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POHow do C# Events work behind the scenes?
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  1. POHow do C# Events work behind the scenes?
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    copied!<p>I'm using C#, .NET 3.5. I understand how to utilize events, how to declare them in my class, how to hook them from somewhere else, etc. A contrived example:</p> <pre><code>public class MyList { private List&lt;string&gt; m_Strings = new List&lt;string&gt;(); public EventHandler&lt;EventArgs&gt; ElementAddedEvent; public void Add(string value) { m_Strings.Add(value); if (ElementAddedEvent != null) ElementAddedEvent(value, EventArgs.Empty); } } [TestClass] public class TestMyList { private bool m_Fired = false; [TestMethod] public void TestEvents() { MyList tmp = new MyList(); tmp.ElementAddedEvent += new EventHandler&lt;EventArgs&gt;(Fired); tmp.Add("test"); Assert.IsTrue(m_Fired); } private void Fired(object sender, EventArgs args) { m_Fired = true; } } </code></pre> <p>However, what I do <em>not</em> understand, is when one declares an event handler</p> <pre><code>public EventHandler&lt;EventArgs&gt; ElementAddedEvent; </code></pre> <p>It's never initialized - so what, exactly, is ElementAddedEvent? What does it point to? The following won't work, because the EventHandler is never initialized:</p> <pre><code>[TestClass] public class TestMyList { private bool m_Fired = false; [TestMethod] public void TestEvents() { EventHandler&lt;EventArgs&gt; somethingHappend; somethingHappend += new EventHandler&lt;EventArgs&gt;(Fired); somethingHappend(this, EventArgs.Empty); Assert.IsTrue(m_Fired); } private void Fired(object sender, EventArgs args) { m_Fired = true; } } </code></pre> <p>I notice that there is an EventHandler.CreateDelegate(...), but all the method signatures suggest this is only used for attaching Delegates to an already existing EventHandler through the typical ElementAddedEvent += new EventHandler(MyMethod).</p> <p>I'm not sure if <em>what</em> I am trying to do will help... but ultimately I'd like to come up with an abstract parent DataContext in LINQ whose children can register which table Types they want "observed" so I can have events such as BeforeUpdate and AfterUpdate, but specific to types. Something like this:</p> <pre><code>public class BaseDataContext : DataContext { private static Dictionary&lt;Type, Dictionary&lt;ChangeAction, EventHandler&gt;&gt; m_ObservedTypes = new Dictionary&lt;Type, Dictionary&lt;ChangeAction, EventHandler&gt;&gt;(); public static void Observe(Type type) { if (m_ObservedTypes.ContainsKey(type) == false) { m_ObservedTypes.Add(type, new Dictionary&lt;ChangeAction, EventHandler&gt;()); EventHandler eventHandler = EventHandler.CreateDelegate(typeof(EventHandler), null, null) as EventHandler; m_ObservedTypes[type].Add(ChangeAction.Insert, eventHandler); eventHandler = EventHandler.CreateDelegate(typeof(EventHandler), null, null) as EventHandler; m_ObservedTypes[type].Add(ChangeAction.Update, eventHandler); eventHandler = EventHandler.CreateDelegate(typeof(EventHandler), null, null) as EventHandler; m_ObservedTypes[type].Add(ChangeAction.Delete, eventHandler); } } public static Dictionary&lt;Type, Dictionary&lt;ChangeAction, EventHandler&gt;&gt; Events { get { return m_ObservedTypes; } } } public class MyClass { public MyClass() { BaseDataContext.Events[typeof(User)][ChangeAction.Update] += new EventHandler(OnUserUpdate); } public void OnUserUpdated(object sender, EventArgs args) { // do something } } </code></pre> <p>Thinking about this made me realize I don't really understand what's happening under the hod with events - and I would like to understand :)</p>
 

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