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449511
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2009-01-16T04:23:47.570
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2009-01-16T04:23:47.570
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Type-cases and object oriented code don't seem to get on that well together in my experience. The approach I prefer in this situation is the <a href="http://c2.com/cgi/wiki?DoubleDispatch" rel="nofollow noreferrer">double dispatch pattern</a>. In short: <ul> <li>Create a listener type with an empty virtual method Process(ExtendedTypeN arg) for each extended type you will be dispatching over. <li>Add a virtual method Dispatch(Listener listener) to the base type that takes a listener as argument. Its implementation will be to call listener.Process((Base) this). <li>Override the Dispatch method in each extended type to call the appropriate overload of Process in the listener type. <li>Extend the listener type by overriding the appropriate Process method for each subtype you are interested in. </ul> The argument shuffling dance eliminates the narrowing cast by folding it into the call to Dispatch -- the receiver knows its exact type, and communicates it by calling back the exact overload of Process for its type. This is also a big performance win in implementations such as .NET Compact Framework, in which narrowing casts are extremely slow but virtual dispatch is fast. The result will be something like this: <pre><code> public class Listener { public virtual void Process(Base obj) { } public virtual void Process(Derived obj) { } public virtual void Process(OtherDerived obj) { } } public class Base { public virtual void Dispatch(Listener l) { l.Process(this); } } public class Derived { public override void Dispatch(Listener l) { l.Process(this); } } public class OtherDerived { public override void Dispatch(Listener l) { l.Process(this); } } public class ExampleListener { public override void Process(Derived obj) { Console.WriteLine("I got a Derived"); } public override void Process(OtherDerived obj) { Console.WriteLine("I got an OtherDerived"); } public void ProcessCollection(IEnumerable collection) { foreach (Base obj in collection) obj.Dispatch(this); } } </code></pre>
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1. POusing the 'is' keyword in a switch in c#
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1. PTAnswer
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1. USJeffrey Hantin
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