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Thread Safe Generic Queue Class

By , 11 Aug 2009
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I've been doing a lot of mult-threading work, recently, using the standard Thead class, the Worker Queue, and the new PLINQ (Parallel LINQ). The problem with most of the built-in generic collections (Queue<>, List<>, Dictionary<>, etc), is that they are not thread safe.

I created a library of thread safe collections which allow me to use the standard generic collection actions (foreach, LINQ, etc), while at the same time being thread safe.

The classes in this library inherit from the appropriate collection interface (IEnumerable, ICollection, etc). Each class also has all the functions and properties that it's original non-thread safe class has.

You can download a copy of the entire library, which includes support for a thread safe List<>, Dictionary<>, and Queue<>, here: Thread Safe Generic Collections

TQueue<> Example:

The first thing we need to do is create a container for the TQueue and a thread lock object. I generally prefer to use the ReaderWriterLockSlim because it is light weight and fast.

/// <summary>

/// The private q which holds the actual data

/// </summary>

private readonly Queue<T> m_Queue;

 

/// <summary>

/// Lock for the Q

/// </summary>

private readonly ReaderWriterLockSlim LockQ = new ReaderWriterLockSlim();

Just like a standard Queue, we have three overloads for the Initialization. These overloads allow an empty Queue to be created, a Queue with a specified capacity, or a Queue with an initial IENumerable collection to populate the Queue.

/// <summary>

/// Initializes the Queue

/// </summary>

public TQueue()

{

    m_Queue = new Queue<T>();

}

 

/// <summary>

/// Initializes the Queue

/// </summary>

/// <param name="capacity">the initial number of elements the queue can contain</param>

public TQueue(int capacity)

{

    m_Queue = new Queue<T>(capacity);

}

 

/// <summary>

/// Initializes the Queue

/// </summary>

/// <param name="collection">the collection whose members are copied to the Queue</param>

public TQueue(IEnumerable<T> collection)

{

    m_Queue = new Queue<T>(collection);

}

 

This next function is probably the most important one. The GetEnumerator() is used during ForEach loops, and returns the next item in the collection. Following Microsoft's example of a thread-safe enumerator, we first get a copy of the current container Queue, then use this copy for iterating. You'll notice the use the Read lock before acquiring the container Queue copy.

/// <summary>

/// Returns an enumerator that enumerates through the collection

/// </summary>

public IEnumerator<T> GetEnumerator()

{

    Queue<T> localQ;

 

    // init enumerator

    LockQ.EnterReadLock();

    try

    {

        // create a copy of m_TList

        localQ = new Queue<T>(m_Queue);

    }

    finally

    {

        LockQ.ExitReadLock();

    }

 

    // get the enumerator

    foreach (T item in localQ)

        yield return item;

}

A Queue must include an Enqueue and a Dequeue, used for adding and removing items from the collection. Just as in every other function, we're using the locks to protect our data access.

/// <summary>

/// Adds an item to the queue

/// </summary>

/// <param name="item">the item to add to the queue</param>

public void Enqueue(T item)

{

    LockQ.EnterWriteLock();

    try

    {

        m_Queue.Enqueue(item);

    }

 

    finally

    {

        LockQ.ExitWriteLock();

    }

}

/// <summary>

/// Removes and returns the item in the beginning of the queue

/// </summary>

public T Dequeue()

{

    LockQ.EnterWriteLock();

    try

    {

        return m_Queue.Dequeue();

    }

 

    finally

    {

        LockQ.ExitWriteLock();

    }

}

I found that many times I have a need to enqueue multiple items at once. This lead to the creation of the EnqueueAll functions. You'll notice the second overload is using the thread safe List (TList).

/// <summary>

/// Enqueues the list of items

/// </summary>

/// <param name="ItemsToQueue">list of items to enqueue</param>

public void EnqueueAll(IEnumerable<T> ItemsToQueue)

{

    LockQ.EnterWriteLock();

    try

    {

        // loop through and add each item

        foreach (T item in ItemsToQueue)

            m_Queue.Enqueue(item);

    }

 

    finally

    {

        LockQ.ExitWriteLock();

    }

}

 

/// <summary>

/// Enqueues the list of items

/// </summary>

/// <param name="ItemsToQueue">list of items to enqueue</param>

public void EnqueueAll(TList<T> ItemsToQueue)

{

    LockQ.EnterWriteLock();

    try

    {

        // loop through and add each item

        foreach (T item in ItemsToQueue)

            m_Queue.Enqueue(item);

    }

 

    finally

    {

        LockQ.ExitWriteLock();

    }

}

And, since we have an EnqueueAll, I also found a need to dequeue everything at once. DequeueAll returns a thread safe list (TList), instead of the standard List.

/// <summary>

/// Dequeues all the items and returns them as a thread safe list

/// </summary>

public TList<T> DequeueAll()

{

    LockQ.EnterWriteLock();

    try

    {

        // create return object

        TList<T> returnList = new TList<T>();

 

        // dequeue until everything is out

        while (m_Queue.Count > 0)

            returnList.Add(m_Queue.Dequeue());

 

        // return the list

        return returnList;

    }

 

    finally

    {

        LockQ.ExitWriteLock();

    }

}


License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

merlin981
Software Developer (Senior)
United States United States
Winner - Best Mobile App - AT&T Developer Summit, Las Vegas, 2013
 
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About Philip Pierce:
 
I am a software developer with twenty years experience in game development, mobile, web, desktop, server, and database. My extensive background highlights an expertise in rapid application development using the latest Microsoft, Mobile, and Game Development technologies, along with the ability to create AI for games and business software, redesign existing software, develop multi-threaded software, and create client/server applications.

Comments and Discussions

 
QuestionConcurrentQueue<T> PinmemberMember 876402327-Sep-12 23:29 
GeneralMy vote of 5 Pinmemberaka MasteR27-Jun-12 23:44 
GeneralI have found that. Pinmemberreborn_zhang12-Feb-10 23:00 
GeneralImplementing your thread-safe Queue in a VB project PinmemberBobishKindaGuy9-Feb-10 8:20 
GeneralGetEnumerator not thread-safe PinmemberGideon Engelberth30-Sep-09 9:40 
GeneralRe: GetEnumerator not thread-safe Pinmembersupercat95-Oct-09 11:07 
Generaloverload PinmemberJean-Paul Mikkers12-Aug-09 14:26 
GeneralLike it Pinmemberkopota11-Aug-09 12:48 
GeneralRe: Like it Pinmembermerlin98111-Aug-09 15:12 

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