当该讯息再度从伫列移除时，相同情况还会发生。
When the message is dequeued again, the same condition occurs.
如果伫列中已有讯息，方法会立即传回。
The method returns immediately if a message already exists in the queue.
如果目前有可用的列举值，让列举值前进至伫列中的下一个讯息。
Advances the enumerator to the next message in the queue, if one is currently available.
讯息会留在伫列中，并且必须由手动方法来移除。
The message is left in the queue and must be removed in a manual way.
讯息会留在伫列中，并且必须由手动方法来移除。
The message is left in the queue and must be removed in a manual way.
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