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POThread-safe raising of F# events
 

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  1. POThread-safe raising of F# events
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    <p>I'm trying to do an F# async computation that calls an C# callback when ready. The code is the following:</p> <pre><code>type Worker() = let locker = obj() let computedValue = ref None let started = ref false let completed = Event&lt;_&gt;() let doNothing() = () member x.Compute(callBack:Action&lt;_&gt;) = let workAlreadyStarted, action = lock locker (fun () -&gt; match !computedValue with | Some value -&gt; true, (fun () -&gt; callBack.Invoke value) | None -&gt; completed.Publish.Add callBack.Invoke if !started then true, doNothing else started := true false, doNothing) action() if not workAlreadyStartedthen async { // heavy computation to calc result let result = "result" lock locker (fun () -&gt; computedValue := Some result completed.Trigger result) } |&gt; Async.Start </code></pre> <p>But there's a problem, I want to trigger the completed event outside the lock, but I want to make sure that the triggering is thread safe (Actually, in this small example I could just trigger the event outside the lock as I know no one else will subscribe to it, but that's not always the case).</p> <p>In C# events this is very easy to accomplish:</p> <pre><code> object locker = new object(); event Action&lt;string&gt; MyEvent; void Raise() { Action&lt;string&gt; myEventCache; lock (locker) { myEventCache = MyEvent; } if (myEventCache != null) { myEventCache("result"); } } </code></pre> <p>How can I do the equivalent with F# events, freezing the list of subscribers inside the lock but invoking it outside the lock?</p>
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    1. COAre you sure the C# freezes the subscriber list? Aren't you just taking a reference to the event?
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      1. USDaniel
    2. COWhenever you add a new handler to an event, you create a new delegate value and replace the previous value, so as I'm copying the value of the delegate, if another handler is added after that, it won't be linked to the copy that was made
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    3. COIf you need access to the subscriber list, then you can use your own `IEvent<_>` implementation instead of the one provided by `Event<_>.Publish`.
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