问如果不为空，则释放锁队列入队

EN

public bool EnqueueIfEmpty(T item) {
  // Return immediately if the queue is not empty.
  // Possibly the first condition is redundant.
  if (head!=tail || head.Next!=null)
      return false;

  SingleLinkNode<T> oldHead = null;

  // create and initialize the new node
  SingleLinkNode<T> node = new SingleLinkNode<T>();
  node.Item = item;

  // loop until we have managed to update the tail's Next link 
  // to point to our new node
  bool Succeeded = false;
  while (head==tail && !Succeeded) {

    // save the current value of the head
    oldHead = head;         

    // providing that the tail still equals to head...
    if (tail == oldHead) {

      // ...and its Next field is null...
      if (oldhead.Next == null) {

        // ...try inserting new node right after the head.
        // Do not insert at the tail, because that might succeed
        // with a non-empty queue as well.
        Succeeded = SyncMethods.CAS<SingleLinkNode<T>>(ref head.Next, null, node);
      }

      // if the head's Next field was non-null, another thread is
      // in the middle of enqueuing a new node, so the queue becomes non-empty
      else {
        return false;
      }
    }
  }

  if (Succeeded) {
    // try and update the tail field to point to our node; don't
    // worry if we can't, another thread will update it for us on
    // the next call to Enqueue()
    SyncMethods.CAS<SingleLinkNode<T>>(ref tail, oldHead, node);
  }
  return Succeeded;
}