Hello. I'm looking for an implementation of thread-safe blocking queue for .NET. By "thread-safe blocking queue" I mean: - thread-safe access to a queue where Dequeue method call blocks a thread untill other thread puts (Enqueue) some value.
By the moment I'v found this one: http://www.eggheadcafe.com/articles/20060414.asp (But it's for .NET 1.1).
Could someone comment/criticize correctness of this implementation. Or suggest some another one. Thanks in advance.
Queue.Synchronized http://msdn.microsoft.com/en-us/library/system.collections.queue.synchronized(VS.71).aspx
Is a starting point anyways, I've never used a Blocking Queue. Sorry for the not so relevant post.
How about this one Creating a blocking Queue in .NET?
If you need it for .NET 1.1 (I wasn't sure from the question), just drop the generics and replace T with object.
Microsoft has a pretty nice sample about this:
//Copyright (C) Microsoft Corporation. All rights reserved.
using System;
using System.Threading;
using System.Collections;
using System.Collections.Generic;
// The thread synchronization events are encapsulated in this
// class to allow them to easily be passed to the Consumer and
// Producer classes.
public class SyncEvents
{
public SyncEvents()
{
// AutoResetEvent is used for the "new item" event because
// we want this event to reset automatically each time the
// consumer thread responds to this event.
_newItemEvent = new AutoResetEvent(false);
// ManualResetEvent is used for the "exit" event because
// we want multiple threads to respond when this event is
// signaled. If we used AutoResetEvent instead, the event
// object would revert to a non-signaled state with after
// a single thread responded, and the other thread would
// fail to terminate.
_exitThreadEvent = new ManualResetEvent(false);
// The two events are placed in a WaitHandle array as well so
// that the consumer thread can block on both events using
// the WaitAny method.
_eventArray = new WaitHandle[2];
_eventArray[0] = _newItemEvent;
_eventArray[1] = _exitThreadEvent;
}
// Public properties allow safe access to the events.
public EventWaitHandle ExitThreadEvent
{
get { return _exitThreadEvent; }
}
public EventWaitHandle NewItemEvent
{
get { return _newItemEvent; }
}
public WaitHandle[] EventArray
{
get { return _eventArray; }
}
private EventWaitHandle _newItemEvent;
private EventWaitHandle _exitThreadEvent;
private WaitHandle[] _eventArray;
}
// The Producer class asynchronously (using a worker thread)
// adds items to the queue until there are 20 items.
public class Producer
{
public Producer(Queue<int> q, SyncEvents e)
{
_queue = q;
_syncEvents = e;
}
public void ThreadRun()
{
int count = 0;
Random r = new Random();
while (!_syncEvents.ExitThreadEvent.WaitOne(0, false))
{
lock (((ICollection)_queue).SyncRoot)
{
while (_queue.Count < 20)
{
_queue.Enqueue(r.Next(0, 100));
_syncEvents.NewItemEvent.Set();
count++;
}
}
}
Console.WriteLine("Producer thread: produced {0} items", count);
}
private Queue<int> _queue;
private SyncEvents _syncEvents;
}
// The Consumer class uses its own worker thread to consume items
// in the queue. The Producer class notifies the Consumer class
// of new items with the NewItemEvent.
public class Consumer
{
public Consumer(Queue<int> q, SyncEvents e)
{
_queue = q;
_syncEvents = e;
}
public void ThreadRun()
{
int count = 0;
while (WaitHandle.WaitAny(_syncEvents.EventArray) != 1)
{
lock (((ICollection)_queue).SyncRoot)
{
int item = _queue.Dequeue();
}
count++;
}
Console.WriteLine("Consumer Thread: consumed {0} items", count);
}
private Queue<int> _queue;
private SyncEvents _syncEvents;
}
public class ThreadSyncSample
{
private static void ShowQueueContents(Queue<int> q)
{
// Enumerating a collection is inherently not thread-safe,
// so it is imperative that the collection be locked throughout
// the enumeration to prevent the consumer and producer threads
// from modifying the contents. (This method is called by the
// primary thread only.)
lock (((ICollection)q).SyncRoot)
{
foreach (int i in q)
{
Console.Write("{0} ", i);
}
}
Console.WriteLine();
}
static void Main()
{
// Configure struct containing event information required
// for thread synchronization.
SyncEvents syncEvents = new SyncEvents();
// Generic Queue collection is used to store items to be
// produced and consumed. In this case 'int' is used.
Queue<int> queue = new Queue<int>();
// Create objects, one to produce items, and one to
// consume. The queue and the thread synchronization
// events are passed to both objects.
Console.WriteLine("Configuring worker threads...");
Producer producer = new Producer(queue, syncEvents);
Consumer consumer = new Consumer(queue, syncEvents);
// Create the thread objects for producer and consumer
// objects. This step does not create or launch the
// actual threads.
Thread producerThread = new Thread(producer.ThreadRun);
Thread consumerThread = new Thread(consumer.ThreadRun);
// Create and launch both threads.
Console.WriteLine("Launching producer and consumer threads...");
producerThread.Start();
consumerThread.Start();
// Let producer and consumer threads run for 10 seconds.
// Use the primary thread (the thread executing this method)
// to display the queue contents every 2.5 seconds.
for (int i = 0; i < 4; i++)
{
Thread.Sleep(2500);
ShowQueueContents(queue);
}
// Signal both consumer and producer thread to terminate.
// Both threads will respond because ExitThreadEvent is a
// manual-reset event--so it stays 'set' unless explicitly reset.
Console.WriteLine("Signaling threads to terminate...");
syncEvents.ExitThreadEvent.Set();
// Use Join to block primary thread, first until the producer thread
// terminates, then until the consumer thread terminates.
Console.WriteLine("main thread waiting for threads to finish...");
producerThread.Join();
consumerThread.Join();
}
}
Please keep in mind that locking in calling code may be a better option if you have full control over it. Consider accessing your queue in a loop: you'll be needlessly acquiring locks multiple times, potentially incurring a performance penalty.
The Microsoft example is a good one but it is not encapsulated into a class. Also, it requires that the consumer thread is running in the MTA (because of the WaitAny call). There are some cases in which you may need to run in an STA (e.g., if you are doing COM interop). In these cases, WaitAny cannot be used.
I have a simple blocking queue class that overcomes this issue here: http://element533.blogspot.com/2010/01/stoppable-blocking-queue-for-net.html
For the reference, .NET 4 introduces the System.Collections.Concurrent.BlockingCollection<T> type to address this. For non-blocking queue, you can use System.Collections.Concurrent.ConcurrentQueue<T>. Note that ConcurrentQueue<T> would likely be used as the underlying datastore for the BlockingCollection<T> for the OP's usage.
Yes, .NET4 contains concurrent collections. BTW, very very nice manual about Parallel Extensions from pfx team - http://www.microsoft.com/downloads/details.aspx?FamilyID=86b3d32b-ad26-4bb8-a3ae-c1637026c3ee&displaylang=en.
pfx is also available for .net 3.5 as part of Reactive Extensions.
I created a blog post about this very problem. You can check it out here:
http://element533.blogspot.com/2010/01/stoppable-blocking-queue-for-net.html
It's a very simple and straight-forward blocking queue that does what you're looking for.