Question

I need to add some extension points to our existing code, and I’ve been looking at MEF as a possible solution. We have an IRandomNumberGenerator interface, with a default implementation (ConcreteRNG) that we would like to be swappable. This sounds like an ideal scenario for MEF, but I’ve been having problems with the way we instantiate the random number generators. Our current code looks like:

public class Consumer
{
    private List<IRandomNumberGenerator> generators;
    private List<double> seeds;

    public Consumer()
    {
        generators = new List<IRandomNumberGenerator>();
        seeds = new List<double>(new[] {1.0, 2.0, 3.0});

        foreach(var seed in seeds)
        {
            generators.Add(new ConcreteRNG(seed));
        }
    }
}

In other words, the consumer is responsible for instantiating the RNGs it needs, including providing the seed that each instance requires.

What I’d like to do is to have the concrete RNG implementation discovered and instantiated by MEF (using the DirectoryCatalog). I’m not sure how to achieve this. I could expose a Generators property and mark it as an [Import], but how do I provide the required seeds?

Is there some other approach I am missing?

Answer 1

Currently there isn’t a direct way to do this in MEF but the MEF team is considering support for this in v.Next. You essentially want to create multiple instances of the same implementation which is traditially done using a Factory pattern. So one approach you could use is something like:

public interface IRandomNumberGeneratorFactory
{
  IRandomNumberGenerator CreateGenerator(int seed);
}

[Export(typeof(IRandomNumberGeneratorFactory))]
public class ConcreateRNGFactory : IRandomNumberGeneratorFactory
{
  public IRandomNumberGenerator CreateGenerator(int seed)
  {
    return new ConcreateRNG(seed);
  }
}

public class Consumer
{
  [Import(typeof(IRandomNumberGeneratorFactory))]
  private IRandomNumberGeneratorFactory generatorFactory;
  private List<IRandomNumberGenerator> generators;    
  private List<double> seeds;    

  public Consumer()    
  {
    generators = new List<IRandomNumberGenerator>();
    seeds = new List<double>(new[] {1.0, 2.0, 3.0});

    foreach(var seed in seeds)
    {            
      generators.Add(generatorFactory.CreateGenerator(seed));
    }
  }
}