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copied!<p>The event keyword creates an accessor for a private delegate object. The exact same thing a property does, it restricts access to a private field. Your code snippet fails with a similar kind of error when you use a property instead of an event:</p> <pre><code> class ClassA { public int Property { get; set; } } class ClassB { public ClassB() { ClassA a = new ClassA(); ClassC c = new ClassC(); c.setValue(ref a.Property); // CS0206 } } class ClassC { public void setValue(ref int value) { value = 42; } } </code></pre> <p>It is easier to see now, there is no way for the compiler to ensure that the setValue() method uses the property setter. Nor could it know that the "value" argument is a property with a setter or a plain field.</p> <p>It is less clear for an event because there is so much syntax sugar at work. This declaration</p> <pre><code>public event EventHandler SomeEvent; </code></pre> <p>actually generates this code:</p> <pre><code>private EventHandler _SomeEvent; public event SomeEvent { add { _SomeEvent += new EventHandler(value); } remove { _SomeEvent -= new EventHandler(value); } } </code></pre> <p>The add and remove accessors are equivalent to the get and set accessors of a property, they prevent code from messing with the private _SomeEvent field. By convention, the add accessor is invoked when you use +=, remove is invoked with -=. Compare this with the earlier example I gave for a property. Same problem, you can't use the ref keyword and ClassC.addListener() would have no way to know that the handler is actually an event instead of a delegate object. If the compiler would pass _SomeEvent instead, the point of using the accessors is lost.</p> <p>You can restructure the code to solve this problem:</p> <pre><code> class ClassC { public EventHandler getListener() { return new EventHandler(onEvent); } private void onEvent(object sender, EventArgs e) { } } ... a.SomeEvent += c.getListener(); </code></pre> <hr> <p>One final note: the symmetry between an event and a property is a bit lost, the C# compiler automatically generates the add/remove accessors if you don't write them explicitly. It doesn't do this for a property. It would have made automatic properties a lot easier:</p> <pre><code>property int Property; </code></pre> <p>But that would have required adding a new keyword to the language, something the C# team really dislikes. Other languages like VB.NET and C++/CLI do have that keyword.</p>

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