AutoResetEvent

AutoResetEvent is a thread synchronization primitive provided by the Windows API and the .NET framework, used to coordinate signaling between threads. It is a type of EventWaitHandle that releases a single waiting thread when signaled and then automatically returns to a non-signaled state. This one-at-a-time signaling makes it useful for hand-off scenarios where only one thread should proceed after an event occurs.

An AutoResetEvent has an initial state, signaled or non-signaled, defined at construction. The Set method transitions the event to signaled. If one or more threads are waiting on the event, exactly one waiting thread is released and the event immediately resets to non-signaled. If no threads are waiting when Set is called, the event remains signaled until a thread waits; that thread will then be released and the event resets. The Reset method explicitly sets the event to non-signaled without releasing any thread. A named AutoResetEvent can be created to enable synchronization across processes.

In contrast, a ManualResetEvent remains signaled until Reset is called, releasing all waiting threads and continuing to let subsequent waits proceed until the reset occurs. The automatic reset behavior of AutoResetEvent makes it suitable for scenarios where sequential, one-at-a-time progress is required, such as handing off control from one task to the next or coordinating stages in a pipeline.

In .NET, AutoResetEvent resides in the System.Threading namespace and wraps a Windows kernel synchronization object. It supports standard wait operations (such as WaitOne) and can participate in cross-thread or cross-process synchronization when used with a named event.