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POCreating custom events - Object Sender or Typed Sender?
 

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  1. POCreating custom events - Object Sender or Typed Sender?
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    <p>I searched through the archives and I found lots of questions about what sender is and why you should use the pattern but I didn't see anything about a custom event and the <em>type</em> if sender.</p> <p>Say I am creating a custom class called Subscription and it implements ISubscription and I have some event args called SubscriptionEventArgs. If Subscription had an event called Changed what is wrong about the event signature Changed(ISubscription sender, SubscriptionEventArgs e)?</p> <p>A little code to help drive the question:</p> <pre><code>public class SubscriptionEventArgs : EventArgs { // guts of event args go here } public interface ISubscription { event Action&lt;ISubscription, SubscriptionEventArgs&gt; Changed; } public class Subscription : ISubscription { public event Action&lt;ISubscription, SubscriptionEventArgs&gt; Changed; private void OnChanged(SubscriptionEventArgs e) { if (Changed!= null) { Changed(this, e); } } } </code></pre> <p>If you just despise the use of action in place of "EventHandler" then you could do the same thing but with a custom generic "EventHandler".</p> <pre><code>public delegate void EventHandler&lt;TSender, TEventArgs&gt;(TSender sender, TEventArgs e); public class SubscriptionEventArgs : EventArgs { // guts of event args go here } public interface ISubscription { event EventHandler&lt;ISubscription, SubscriptionEventArgs&gt; Changed; } public class Subscription : ISubscription { public event EventHandler&lt;ISubscription, SubscriptionEventArgs&gt; Changed; private void OnChanged(SubscriptionEventArgs e) { if (Changed!= null) { Changed(this, e); } } } </code></pre> <p>In response to Hans' request for a sample event handler:</p> <pre><code>public class SubscriptionCollection { // what is actually holding the subscriptions is not really relevant to the question private List&lt;ISubscription&gt; _subscriptions; public SubscriptionCollection() { _subscriptions = new List&lt;ISubscription&gt;(); } public void Add(ISubscription subscription) { subscription.Changed += new EventHandler&lt;ISubscription, SubscriptionEventArgs&gt;(Subscription_Changed); _subscriptions.Add(subscription); } private void Subscription_Changed(ISubscription sender, SubscriptionEventArgs e) { // Now when the subscription changed event is being handled by the collection // I don't have to look up the subscription in the list by some key and I don't // have to cast sender to the correct type because the event handler was typed // correctly from the beginning. } } </code></pre> <p>The lookup of the subscription in the list might seem trivial but what if I am working with very large sets of data and new volumes of data are coming at the application through a real-time stream. The cost of having to stop and get a reference out of a list or go through the steps of casting don't make sense. They gave us generics in 2.0 to solve that issue so I don't understand why we didn't get a generic event handler too and this led me to question what is wrong with a generic event handler?</p>
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    1. USRyan Pedersen
    1. USRyan Pedersen
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      1. USKris van der Mast
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    1. COI'd like the second block a lot more if a "where TEventArgs : EventArgs" were added.
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      1. USMatt DeKrey
    2. COJust a tip, but if you use IObservable rather than ISubscription for your interface type, I think you'd be following design pattern naming a bit better, and may be a bit more clear at a glance what's going on.
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      1. USMatt DeKrey
 

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