Question

What is the best way to implement polymorphic behavior in classes that I can’t modify? I currently have some code like:

if(obj is ClassA) {     // ... } else if(obj is ClassB) {     // ... } else if ... 

The obvious answer is to add a virtual method to the base class, but unfortunately the code is in a different assembly and I can’t modify it. Is there a better way to handle this than the ugly and slow code above?

Answer 1

Hmmm… seems more suited to Adapter.

public interface ITheInterfaceYouNeed {     void DoWhatYouWant(); }  public class MyA : ITheInterfaceYouNeed {     protected ClassA _actualA;      public MyA( ClassA actualA )     {         _actualA = actualA;     }      public void DoWhatYouWant()     {         _actualA.DoWhatADoes();     } }  public class MyB : ITheInterfaceYouNeed {     protected ClassB _actualB;      public MyB( ClassB actualB )     {         _actualB = actualB;     }      public void DoWhatYouWant()     {         _actualB.DoWhatBDoes();     } } 

Seems like a lot of code, but it will make the client code a lot closer to what you want. Plus it’ll give you a chance to think about what interface you’re actually using.