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

Answer 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();
    }
}