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    <blockquote> <p>I have an event that when fired the subscribers execute some DB code that may take a while to return. I want to execute all the subscriber delegates in parallel. I am trying to use Parallel.Foreach but I cant get it quite right any suggestions?</p> </blockquote> <p>I would recommend pushing the parallelism into the subscriber's side, and not try to invoke the delegates in parallel.</p> <p>Just have the subscribers do:</p> <pre><code>private void OnMyEvent(object sender, EventArgs e) { Task.Factory.StartNew( () =&gt; { // Your DB code logic here... }); } </code></pre> <p>This will cause the event to occur asynchronously.</p> <p>If you must wait for the return type, I personally would avoid events all together. The issue here is that there is an inherent expectation that an event subscriber will not block. Allowing the event to block will violate expectations, and lead to problems. </p> <p>Instead, I would recommend using a form of inversion of control, and handling this via an interface instead of an event. This then becomes a somewhat different variation of the <a href="http://en.wikipedia.org/wiki/Observer_pattern" rel="nofollow">Observer Pattern</a> than standard event usage:</p> <pre><code>// Publisher side - add methods to add/remove "subscribers": public void AddListener(IDatabaseOperation op) { lock(syncObj) // thread safety may be an issue... this.listeners.Add(op); } private void OnEvent() { lock(syncObj) { Parallel.ForEach(this.listeners, l =&gt; l.DoOperation()); } } </code></pre> <hr> <p>If you must multithread your invocations on the publishing side, it can be done via:</p> <pre><code>private void RaiseMyEvent(MyEventArgs e) { var handler = this.TheEvent; if (handler != null) { var tasks = new List&lt;Task&gt;(); foreach(var yourEvent in handler.GetInvocationList().Cast&lt;EventHandler&lt;MyEventArgs&gt;()) tasks.Add(Task.Factory.StartNew( () =&gt; yourEvent(this, e))); // Optionally wait here... Removing the following makes this asynchronous Task.WaitAll(tasks.ToArray()); } } </code></pre>
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    1. POhow can I handle an event in parallel in C# and TPL
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    1. COWhy do you recommend pushing it to the subscriber side? I do not need to wait for the return .
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    2. CO@Wegged: Because multi-threaded event handling causes all sorts of complexities that are unexpected on the subscription side. It tends to lead to code that's unexpected, and causes bugs to creep it. Pushing it to the subscriber, as shown above, makes it completely async. and is simple - most importantly, it's obvious what is occurring if you look at the code.
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    3. COmy problem is that I would have to make changes in many places (not just my code) to implement it that way. Also if I did decide to go that route would I be able to control how many threads are working at once so that I do not end up with alot of connections to the database?
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