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  1. PO
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    <p>What you described is pretty much exactly the use case for the <a href="http://en.wikipedia.org/wiki/Visitor_pattern" rel="nofollow">Visitor Pattern</a>.</p> <p><strong>EDIT</strong>: For your concrete example, you could apply the visitor pattern like this:</p> <pre><code>// interface used to add external functionality to pricing models public interface PricingModelVisitor { void visitPricingModel1(PricingModel1 m); void visitPricingModel2(PricingModel2 m); ... } // your existing base-class, with added abstract accept() method to accept a visitor public abstract class PricingModelBase { public abstract void accept(PricingModelVisitor v); ... } // concrete implementations of the PricingModelBase implement accept() by calling the // appropriate method on the visitor, passing themselves as the argument public class PricingModel1 : PricingModelBase { public void accept(PricingModelVisitor v) { v.visitPricingModel1(this); } ... } public class PricingModel2 : PricingModel { public void accept(PricingModelVisitor v) { v.visitPricingModel2(this); } ... } // concrete implementation of the visitor interface, in this case with the new // functionality of adding the appropriate controls to a parent control public class ParameterGuiVisitor : PricingModelVisitor { private Control _parent; public ParameterGuiVisitor(Control parent) { _parent = parent; } visitPricingModel1(PricingModel1 m) { // add controls to _parent for PricingModel1 } visitPricingModel2(PricingModel2 m) { // add controls to _parent for PricingModel1 } } </code></pre> <p>now, instead of using a big <code>if</code>-<code>else</code> block when you need to display the edit-controls for the parameters of a specific subtype of <code>PricingModelVisitor</code>, you can simply call</p> <pre><code>somePricingModel.accept(new ParameterGuiVisitor(parentControl)) </code></pre> <p>and it will populate the appropriate GUI for you.</p>
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    1. POHow do you implement type-specific functionality when using polymorphism does NOT make sense?
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    1. USLuke Hutteman
    1. USLuke Hutteman
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    1. COVisitor is probably not flexible enough. You can't add visited types dynamically. You only can dynamically add implementations of visitors.
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      1. PO
      1. USStefan Steinegger
    2. COWhat do you mean by "adding visited types dynamically"? The visitor pattern easily allows you to add new types by simply adding a new method to the visitor interface that deals with this new type. This will then at compile-time force you to update all existing visitor implementations to appropriately handle this new type, something that might be easily overlooked with a dictionary-based solution.
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      1. PO
      1. USLuke Hutteman
    3. COSometimes it needs injecting, easy registering of types, adding types at runtime, adding types which are not known by the assembly which specifies the visitor interface etc. I don't say that dynamic is better then static, I just say that it is *probably not flexible enough* for the problem described here.
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      1. USStefan Steinegger
 

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