AsyncAutoResetEvent

Overview

This is the async-ready equivalent of AutoResetEvent, similar to Stephen Toub's AsyncAutoResetEvent.

Like other "events", an AsyncAutoResetEvent is either set or unset at any time. An AsyncAutoResetEvent can be changed from unset to set by calling its Set method. When a WaitAsync operation completes, the AsyncAutoResetEvent is automatically changed back to the unset state.

Moving an AsyncAutoResetEvent to the set state can only satisfy a single waiter. If there are multiple waiters when Set is called, only one will be released. (If this is not the behavior you want, use AsyncManualResetEvent instead).

When an AsyncAutoResetEvent is in the set state (with no waiters), Set is a noop. The AsyncAutoResetEvent will not remember how many times Set is called; those extra signals are "lost". (If this is not the behavior you want, use AsyncSemaphore instead).

The task returned from WaitAsync will enter the Completed state when the wait is satisfied and the AsyncAutoResetEvent has been automatically reset. That same task will enter the Canceled state if the CancellationToken is signaled before the wait is satisfied; in that case, the AsyncAutoResetEvent has not been automatically reset.

API

// An async-compatible auto-reset event.
public sealed class AsyncAutoResetEvent
{
  // Creates an async-compatible auto-reset event.
  public AsyncAutoResetEvent(bool set = false);

  // Gets a semi-unique identifier for this asynchronous auto-reset event.
  public int Id { get; }

  // Asynchronously waits for this event to be set.
  // If the event is set, this method will auto-reset it and return immediately, even if the cancellation token is already signalled.
  // If the wait is canceled, then it will not auto-reset this event.
  public Task WaitAsync(CancellationToken cancellationToken = new CancellationToken());

  // Sets the event, atomically completing a task returned by WaitAsync.
  // If the event is already set, this method does nothing.
  public void Set();
}

Advanced Usage

You can call WaitAsync with an already-cancelled CancellationToken to attempt to acquire the AsyncAutoResetEvent immediately without actually entering the wait queue.

Platform Support

The full API is supported on all platforms.