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POHow to reference an event in C#
 

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    <p>I have the following class, which has one public event called <code>LengthChanged</code>:</p> <pre><code>class Dimension { public int Length { get { return this.length; } set { if (this.length != value) { this.length = value; this.OnLengthChanged (); } } protected virtual void OnLengthChanged() { var handler = this.LengthChanged; if (handler != null) { handler (this, System.EventArgs.Empty); } } public event System.EventHandler LengthChanged; private int length; } </code></pre> <p>I would like to be able to register/unregister handlers for this event in a method called <code>Observer</code>, which does not know anything about the <code>Dimension</code> class. I have come up with two scenarios, none of which are really satisfying:</p> <ol> <li><p>Define an interface <code>ILengthChanged</code> with the <code>LengthChanged</code> event, then make sure <code>Dimension</code> implements <code>ILengthChanged</code>. Then I have to provide one implementation of the <code>Observer</code> method for every interface I define. This by no way generic enough. I'd really want to be able to simply pass in a reference to a <code>System.EventHandler</code> event.</p></li> <li><p>Use <code>System.Action&lt;System.EventHandler&gt;</code> callbacks for registering and unregistering the event handler in the <code>Observer</code> method, just like that:</p> <p>class Foo { public void Observer(System.Action register, System.Action unregister) { register (this.MyEventHandler);</p> <pre><code> // keep track of the unregister callback, so that we can unregister // our event handler later on, if needed... } private void MyEventHandler(object sender, System.EventArgs e) { ... } </code></pre> <p>}</p></li> </ol> <p>which would then be invoked like this:</p> <pre><code>Foo foo = ...; Dimension dim = ...; foo.Observer (x =&gt; dim.LengthChanged += x, x =&gt; dim.LengthChanged -= x); </code></pre> <p>and which, when executed, will indeed end up wiring the <code>LengthChanged</code> event with the internal event handler <code>MyEventHandler</code>. But this is not very elegant. I would have loved to be able to write this instead:</p> <pre><code>Foo foo = ...; Dimension dim = ...; foo.Observer (dim.LengthChanged); </code></pre> <p>but I've no idea how this could be achieved. Maybe I am missing something really obvious here? I guess that some <code>dynamic</code> magic could do the trick, somehow, but this would not enforce compile-time type checking: I don't want the users of <code>Observer</code> to pass in references to events which do not satisfy the <code>System.EventHandler</code> event signature.</p>
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