AsyncAutoResetEvent.cs

public sealed class AsyncAutoResetEvent
{
    private readonly Lock @lock = new();
    private Task? currentWait;
    private TaskCompletionSource? taskCompletionSource;

    public void Set()
    {
        lock (@lock)
        {
            if (currentWait is null)
            {
                currentWait = Task.CompletedTask;
            }
            else if (!currentWait.IsCompleted)
            {
                currentWait = null;
                taskCompletionSource!.TrySetResult();
            }
        }
    }

    public Task WaitAsync()
    {
        lock (@lock)
        {          
            if (currentWait is null)
            {
                taskCompletionSource = new();
                currentWait = taskCompletionSource.Task;
            }
            else if (currentWait.IsCompleted)
            {
                var originalTask = currentWait;
                currentWait = null;
                return originalTask;
            }

            return currentWait;
        }
    }
}

AsyncAutoResetEventTests.cs

using Shouldly;

public class AsyncAutoResetEventTests
{
    [Test]
    public void The_event_is_initially_not_signaled()
    {
        var ev = new AsyncAutoResetEvent();
        ev.WaitAsync().IsCompleted.ShouldBeFalse();
    }

    [Test]
    public void The_event_can_be_signaled_before_waiting_starts()
    {
        var ev = new AsyncAutoResetEvent();
        ev.Set();
        ev.WaitAsync().IsCompleted.ShouldBeTrue();
    }

    [Test]
    public void The_event_can_be_signaled_after_waiting_starts()
    {
        var ev = new AsyncAutoResetEvent();
        var waitTask = ev.WaitAsync();
        ev.Set();
        waitTask.IsCompleted.ShouldBeTrue();
    }

    [Test]
    public void The_event_automatically_resets_when_signaled_after_the_first_wait_starts()
    {
        var ev = new AsyncAutoResetEvent();
        ev.WaitAsync();
        ev.Set();
        ev.WaitAsync().IsCompleted.ShouldBeFalse();
    }

    [Test]
    public void The_event_automatically_resets_when_signaled_before_the_first_wait_starts()
    {
        var ev = new AsyncAutoResetEvent();
        ev.Set();
        ev.WaitAsync();
        ev.WaitAsync().IsCompleted.ShouldBeFalse();
    }

    [Test]
    public void Setting_multiple_times_does_not_signal_more_than_one_wait()
    {
        var ev = new AsyncAutoResetEvent();
        ev.Set();
        ev.Set();
        ev.WaitAsync();
        ev.WaitAsync().IsCompleted.ShouldBeFalse();
    }

    [Test]
    public void Multiple_waits_for_the_same_signal_are_the_same_task()
    {
        var ev = new AsyncAutoResetEvent();
        ev.WaitAsync().ShouldBeSameAs(ev.WaitAsync());
        ev.Set();
        ev.WaitAsync().ShouldBeSameAs(ev.WaitAsync());
    }
}