MQTTnet/Frameworks/MQTTnet.NetStandard/Internal/AsyncLock.cs

using System;
using System.Threading;
using System.Threading.Tasks;

namespace MQTTnet.Internal
{
    // From Stephen Toub (https://blogs.msdn.microsoft.com/pfxteam/2012/02/12/building-async-coordination-primitives-part-6-asynclock/)
    public sealed class AsyncLock
    {
        private readonly SemaphoreSlim m_semaphore = new SemaphoreSlim(1, 1);
        private readonly Task<IDisposable> m_releaser;

        public AsyncLock()
        {
            m_releaser = Task.FromResult((IDisposable)new Releaser(this));
        }

        public Task<IDisposable> LockAsync(CancellationToken cancellationToken)
        {
            var wait = m_semaphore.WaitAsync(cancellationToken);
            return wait.IsCompleted ?
                        m_releaser :
                        wait.ContinueWith((_, state) => (IDisposable)state,
                            m_releaser.Result, cancellationToken,
                            TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
        }

        private sealed class Releaser : IDisposable
        {
            private readonly AsyncLock m_toRelease;
            internal Releaser(AsyncLock toRelease) { m_toRelease = toRelease; }
            public void Dispose() { m_toRelease.m_semaphore.Release(); }
        }
    }
}