Question

The problem

I am trying to avoid code that looks like the following:

If(object Is Man)
  Return Image("Man")
ElseIf(object Is Woman)
  Return Image("Woman")
Else
  Return Image("Unknown Object")

I thought I could do this through method overloading, but it always picks the least derived type, I assume this is because the overloading is determined at compile time (unlike overriding), and therefore only the base class can be assumed in the following code:

Code structure:

NS:Real
   RealWorld (Contains a collection of all the RealObjects)
   RealObject
     Person
       Man
       Woman
NS:Virtual
   VirtualWorld (Holds a reference to the RealWorld, and is responsible for rendering)
   Image (The actual representation of the RealWorldObject, could also be a mesh..)
   ArtManager (Decides how an object is to be represented)

Code Implementation (key classes):

class VirtualWorld
{
    private RealWorld _world;

    public VirtualWorld(RealWorld world)
    {
        _world = world;
    }

    public void Render()
    {
        foreach (RealObject o in _world.Objects)
        {
            Image img = ArtManager.GetImageForObject(o);
            img.Render();
        }
    }
}

static class ArtManager
{
    public static Image GetImageForObject(RealObject obj)// This is always used
    {
        Image img = new Image("Unknown object");
        return img;
    }

    public static Image GetImageForObject(Man man)
    {
        if(man.Age < 18)
            return new Image("Image of Boy");
        else
            return new Image("Image of Man");
    }

    public static Image GetImageForObject(Woman woman)
    {
        if (woman.Age < 70)
            return new Image("Image of Woman");
        else
            return new Image("Image of Granny");
    }
}

My scenario:
Essentially I am creating a game, and want to decouple real-world classes (such as a man), from on-screen classes (an image of a person). The real world object should have no knowledge of it’s on-screen representation, the representation will need to be aware of the real object (to know how old the man is, and therefore how many wrinkles are drawn). I want to have the fallback where if a RealObject is of an unknown type, it still displays something (like a big red cross).

Please note that this code is not what i’m using, it’s a simplified version to keep the question clear. I may need to add details later if applicable, I’m hoping the solution to this code will also work in the application.

What’s the most elegant way to solve this? – Without the RealObject itself holding information on how it should be represented. The XNA game is a proof of concept which is very AI heavy, and if it proves doable, will be changed from 2D to 3D (probably supporting both for lower end computers).

Answer 1

Use a factory:

public class ImageFactory
{
    Dictionary<Type, Func<IPerson, Image>> _creators;

    void Assign<TPerson>(Func<IPerson, Image> imageCreator) where T : IPerson
    {
       _creators.Add(typeof(TPerson), imageCreator);
    }

   void Create(Person person)
   {
       Func<IPerson, Image> creator;
       if (!_creators.TryGetValue(person.GetType(), out creator))
          return null;

       return creator(person);
   }
}

Assign factory methods:

imageFactory.Assign<Man>(person => new Image("Man");
imageFactory.Assign<Woman>(person => new Image("Big bad mommy");
imageFactory.Assign<Mice>(person => new Image("Tiny little mouse");

And use it:

var imageOfSomeone = imageFactory.Create(man);
var imageOfSomeone2 = imageFactory.Create(woman);
var imageOfSomeone3 = imageFactory.Create(mice);

To be able to return different images for men you can use a condition:

factory.Assign<Man>(person => person.Age > 10 ? new Image("Man") : new Image("Boy"));

For clarity you can add all more complex methods to a class:

public static class PersonImageBuilders
{
    public static Image CreateMen(IPerson person)
    {
        if (person.Age > 60)
            return new Image("Old and gready!");
        else
            return new Image("Young and foolish!");

    }
}

And assign the method

imageFactory.Assign<Man>(PersonImageBuilders.CreateMen);