What happens if I call Dispose() on locked object?
lock (obj)
{
obj.Dispose();
}
And what happens if I skip Monitor.Exit() call in this case:
Monitor.Enter(obj);
obj.Dispose();
Implementations of Dispose are not generally thread-safe.
This is because you should not dispose of an object while there are still references which may be used. This implies you should not dispose of an object which any other thread holds a reference to.
IDisposable/Dispose is not a method of managing object lifetime, either in general or across threads - it is a paradigm for releasing the object's resources once its lifetime is over. (The using statement is an idiom/paradigm for use where the object's proper lifetime is the scope of the statement.)
So if you are calling dispose while another thread has the object locked, something has already gone very wrong.
What happens if I call Dispose() on locked object?
First, the object itself is not locked (protected) to begin with. The reference used in the lock keyword is used to mark or tag a section of the code that should not run simultaneously with any other (or same) section of code that used the same object reference. It does not actually affect the object itself. This is a pretty common misconception about how locks work in .NET.
In this context calling Dispose is no different than calling any other method. There is nothing special that happens. It just means that two different threads cannot be executing Dispose simultaneously. What you are doing is not only acceptable, but is actually recommend if the class is not thread-safe.
And what happens if I skip Monitor.Exit() call in this case:
You should always release locks. Keeping in mind that locks do not act upon the object reference itself it should be easier to understand that you would be leaving a lock in an acquired state. It does not matter that the object is disposed. Remember, Monitor (or lock) does not lock or protect the object itself. It only marks or tags a section of code. If a thread attempts to acquire a lock with the same object again then that thread will be forced to wait indefinitely possibly resulting in a deadlock.
A more interesting question is whether this could lead to a memory leak. Does Monitor.Enter root the object? The answer is no. This can be demonstrated with the following example.
public class Program
{
public static void Main(string[] args)
{
var foo = new Foo();
Monitor.Enter(foo);
foo = null;
GC.Collect();
GC.WaitForPendingFinalizers();
Console.ReadLine();
}
}
internal class Foo
{
~Foo()
{
Console.WriteLine("~Foo");
}
}
If you compile and run this you will observe that "~Foo" is printed out. This suggests that Monitor.Enter does not hold a reference internally. So while it is not advisable to skip the Monitor.Exit call you can have some comfort in knowing that it is not going to cause a memory leak.1
1Actually, this may not be entirely true. While it is fairly easy to demonstrate that there is no managed memory leak things might be different in the unmanaged realm. Short of looking at the SSCLI code we really do not know what Monitor.Enter is doing internally. Maybe it is allocating one additional array slot (or whatever) in the unmanaged heap or stack. I would like to think Microsoft considered this obscure scenario, but who knows. The point is that Monitor.Enter and Monitor.Exit calls should always be paired so that you do not have to worry about it.
The Dispose paradigm is distinct from the lock one. You can safely dispose an object on which you currently hold a mutual-exclusion lock; however, you should still release the lock afterwards. If you fail to do this, other threads that call Monitor.Enter on the (now disposed) object would block indefinitely.
You can always lock on a existing object, The Dispose method won't delete your object, so the object's reference still exists. The Dispose() method isn't affecting the instance of the object.
For your first question - what happens is that only one threat at any given moment may call Dispose().
For your second question - if multiple threads run this code, the 1st one of them will invoke the Dispose method. The rest will block forever.
There is nothing special in the dispose method. It is just like any other method, except that it participates in some syntactic sugar (using statements, for example).
Related
I have a method Foo that returns a IDisposable object. Oftentimes when this method is called, the return value is used. However, in this case I need the method to be called but don't need the return value. Is the correct thing to do to immediately call Dispose on it? If so, does it matter which of the below I do?
Foo().Dispose();
or
using(Foo())
{
}
or just
Foo();
If an object implements IDisposable then you should call the Dispose method when you are finished with the object. Dispose is meant to release resources that should not be held until a GC (Garbage Collector) cycle (or things that are not covered by the GC). When the GC detects that an object is no longer used it will call the objects finalizer, but there is no way to predict when that will happen. Therefore, the IDisposable interface was created so that programmers could explicitly dispose of an object's resources (it doesn't delete the object, the GC still does that).
I think I would prefer
Foo().Dispose();
but it really doesn't matter (between the first two). If you just use "Foo();" then you are not respecting the Dispose pattern which would be incorrect. You may want to try to determine what sort of resources the object is creating, perhaps there is a way to get the information you need without creating an object that needs disposing.
I always thought that the answer to this was no, but I cannot find any source stating this.
In my class below, can I access (managed) fields/properties of an instance of C in the finalizer, i.e. in ReleaseUnmanaged()? What restrictions, if any, are there? Will GC or finalization ever have set these members to null?
The only thing I can find is that stuff on the finalizer queue can be finalized in any order. So in that case, since the recommendation is that types should allow users to call Dispose() more than once, why does the recommended pattern bother with a disposing boolean? What bad things might happen if my finalizer called Dispose(true) instead of Dispose(false)?
public class C : IDisposable
{
private void ReleaseUnmanaged() { }
private void ReleaseOtherDisposables() { }
protected virtual void Dispose(bool disposing)
{
ReleaseUnmanaged();
if (disposing)
{
ReleaseOtherDisposables();
}
}
~ C()
{
Dispose(false);
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}
In general case - yes, you can. If a class has nonempty finalizer, first time GC would collect instance of this class, it calls finalizer instead (only if you didn't call GC.SuppressFinalize on it earlier). Object instance seen from finalizer looks just like it did last time you touched it. You can even create new (direct or indirect) link from root to your instance and thus resurrect it.
Even if you hold unmanaged pointer to unpinned object and inspect raw memory contents, you shouldn't be able to see partially-deallocated object, because .NET uses copying GC. If an instance is alive during collection, it is either promoted to next generation or moved to completely new memory block along with other instances. If it is not reachable it is either left where it was or the whole heap is released and returned to OS. Keep in mind, however, that finalizers can and will be called on instances of objects that failed to construct (i.e. when was an exception thrown during object construction).
Edit: As for Dispose(true) vs Dispose(false) in well-written classes there shouldn't be much of a difference in the long run. If your finalizer called Dispose(true), it would only remove links from your object to other objects, but since your object is already nonreachable, releasing other instances referenced by your object won't matter their reachability.
For more details on .NET GC implementation details, I recommend C# 5.0 in a Nutshell by Joseph and Ben Albahari.
The GC will never collect any object if there is any way via which any code could ever get a reference to it. If the only references that exist to an object are weak references, the GC will invalidate them so that there's no way any code could ever get a reference to that object, whereupon it will then be able to collect it.
If an object has an active finalizer, then if the GC would collect it (but for the existence of the finalizer), the GC will instead add it to a queue of objects whose finalizers should be run ASAP and, having done so, de-activate it. The reference within the queue will prevent the GC from collecting the object until the finalizer has run; once the finalizer finishes, if no other references exist to the object and it hasn't re-registered its finalizer, it will cease to exist.
The biggest problems with finalizers accessing outside objects are:
A finalizer will be run in a threading context which is unrelated to any threading context where the object was used, but should not perform any operations which cannot be guaranteed to complete quickly. This often creates contradictory requirement that code use locking when accessing other objects, but that code not block while waiting on locks.
If a finalizer has a reference to another object which also has a finalizer, there's no guarantee as to which will run first.
Both of these factors severely limit the ability of objects with finalizers tosafely access outside objects they don't own. Further, because of the way finalization is implemented, it's possible for the system to decide to run a finalizer when no strong references exist, but for external strong references to be created and used before the finalizer runs; the object with the finalizer has no say in whether that happens.
Assuming I have an object A containing
// ...
private List<double> someList = new List<double>();
// ...
public List<double> SomeList
{
get { lock (this) { return someList; } }
}
// ...
would it be thread safe to perform operation on the list as in the code below. Knowing that several operations could be executed simultaneously by different threads.
A.SomeList.Add(2.0);
or
A.SomeList.RemoveAt(0);
In other words, when is the lock released?
There is no thread safety here.
The lock is released as soon as the block it protects is finished, just before the property returns, so the calls to Add ad RemoveAt are not protected by the lock.
The lock you shown in the question isn't of much use.
To make list operations thread safe you need to implement your own Add/Remove/etc methods wrapping those of the list.
public void Add(double item)
{
lock(_list)
{
_list.Add(item);
}
}
Also, it's a good idea to hide the list itself from the consumers of your class, i.e. make the field private.
The lock is released when you exit the body of the lock statement. That means that your code is not thread-safe.
In other words, you can be sure that two threads won't be executing return someList on the same object at the same time. But it's certainly possible that one thread will execute Add() at the same time as another thread will execute RemoveAt(), which is what makes it non thread-safe.
The lock is released when the code inside the lock is finished executing.
Also, locking on this will only affect the current instance of the object
Ok, just for the hell of it.
There IS a way to make your object threadsafe, by using architectures that are already threadsafe.
For example, you could make your object a single threaded COM object. The COM object will be thread safe, but you'll pay with performance (the price of being lazy and not managing your own locks).
Create a COM Object in C#
...others said already, but just to formalize a problem a bit...
First, lock (this) {...} suggests a 'scope' - e.g. like using (){} - it only locks (this in this case) for variables inside. And that's a 'good thing' :) actually, as if you couldn't rely on that the whole locks/synchronization concept would be very much useless,
lock (this) { return something; } is an oxymoron of a sort - it's returning something that unlocks the very same moment it returns,
And the problems I think is the understanding of how it works. 'lock()' is not 'persisted' in a state of the object, so that you could return it etc. Take a look here how it's implemented How does lock work exactly? - answer explains it. It's more of a 'critical section' - i.e. you protect certain parts of 'code', which uses the variable - not the variable itself. Any type of synchronization requires 'synchronization objects' to hold the locks - and to be disposed of once lock is no longer needed. Take a look at this post https://stackoverflow.com/a/251668/417747, Esteban formulated that very well,
"Finally, there is the common misconception that lock(this) actually modifies the object passed as a parameter, and in some way makes it read-only or inaccessible. This is false. The object passed as a parameter to lock merely serves as a key" (this is a quote)
you either (usually) lock 'private code' of a class method, property... - to synchronize access to something you're doing inside - and access being to a private member (again usually) so that nobody else can access it w/o going through your synchronized code.
Or you make a thread-safe 'structure' - like a list - which is already 'synchronized' inside so that you can access it in a thread-safe manner. But there are no such things (or not used, almost never) as passing locks around or having one place lock the variable, while the other part of the code unlocks it etc. (in that case it's some type of EventWaitHandle that's rather used to synchronize things in between 'distant' code where one fires off on another etc.)
In your case, the choice is I think to go with the 'synchronized structure', i.e. the list that's internally handled,
I have a timer in C# which executes some code inside it's method. Inside the code I'm using several temporary objects.
If I have something like Foo o = new Foo(); inside the method, does that mean that each time the timer ticks, I'm creating a new object and a new reference to that object?
If I have string foo = null and then I just put something temporal in foo, is it the same as above?
Does the garbage collector ever delete the object and the reference or objects are continually created and stay in memory?
If I just declare Foo o; and not point it to any instance, isn't that disposed when the method ends?
If I want to ensure that everything is deleted, what is the best way of doing it:
with the using statement inside the method
by calling dispose method at the end
by putting Foo o; outside the timer's method and just make the assignment o = new Foo() inside, so then the pointer to the object is deleted after the method ends, the garbage collector will delete the object.
1.If I have something like Foo o = new Foo(); inside the method, does that
mean that each time the timer ticks,
I'm creating a new object and a new
reference to that object?
Yes.
2.If I have string foo = null and then I just put something temporal in foo,
is it the same as above?
If you are asking if the behavior is the same then yes.
3.Does the garbage collector ever delete the object and the reference or
objects are continually created and
stay in memory?
The memory used by those objects is most certainly collected after the references are deemed to be unused.
4.If I just declare Foo o; and not point it to any instance, isn't that
disposed when the method ends?
No, since no object was created then there is no object to collect (dispose is not the right word).
5.If I want to ensure that everything is deleted, what is the best way of
doing it
If the object's class implements IDisposable then you certainly want to greedily call Dispose as soon as possible. The using keyword makes this easier because it calls Dispose automatically in an exception-safe way.
Other than that there really is nothing else you need to do except to stop using the object. If the reference is a local variable then when it goes out of scope it will be eligible for collection.1 If it is a class level variable then you may need to assign null to it to make it eligible before the containing class is eligible.
1This is technically incorrect (or at least a little misleading). An object can be eligible for collection long before it goes out of scope. The CLR is optimized to collect memory when it detects that a reference is no longer used. In extreme cases the CLR can collect an object even while one of its methods is still executing!
Update:
Here is an example that demonstrates that the GC will collect objects even though they may still be in-scope. You have to compile a Release build and run this outside of the debugger.
static void Main(string[] args)
{
Console.WriteLine("Before allocation");
var bo = new BigObject();
Console.WriteLine("After allocation");
bo.SomeMethod();
Console.ReadLine();
// The object is technically in-scope here which means it must still be rooted.
}
private class BigObject
{
private byte[] LotsOfMemory = new byte[Int32.MaxValue / 4];
public BigObject()
{
Console.WriteLine("BigObject()");
}
~BigObject()
{
Console.WriteLine("~BigObject()");
}
public void SomeMethod()
{
Console.WriteLine("Begin SomeMethod");
GC.Collect();
GC.WaitForPendingFinalizers();
Console.WriteLine("End SomeMethod");
}
}
On my machine the finalizer is run while SomeMethod is still executing!
The .NET garbage collector takes care of all this for you.
It is able to determine when objects are no longer referenced and will (eventually) free the memory that had been allocated to them.
Objects are eligable for garbage collection once they go out of scope become unreachable (thanks ben!). The memory won't be freed unless the garbage collector believes you are running out of memory.
For managed resources, the garbage collector will know when this is, and you don't need to do anything.
For unmanaged resources (such as connections to databases or opened files) the garbage collector has no way of knowing how much memory they are consuming, and that is why you need to free them manually (using dispose, or much better still the using block)
If objects are not being freed, either you have plenty of memory left and there is no need, or you are maintaining a reference to them in your application, and therefore the garbage collector will not free them (in case you actually use this reference you maintained)
Let's answer your questions one by one.
Yes, you make a new object whenever this statement is executed, however, it goes "out of scope" when you exit the method and it is eligible for garbage collection.
Well this would be the same as #1, except that you've used a string type. A string type is immutable and you get a new object every time you make an assignment.
Yes the garbage collector collects the out of scope objects, unless you assign the object to a variable with a large scope such as class variable.
Yes.
The using statement only applies to objects that implement the IDisposable interface. If that is the case, by all means using is best for objects within a method's scope. Don't put Foo o at a larger scope unless you have a good reason to do so. It is best to limit the scope of any variable to the smallest scope that makes sense.
Here's a quick overview:
Once references are gone, your object will likely be garbage collected.
You can only count on statistical collection that keeps your heap size normal provided all references to garbage are really gone. In other words, there is no guarantee a specific object will ever be garbage collected.
It follows that your finalizer will also never be guaranteed to be called. Avoid finalizers.
Two common sources of leaks:
Event handlers and delegates are references. If you subscribe to an event of an object, you are referencing to it. If you have a delegate to an object's method, you are referencing it.
Unmanaged resources, by definition, are not automatically collected. This is what the IDisposable pattern is for.
Finally, if you want a reference that does not prevent the object from getting collected, look into WeakReference.
One last thing: If you declare Foo foo; without assigning it you don't have to worry - nothing is leaked. If Foo is a reference type, nothing was created. If Foo is a value type, it is allocated on the stack and thus will automatically be cleaned up.
Yes
What do you mean by the same? It will be re-executed every time the method is run.
Yes, the .Net garbage collector uses an algorithm that starts with any global/in-scope variables, traverses them while following any reference it finds recursively, and deletes any object in memory deemed to be unreachable. see here for more detail on Garbage Collection
Yes, the memory from all variables declared in a method is released when the method exits as they are all unreachable. In addition, any variables that are declared but never used will be optimized out by the compiler, so in reality your Foo variable will never ever take up memory.
the using statement simply calls dispose on an IDisposable object when it exits, so this is equivalent to your second bullet point. Both will indicate that you are done with the object and tell the GC that you are ready to let go of it. Overwriting the only reference to the object will have a similar effect.
The garbage collector will come around and clean up anything that no longer has references to it. Unless you have unmanaged resources inside Foo, calling Dispose or using a using statement on it won't really help you much.
I'm fairly sure this applies, since it was still in C#. But, I took a game design course using XNA and we spent some time talking about the garbage collector for C#. Garbage collecting is expensive, since you have to check if you have any references to the object you want to collect. So, the GC tries to put this off as long as possible. So, as long as you weren't running out of physical memory when your program went to 700MB, it might just be the GC being lazy and not worrying about it yet.
But, if you just use Foo o outside the loop and create a o = new Foo() each time around, it should all work out fine.
As Brian points out the GC can collect anything that is unreachable including objects that are still in scope and even while instance methods of those objects are still executing. consider the following code:
class foo
{
static int liveFooInstances;
public foo()
{
Interlocked.Increment(ref foo.liveFooInstances);
}
public void TestMethod()
{
Console.WriteLine("entering method");
while (Interlocked.CompareExchange(ref foo.liveFooInstances, 1, 1) == 1)
{
Console.WriteLine("running GC.Collect");
GC.Collect();
GC.WaitForPendingFinalizers();
}
Console.WriteLine("exiting method");
}
~foo()
{
Console.WriteLine("in ~foo");
Interlocked.Decrement(ref foo.liveFooInstances);
}
}
class Program
{
static void Main(string[] args)
{
foo aFoo = new foo();
aFoo.TestMethod();
//Console.WriteLine(aFoo.ToString()); // if this line is uncommented TestMethod will never return
}
}
if run with a debug build, with the debugger attached, or with the specified line uncommented TestMethod will never return. But running without a debugger attached TestMethod will return.
I have an application with two classes, A and B. The class A has inside a reference to class B. The destructors of the classes do some cleanup of resources but they have to be called in the right order, first the destructor of A and then the destructor of B.
What is happening is that somehow the destructor of B is called first and then the destructor of A is crashing because is trying to execute methods from a disposed object.
Is this behavior of the GC correct? I expected the GC to detect that A has a reference to B and then call the A destructor first. Am I right?
Thanks mates!
PD: In case of doubt about destructor/finalizer/disposer etc that's what we have:
~A()
{
this.Dispose();
}
~B()
{
this.Dispose();
}
As others have noted, your finalizers are wrong, wrong, wrong. You cannot simply call Dispose in a finalizer and expect good things to happen. Read up on the correct way to implement the disposable pattern.
Getting that right is the beginning, not the end, of the work you have to do. In addition to all the other correct answers here, I note that:
the finalizer can (and usually does) run on a different thread. Finalizing resources that have an affinity to a particular thread is dangerous, you can run into deadlocks if you are not careful, and so on.
finalization can "resurrect" a dead object by assigning a reference to it to a variable that is in a live object. Do not do that. It is incredibly confusing.
finalizers can run on objects that were partially constructed when a thread abort exception happened. You cannot assume that any invariant that is true of a fully-constructed object is true of an object being finalized.
Writing a finalizer correctly is extremely difficult for all these reasons. Avoid, avoid, avoid.
The GC is nondeterministic: it is not guaranteed to garbage-collect your objects in any particular order, or at any time, or even ever---it could decide to keep them around after your program is completed, if it wanted to. Thus finalizers are very rarely useful or used.
If you want deterministic finalization, you should use the IDisposable pattern.
From http://msdn.microsoft.com/en-us/magazine/bb985010.aspx:
The runtime doesn't make any guarantees as to the order in which Finalize methods are called. For example, let's say there is an object that contains a pointer to an inner object. The garbage collector has detected that both objects are garbage. Furthermore, say that the inner object's Finalize method gets called first. Now, the outer object's Finalize method is allowed to access the inner object and call methods on it, but the inner object has been finalized and the results may be unpredictable. For this reason, it is strongly recommended that Finalize methods not access any inner, member objects.
You should not write code that depends on the deconstructors in C#, they might never be run so can not be depended upon.
The correct way to write finalizers is to not call anything on related objects, you should only destroy unmanaged resources through the finalizer, precisely because, as you've noted, things happen out of order.
As others have noted, this is documented as part of the specification, so this is not a bug, nor is it something that will change to "do the right thing". In particular, think about two objects both having a reference to the other, it would be impossible for the runtime to figure out what the correct order would be here.
In any case, here's how you should implement a finalizer. Note that you should only implement a finalizer if you need to destroy unmanaged resources (note, there are some edge-cases where you might want a finalizer for a very special-purpose class, but I won't mention these here any more than this):
public class Test : IDisposable
{
~Test()
{
Dispose(false);
}
public void Dispose()
{
Dispose(true);
}
protected virtual void Dispose(bool disposing)
{
// TODO: Make sure calling Dispose twice is safe
if (disposing)
{
// call Dispose method on other objects
GC.SuppressFinalize(this);
}
// destroy unmanaged resources here
}
}
You can find more information here: Implementing a Dispose Method.