ConcurrentQueue ToList() 与 ToArray().ToList()

Question

我不是 100% 的，所以我需要专家的意见。

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

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

我知道该ToArray()方法会复制一份（因为它是 内的一个内部方法ConcurrentQueue），但是ToList()直接调用该方法会做同样的事情吗？

简单地说，将代码从 A 重构到 B 是否安全？

Answer 1

如果我们查看源代码，我们会发现 GetEnumerator 也是线程安全的，所以我假设 A 和 B 都是线程安全的。

当您调用 .ToList() Linq 时调用 List 的构造函数

 public List(IEnumerable<T> collection) {

所以代码实际上使副本看起来像线程安全：

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

ConcurrentQueue 的源代码

List的源代码

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);
}

枚举器的评论也说我们可以同时使用它：

    /// 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.