标题：ThreadPool Class
地址：https://docs.microsoft.com/zh-cn/dotnet/api/system.threading.threadpool?redirectedfrom=MSDN&view=netframework-4.8
百度关键字看到了：ManualResetEvent
Provides a pool of threads that can be used to execute tasks, post work items, process asynchronous I/O, wait on behalf of other threads, and process timers.
提供一个线程池，可用于执行任务、发布工作项、处理异步I/O、代表其他线程等待和执行定时器。
线程池运行以下情况：
When you create a Task or Task<TResult> object to perform some task asynchronously, by default the task is scheduled to run on a thread pool thread.
您创建一个任务或任务<TResult>对象来异步执行某些任务时，默认情况下，该任务将在线程池线程上运行。
Asynchronous timers use the thread pool. Thread pool threads execute callbacks from the System.Threading.Timer class and raise events from the System.Timers.Timer class.
异步计时器使用线程池。线程池线程从System.Threading.Timer类执行回调，并从System.Timers.Timer类引发事件。
When you use registered wait handles, a system thread monitors the status of the wait handles. When a wait operation completes, a worker thread from the thread pool executes the corresponding callback function.
使用已注册的等待句柄时，系统线程将监视等待句柄的状态。当等待操作完成时，线程池中的工作线程执行相应的回调函数。
When you call the QueueUserWorkItem method to queue a method for execution on a thread pool thread. You do this by passing the method a WaitCallback delegate. The delegate has the signature
当您调用QueueUserWorkItem方法对线程池线程上的执行方法进行排队时。您可以通过传递方法WaitCallback委托来实现这一点。委托具有签名。
void WaitCallback(Object state)
其中状态是一个对象，它包含要由委托使用的数据。可以通过调用QueueUserWorkItem(WaitCallback，Object)方法将实际数据传递给委托。
注意：
The threads in the managed thread pool are background threads. That is, their IsBackground properties are true. This means that a ThreadPool thread will not keep an application running after all foreground threads have exited.
托管线程池中的线程是后台线程。也就是说，他们的IsBackplace属性是真实的。这意味着在所有前台线程退出之后，线程将不会继续运行应用程序。