Queue null value return and throws NullReferenceException

Question

I have Queue code but throws NullReferenceException error.

public class BlockingQueue<T>
{
    private Queue<T> _queue;
    private Semaphore _sem;

    public int Count
    {
        get
        {
            return this._queue.Count;
        }
    }


    public T Dequeue()
    {
        T t;
        this._sem.WaitOne();
        lock (this._queue)
        {
            t = this._queue.Dequeue();
        }
        return t;
    }

    public void Enqueue(T item)  
    {
        // item have proper value but 
        // when insert value to Queue<T> _queue is null and I get NRE
        lock (this._queue)
        {
            this._queue.Enqueue(item);
        }
        this._sem.Release();
    }
}

item have two value but when insert value to Queue _queue null value return error

score 1 · Accepted Answer · answered Feb 25 '19 at 01:42

1

public class BlockingQueue<T>
{
    private Queue<T> _queue = new Queue<T>;
    private Semaphore _sem -= new Semaphore();

    public int Count
    {
        get
        {
            return this._queue.Count;
        }
    }


    public T Dequeue()
    {
        T t;
        this._sem.WaitOne();
        lock (this._queue)
        {
            t = this._queue.Dequeue();
        }
        return t;
    }

    public void Enqueue(T item)  // < = item have two value but when insert value to Queue<t> _queue null value return error
    {
        lock (this._queue)
        {
            this._queue.Enqueue(item);
        }
        this._sem.Release();
    }
}

answered Feb 25 '19 at 01:42

Alan Turing

2,482
17
20

To clarify this answer, the difference between your code and this is the private fields have been initialized here. Merely defining the field does not create an instance of the object. – Chris Feb 25 '19 at 01:47
thanks so much for your help! i changed bit private Queue _queue = new Queue(); private Semaphore _sem = new Semaphore(1,2); – jeon Feb 25 '19 at 02:20

score 1 · Answer 2 · edited Jul 28 '21 at 22:07

Your problem is initialisation.

However, if you are trying to create a thread-safe queue, I'd suggest using a BCL (FIFO) collection if possible, all the hard work has been done and tested for you.

ConcurrentQueue Class

Represents a thread-safe first in-first out (FIFO) collection.

It will be a more solid solution then what you have at this point, and has methods that will allow you more control and power of the queue.

Example

 // Construct a ConcurrentQueue.
  ConcurrentQueue<int> cq = new ConcurrentQueue<int>();

  // Populate the queue.
  for (int i = 0; i < 10000; i++)
  {
      cq.Enqueue(i);
  }

  // Peek at the first element.
  int result;
  if (!cq.TryPeek(out result))
  {
     Console.WriteLine("CQ: TryPeek failed when it should have succeeded");
  }
  else if (result != 0)
  {
     Console.WriteLine("CQ: Expected TryPeek result of 0, got {0}", result);
  }

  int outerSum = 0;
  // An action to consume the ConcurrentQueue.
  Action action = () =>
  {                
     int localSum = 0;
     int localValue;
     while (cq.TryDequeue(out localValue)) localSum += localValue;
     Interlocked.Add(ref outerSum, localSum);
  };

  // Start 4 concurrent consuming actions.
  Parallel.Invoke(action, action, action, action);

  Console.WriteLine("outerSum = {0}, should be 49995000", outerSum);

Queue null value return and throws NullReferenceException

2 Answers2