ConcurrentQueue ToList() vs ToArray().ToList()

Question

I'm not 100% on this, so I want an expert's input.

ConcurrentQueue<object> queue = new ConcurrentQueue<object>();

List<object> listA = queue.ToArray().ToList();   // A
List<object> listB = queue.ToList();             // B

I understand that ToArray() method will make a copy (as it is an internal method within ConcurrentQueue), but will calling the ToList() method directly do the same thing?

Simply, is it safe to refactor code from A to B?

Answer 1

If we look to the source code we wiil see that GetEnumerator is thread safe also, so I supose A and B are threadsafe both.

When you call .ToList() Linq call to constructor of List

 public List(IEnumerable<T> collection) {

so the code are actually make a copy looks like a thread safe:

        using(IEnumerator<T> en = collection.GetEnumerator()) {
            while(en.MoveNext()) {
                Add(en.Current);                                    
            }

source code of ConcurrentQueue

source code of List

public IEnumerator<T> GetEnumerator()
{
    // Increments the number of active snapshot takers. This increment must happen before the snapshot is 
    // taken. At the same time, Decrement must happen after the enumeration is over. Only in this way, can it
    // eliminate race condition when Segment.TryRemove() checks whether m_numSnapshotTakers == 0. 
    Interlocked.Increment(ref m_numSnapshotTakers);

    // Takes a snapshot of the queue. 
    // A design flaw here: if a Thread.Abort() happens, we cannot decrement m_numSnapshotTakers. But we cannot 
    // wrap the following with a try/finally block, otherwise the decrement will happen before the yield return 
    // statements in the GetEnumerator (head, tail, headLow, tailHigh) method.           
    Segment head, tail;
    int headLow, tailHigh;
    GetHeadTailPositions(out head, out tail, out headLow, out tailHigh);

    //If we put yield-return here, the iterator will be lazily evaluated. As a result a snapshot of
    // the queue is not taken when GetEnumerator is initialized but when MoveNext() is first called.
    // This is inconsistent with existing generic collections. In order to prevent it, we capture the 
    // value of m_head in a buffer and call out to a helper method.
    //The old way of doing this was to return the ToList().GetEnumerator(), but ToList() was an 
    // unnecessary perfomance hit.
    return GetEnumerator(head, tail, headLow, tailHigh);
}

remarks of the enumerator also says that we can use it concurently:

    /// The enumeration represents a moment-in-time snapshot of the contents
    /// of the queue.  It does not reflect any updates to the collection after 
    /// <see cref="GetEnumerator"/> was called.  The enumerator is safe to use
    /// concurrently with reads from and writes to the queue.