Dispose method (either explicitly or via the 'using' statement).
But what happens if V1 of a class (in your public API) didn't need IDisposable, but V2 does? Technically it doesn't break backward compatibility to add an interface to a class, but because of that convension, it is! Because old clients of your code won't call its Dispose method and may cause resources to not get freed.
The (almost) only way to avoid it is to implement IDisposable in any case you suspect that you'll need it in the future, to make sure that your clients always call your Dispose method, that some day may be really needed.
The other (and probably better) way is to implemet the lambda pattern mentioned by JaredPar above in the V2 of the API.
I was wondering if this statement would cause sync issues:
List<Character> characters = World.CharacterManager.Characters;
'Characters' is a class
'CharacterManager.Characters' would look something like this:
public List<Character> Characters
{
get
{
lock (this.objLock) { return this.characters; }
}
}
would this cause synchronization problems?
I want to use the referenced List to iterate through to find the character I am looking for.
The problem is that you are locking during the get, but once each thread has a reference to the collection, they can both act against it at the same time. Since the List<T>'s members aren't thread safe, you will encounter random bugs and exceptions when iterating, adding, removing, etc the collection.
You probably need to return a thread-safe collection. There isn't a 100% compatible thread-safe version, so you need to look through System.Collections.Concurrent and find a version that you can use.
The lock is useless that way. You will have to use a thread-safe collection such as Will suggested, or if you don't need write access you can expose only a read only version of your list like so:
public ReadOnlyCollection<Character> Characters {
get {
lock (locker) { return this.characters.AsReadOnly(); }
}
}
These collections cannot be modified, so if your Character type is immutable, you don't have any synchronization issues. If Character is mutable, you have again a problem, but you would have that problem even with a thread-safe collection. I hope you're aware of that. You can also expose the property returning an IList<Character>, but usually I find it better to tell the caller that the object is read only.
If you need write access, you could also do that by providing the appropriate methods at the scope of the CharacterManager and synchronize them. Jesse has written a nice example on how to do this.
EDIT: SyncRoot is not present on ICollection<T>.
Does the calling code actually need to be able to add and remove from the list? If so, that's not considered a best practice. Here's a (possible) way to implement without that requirement, instead putting the adding and removing of Character items into the CharacterManager class itself:
internal sealed class CharacterManager
{
private readonly IList<Character> characters = new List<Character>();
public ReadOnlyCollection<Character> Characters
{
get
{
lock (this.characters)
{
return this.characters.AsReadOnly();
}
}
}
public void Add(Character character)
{
lock (this.characters)
{
this.characters.Add(character);
}
}
public void Remove(Character character)
{
lock (this.characters)
{
this.characters.Remove(character);
}
}
}
If you are only wanting the caller to be able to enumerate the list then your property should have the type IEnumerable instead. If this is the case, then I would also make a copy of that list and return the copy. If the list is changed while you are enumerating it, then it will become invalid and throw an exception. The trade off is that the caller could possibly not have the latest version of the list. I would be inclined to convert it to a method named GetCharactersAsOfNow() instead of a property to help show that the caller needs to get an updated list for each call.
However if you are planning on allowing the caller to modify the list, then you have to make note that the list is not thread safe and require your caller to perform thread synchronization. Given that the caller now has this responsibility then you no longer need the lock in your property getter.
I have a a list of disposable items that I am adding to collection that already contains a number of disposable items. I wrap the code in a try...finally block so that if an exception is thrown while I am copying the items from the list to the collection all the objects in the list get disposed of correctly:
private static void LoadMenuItems(ApplicationMenu parent)
{
List<ApplicationMenuItem> items = null;
try
{
items = DataContext.GetMenuItems(parent.Id);
foreach (var item in items)
{
parent.Items.Add(item);
}
items = null;
}
finally
{
if (items != null)
{
foreach (var item in items)
{
item.Dispose();
}
}
}
}
If an exception occurs after adding a number of the objects to the collection, I'll have a situation where the collection contains some disposed objects. Which could give rise to those disposed objects being disposed of again in the following try...catch block:
try
{
// Assume that menu.Items contains some items prior
// to the call to LoadMenuItems.
LoadMenuItems(menu);
}
catch
{
// The Dispose() iterates through menu.Items calling
// Dispose() on each.
menu.Dispose();
}
So what I am looking for possible solutions to stop Dispose() being called twice. I have a solution in mind, but thought I would give it to the community to see if there are any alternatives.
Which could give rise to those disposed objects being disposed of again
Which should be quite alright. The contract for Dispose() is very specific: it should be safe to call it multiple times.
But another way to get rid of it:
Analyzing your logic I would say the finally part is superfluous, maybe the analyzer thinks so too. You really do solve the same problem twice.
Most cases where one might be worried about "accidentally" disposing an object multiple times come about because there is confusion about who "owns" the object in question, and such confusion will likely create other problems in addition to repeated disposal. While one could avoid having the multiple-disposal itself cause problems by having the disposal method use a flag so the second dispose attempt will return harmlessly, doing that without resolving the confusion about IDisposable ownership would not lave the more serious issues unresolved.
The primary scenarios where repeated disposal attempts should not be regarded as indicating broader problems are
Situations where creation of an object fails with a partially-constructed object; while one could probably define policies as to which parts of a partially-constructed object are responsible for cleaning up which other parts, it may be easier to simply have each part that's told to perform an "emergency" cleanup to tell every part it knows about to do likewise if it hasn't already. In most disposal scenarios, confusions regarding object ownership can result in objects being disposed while still in use, but if an object factory fails, that generally implies that no references to the object have been released to anyone who is going to use them.
Situations where disposal of an object which is in use is a legitimate usage scenario with predictable semantics. For example, some data sources have blocking wait methods, and explicitly specify that disposal of the data source while a blocking method is waiting on it will that method to fail without further delay. In some cases, it may well be that yanking the disposable resource out from under the task that's using it is the only way for such a task to become unstuck.
Your scenario seems somewhat like the first one, except that it looks like you'll be disposing of each item after it gets added to parent.Items, which would suggest that you may have muddled issues of object ownership.