Question

Is it possible to obtain a reference/pointer to a class type and enforce that it derives from a particular base class?

I’m writing a client library that needs to negotiate with a server to pick an algorithm to use for communication. I want the user of the library to be able to select a subset of algorithms to use and not be fixed to the set I initially provide (ie. not fixed in some kind of factory class).

Ideally this would be done by passing in a list of classes that derive from some common “Algorithm” subtype. I’ve seen the “Type” object but I would have to check all the types myself. Is there a way to have the compiler do this for me? What I want is something like “Type<Algorithm>” but I can’t find anything like that. Or is there different way entirely to do this?

An example of what I’ve thought of so far:

public class Algorithm {
    public static abstract Name;
}

public class Client {
    public MyLib(Type[] algorithms) {
      m_algorithms = algorithms;
      // ... Check they all derive from Algorithm
    }

    public Communicate() {
      // ... Send list of algorithm names to server
      // ... Create instance of algorithm dictated by server response
    }
}

Answer 1

You unfortunately can’t check that the types are all Algorithms at compile time. This is one of the (few) features I miss about Java. However, there are some good workarounds, which (depending on your situation) may be better than the solution you were hoping for. For example:

public abstract class Algorithm {
}

public class AlgorithmA : Algorithm { }
public class AlgorithmB : Algorithm { }

public interface IAlgorithmFactory
{
    string Name {get;}
    Algorithm GetAlgorithm();
}

public class AlgorithmFactory<T> : IAlgorithmFactory where T : Algorithm, new()
{
    public string Name {get { return typeof(T).Name; }}
    public Algorithm GetAlgorithm()
    {
        return new T();
    }
}

public class Client {
    public void MyLib(IEnumerable<IAlgorithmFactory> algorithms) {

    // ... Check they all derive from Algorithm
    }

    public void Communicate() {
    // ... Send list of algorithm names to server
    // ... Create instance of algorithm dictated by server response
    }
}

Then you can use your client class like this:

new Client().MyLib(new IAlgorithmFactory[] 
                   {
                       new AlgorithmFactory<AlgorithmA>(), 
                       new AlgorithmFactory<AlgorithmB>()
                   });