Yesterday, there was a correct answer. Now it's gone, so I'm going to reproduce it here:
Change your in-dll-code to
public abstract class Class1
public abstract int ADDNumbers(int a, int b,int c);
This doesn't provide any implementation for
at all, but is enforcing that you override it in every derived class.
Your actual application includes the dll. Do the subclassing there including the implementation of
If that's all you want from Class1, you can also turn it into an interface
instead of a class. That would allow the classes in your application to derive from something else and
implement this interface at the same time.
Oh, and the derived class would be something like
public class Class2 : Class1
public override int ADDNumbers(int a, int b, int c)
return( a + b + c);