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    <p>We need to thank Stephen Toub here, as he has some very useful async locking constructs demonstrated in a blog series, including an <a href="http://blogs.msdn.com/b/pfxteam/archive/2012/02/12/10266988.aspx">async lock</a> block.</p> <p>Here is the code from that article (including some code from the previous article in the series):</p> <pre><code>public class AsyncLock { private readonly AsyncSemaphore m_semaphore; private readonly Task&lt;Releaser&gt; m_releaser; public AsyncLock() { m_semaphore = new AsyncSemaphore(1); m_releaser = Task.FromResult(new Releaser(this)); } public Task&lt;Releaser&gt; LockAsync() { var wait = m_semaphore.WaitAsync(); return wait.IsCompleted ? m_releaser : wait.ContinueWith((_, state) =&gt; new Releaser((AsyncLock)state), this, CancellationToken.None, TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default); } public struct Releaser : IDisposable { private readonly AsyncLock m_toRelease; internal Releaser(AsyncLock toRelease) { m_toRelease = toRelease; } public void Dispose() { if (m_toRelease != null) m_toRelease.m_semaphore.Release(); } } } public class AsyncSemaphore { private readonly static Task s_completed = Task.FromResult(true); private readonly Queue&lt;TaskCompletionSource&lt;bool&gt;&gt; m_waiters = new Queue&lt;TaskCompletionSource&lt;bool&gt;&gt;(); private int m_currentCount; public AsyncSemaphore(int initialCount) { if (initialCount &lt; 0) throw new ArgumentOutOfRangeException("initialCount"); m_currentCount = initialCount; } public Task WaitAsync() { lock (m_waiters) { if (m_currentCount &gt; 0) { --m_currentCount; return s_completed; } else { var waiter = new TaskCompletionSource&lt;bool&gt;(); m_waiters.Enqueue(waiter); return waiter.Task; } } } public void Release() { TaskCompletionSource&lt;bool&gt; toRelease = null; lock (m_waiters) { if (m_waiters.Count &gt; 0) toRelease = m_waiters.Dequeue(); else ++m_currentCount; } if (toRelease != null) toRelease.SetResult(true); } } </code></pre> <p>Now applying it to your case:</p> <pre><code>private readonly AsyncLock m_lock = new AsyncLock(); private async void OnMessageReceived(object sender, EventArgs e) { using(var releaser = await m_lock.LockAsync()) { await Task.Run(() =&gt; this.model.Apply(e)); } } </code></pre>
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    1. POHow to avoid reentrancy with async void event handlers?
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    1. COOther options include the [`SemaphoreSlim` type in .NET 4.5](http://msdn.microsoft.com/en-us/library/vstudio/system.threading.semaphoreslim(v=vs.100).aspx) and the [`AsyncLock` type in my AsyncEx library](http://nitoasyncex.codeplex.com/wikipage?title=AsyncLock).
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      1. USStephen Cleary
    2. CO@StephenCleary Won't the Semaphore solution do a blocking wait, not an async wait?
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    3. CO@Servy Not if you use its `WaitAsync()` method.
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