WaitHandle that is used to wait for an asynchronous operation to complete.
获取一个值，该值指示异步操作是否同步完成。
The method returns an IAsyncResult, which can be used to monitor completion, wait on the WaitHandle, or complete the asynchronous call.
这个方法返回一个IAsyncResult，它可以被用在监测完成，也可以等待在WaitHandel，或者完成异步调用。
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