Is it safe to use the following pattern in a multithreaded scenario?:
var collection = new List<T>(sharedCollection);
Where sharedCollection can be modified at the same time by another thread (i.e. have elements added or removed from it)?
The scenario I'm currently dealing with is copying the items from a BindingList, but the question should be relative to any standard collection type.
If it isn't thread safe, should I put a lock on the sharedCollection, or are there better solutions?
You seem to have answered your own question(s). No, copying a changing list to another list is not thread-safe, and yes, you could lock on sharedCollection. Note that it's not enough to lock sharedCollection while copying it; you need to lock it anytime you read or change its contents as well.
Edit: just a note about when it's bad to lock on the object you're modifying--if the object reference itself can be changed (like `sharedCollection = new List) or if it can be null, then make a separate object to lock on as a member of the class where the reading/writing is happening.
You can lock the SyncRoot object of sharedCollection.
Explain here :
Lock vs. ToArray for thread safe foreach access of List collection
Related
Please explain the difference between these two types of locking.
I have a List which I want to access thread-safe:
var tasks = new List<string>();
1.
var locker = new object();
lock (locker)
{
tasks.Add("work 1");
}
2.
lock (tasks)
{
tasks.Add("work 2");
}
My thoughts:
Prevents two different threads from running the locked block of code at the same time.
But if another thread runs a different method where it tries to access task - this type of lock won't help.
Blocks the List<> instance so other threads in other methods will be blocked untill I unlock tasks.
Am I right or mistaking?
(2) only blocks other code that explicitly calls lock (tasks). Generally, you should only do this if you know that tasks is a private field and thus can enforce throughout your class that lock (tasks) means locking operations on the list. This can be a nice shortcut when the lock is conceptually linked with access to the collection and you don't need to worry about public exposure of the lock. You don't get this 'for free', though; it needs to be explicitly used just like locking on any other object.
They do the same thing. Any other code that tries to modify the list without locking the same object will cause potential race conditions.
A better way might be to encapsulate the list in another object that obtains a lock before doing any operations on the underlying list and then any other code can simple call methods on the wrapper object without worrying about obtaining the lock.
I am facing the problem that this code:
enumerable.OfType<Foo>().Any(x => x.Footastic == true);
Isnt thread safe and throws an enumeration has changed exception.
Is there a good way to overcome this issue?
Already tried the following but it didnt always work (seems to not fire this often)
public class Foo
{
public void DoSomeMagicWithCollection(IEnumerable enumerable)
{
lock (enumerable)
{
enumerable.OfType<Foo>().Any(x => x.Footastic == true);
}
}
}
If you're getting an exception that the underlying collection has changed while enumerating it, given that this code clearly doesn't mutate the collection itself, it means that another thread is mutating the collection while you're trying to iterate it.
There is no possible solution to this problem other than simply not doing that. What's happening is that the enumerator of the List (or whatever collection type that is) is throwing the exception and preventing further enumeration because it can see that the list was modified during the enumeration. There is no way for the enumerators of OfType of Any that wrap it to possibly recover from that. The underlying enumerator is refusing to give them the data from the list. They can't do anything about that.
You need to use some sort of synchronization mechanism to prevent another thread from mutating the collection wnile this thread is enumerating this collection. Your lock doesn't prevent another thread from using the collection, it simply prevents any code that locks on the same instance from running. You need to have any code that could possibly mutate the list also lock on the same object to properly synchronize them.
Another possibility would be to use a collection that is inherently designed to be accessed from multiple threads at the same time. There are several such collections in the System.Collections.Concurrent namespace. They may or may not fit your needs. They will take care of synchronizing access to their data (to a point) on their own, without you needing to explicitly lock when accessing them.
Is there a difference in the below code segments in the way we lock?
public Hashtable mySet= new Hashtable() //mySet is visible to other threads.
lock (mySet)
{
mySet.Add("Hello World");
}
and
public Hashtable mySet= new Hashtable();
lock(mySet.SyncRoot)
{
mySet.Add("Hello World");
}
lock doesn't actually lock the object in question, so it makes no difference which object is used. Instead it uses the object to establish a protocol and as long as all threads use the same object the protocol guarantees that only one thread will execute code guarded by that lock.
You can think of the object as the microphone on a talk show. Whoever holds the microphone is the only one allowed to talk (I know that is not always how it turns out on some of the shows, but that's the idea anyway).
As the object passend to the lock will only be used as a "flag holder", this will not make any difference.
Please see this
According to the MSDN documentation here only a lock on the SyncRoot of a collection does guarantee thread safety.
Enumerating through a collection is intrinsically not a thread-safe
procedure. Even when a collection is synchronized, other threads can
still modify the collection, which causes the enumerator to throw an
exception. To guarantee thread safety during enumeration, you can
either lock the collection during the entire enumeration or catch the
exceptions resulting from changes made by other threads.
Consider the following code, which takes place in a background thread ("thread B"):
List<T> invocationQueueCopy;
lock (invocationQueue)
{
invocationQueueCopy = invocationQueue;
invocationQueue = new List<T>();
}
In another thread ("thread A") I simply lock "invocationQueue" before adding to it:
lock (invocationQueue)
{
invocationQueue.Add(args);
}
I have read that reference assignment is atomic, but could it ever occur that "thread A" will end up writing to the old list (the one replaced in "thread B") after receiving the lock? I have read other answers that imply it could, if the value of the reference were stored in a register on "thread A" then it would not know that "thread B" had modified the value in the class. If this is so, would declaring "invocationQueue" volatile prevent this?
Notes:
I know I could clone then clear the list.
I
know I could have a separate lock
object for the list.
But I'd rather not do either of these things unless it is required.
Thanks in advance.
Edit:
Just to clarify from Adam's comments: invocationQueue is a private field which is created internally to this class and never exposed to the outside world so nothing could lock on it except these two methods.
EDIT: Your solution will work. Lock creates a full fence so any caching is prevented, basically meaning you'll always get the most recent value for the list reference. The only thing, as suggested in the comments is the fact that you should do the locking on a neutral object, not the list itself.
The following is wrong!! But I let it here anyway to show how fu*** hard threading might be... the fact is the following reasoning is defeated by the fact that lock creates a full fence.
Yeah, it can happen so don't do it
that way.
It won't get better even if you did
the lock into a readonly whatever
object.
See what might happen (although most
of the time it WON'T happen).
ThreadA and ThreadB are executing on
different processors, each one with
its own cache memory which holds the
reference to incovationQueue.
ThreadB locks invocationQueue, the lock is done to a reference which is
taken for the cache of processor1, not
to a variable name.
ThreadB copies the invocationQueue.
ThreadA locks invocationQueue, the lock is done to a reference which is
taken for the cache on processor2 and
which, in this moment is the same as
the one in processor1, and starts
waiting.
ThreadB creates a new List and assigns it to the invocationQueue, the
cache in the processor1 is updated but
since the variable is NOT volatile
that's all that happens.
ThreadA enters the lock and gets the reference from his cache, which points
to the old reference, therefore you
end up adding the variable to the old
list.
So you need to make the list volatile
AND use the lock if you're going to be
playing with the reference itself.
I have a Dictionary that tracks objects (ClientObject). Both the dictionary and ClientObject's are accessed by multiple threads. When I modify or read any object in this dictionary, I obtain a read or write lock on the dictionary using ReaderWriterLockSlim (rwl_clients) then obtain an exclusive lock on the actual object.
I just wanted to know if I am using these .net threading facilities correctly
Example:
rwl_clients.EnterReadLock();
ClientObject clobj;
if(!m_clients.TryGetValue(key, out clobj))
return;
rwl_clients.ExitReadLock();
SomeMethod(clobj);
SomeMethod(ClientObject clobj) would do something like:
lock(clobj) {
/// Read / Write operations on clobj
}
Does getting and locking a value (ClientObject) from the dictionary in one thread mean that other threads will respect that lock? In other words, does .net see a value in the dictionary as a single resource (and not a copy) and will therefore respect a lock on that resource in all threads?
One more question, when removing a resource from the dictionary - should I lock it before performing Remove()
Example:
rwl_clients.EnterWriteLock();
ClientObject clobj;
if(m_clients.TryGetValue(key, out clobj)) {
lock(clobj) {
m_clients.Remove(key);
}
}
rwl_clients.ExitWriteLock();
I have learned so much from this site and appreciate any responses!
Thanks.
Does getting and locking a value (ClientObject) from the dictionary in one thread mean that other threads will respect that lock? In other words, does .net see a value in the dictionary as a single resource (and not a copy) and will therefore respect a lock on that resource in all threads?
It depends on the type - if a reference type then yes, if a value type no. This is also why you should never, ever lock on a value type since the value type will be boxed and any subsequent attempts to lock on that value will actually acquire a lock on a different object.
One more question, when removing a resource from the dictionary - should I lock it before performing Remove()
Yes, you should lock before any operation that mutates the state of the object.
As a side note - are you sure that this setup is the best possible solution to your problem? Mutable objects shared across threads tend to create more problems then they solve.
If you are adding or removing items from the dictionary, lock the dictionary.
When you put an object in the dictionary, you are putting a REFERENCE to that object in the dictionary. To prevent that object from being changed by a second thread while the first thread is in the process of changing it, lock the object, not the dictionary.