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PO
 

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  1. PO
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    <p>I'm pretty sure <code>BlockingCollection&lt;T&gt;</code> can't do this, you'd have to roll your own. I came up with this:</p> <pre><code>class NotifyingCollection&lt;T&gt; { private ConcurrentQueue&lt;Action&lt;T&gt;&gt; _subscribers = new ConcurrentQueue&lt;Action&lt;T&gt;&gt;(); private ConcurrentQueue&lt;T&gt; _overflow = new ConcurrentQueue&lt;T&gt;(); private object _lock = new object(); public void Add(T item) { _overflow.Enqueue(item); Dispatch(); } private void Dispatch() { // this lock is needed since we need to atomically dequeue from both queues... lock (_lock) { while (_overflow.Count &gt; 0 &amp;&amp; _subscribers.Count &gt; 0) { Action&lt;T&gt; callback; T item; var r1 = _overflow.TryDequeue(out item); var r2 = _subscribers.TryDequeue(out callback); Debug.Assert(r1 &amp;&amp; r2); callback(item); // or, optionally so that the caller thread's doesn't take too long ... Task.Factory.StartNew(() =&gt; callback(item)); // but you'll have to consider how exceptions will be handled. } } } public void TakeAsync(Action&lt;T&gt; callback) { _subscribers.Enqueue(callback); Dispatch(); } } </code></pre> <p>I've used the thread which calls <code>TakeAsync()</code> or <code>Add()</code> to serve as the callback thread. When you call <code>Add()</code> or <code>TakeAsync()</code> it will attempt to dispatch all the queued-up items to the queued-up callbacks. This way there's no thread created that just sits there sleeping, waiting to be signalled.</p> <p>That lock is kind of ugly, but you'll be able to enqueue and subscribe on multiple threads without locking. I couldn't figure out a way to do the equivalent of <em>only dequeue one if there's something available on the other queue</em> without using that lock.</p> <p><em>Note: I only tested this minimally, with a few threads.</em></p>
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    1. POAsynchronous Take from blocking collection
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    1. USatanamir
    1. USatanamir
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    1. COWell, yes. The only problem is that you'll create a locked thread using your approach which would be dangerous considering a very high load. The `TryTake` call blocks. In a situation where you have thousands of these calls you'd run out of threads in the task thread pool and lock your application.
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      1. USDervall
    2. COAh! you wanted no blocking threads altogether; I thought you had just wanted to ensure the calling thread wouldn't get blocked. I get it now.
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      1. USatanamir
    3. COOK, modified my answer with one which I think will do what you're looking for...
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      1. USatanamir
 

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