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I've recently completed work on a molecular/protein viewer for a computer graphics class. Atoms are obviously a very important part of such a program. Currently, I have a big enum that contains each Element (Hydrogen, Helium, etc). Each Element has associated information such as color, radius, etc. I'm using an extension class so I can write:

float r = Element.Neon.Radius();

I also need to be able to get the corresponding enum from it's symbol. I stuck that method in the extension class which is a bit messy:

Element carbon = ElementExtensions.FromAbbreviation("C");

I want to associate a lot more data with particular Element entries but I'm not sure if this is a good design or not. Currently each set of associated data requires a Dictionary to grab the associated data. Perhaps I could use a Dictionar?

If I went with an Element class I'd need to make sure there's only even one instance of hydrogen, one of helium, etc. I like how System.Drawing.Color is designed but I don't think that's exactly what I need (you'd never have to look up associated data for ForestGreen for example).

What's the best way to support a large but well-defined and finite number of specific instances?

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An enum doesn't seem a very adequate approach to this problem; instead you should create an Element class with all the information associated to each element, and static readonly properties for each known element:

public class Element { private readonly string _symbol; private readonly int _atomicNumber; private readonly Color _color // Private constructor, since you don't want people to be able to "invent" random elements... private Element(string symbol, int atomicNumber, Color color) { _symbol = symbol; _atomicNumber = atomicNumber; _color = color; } public string Symbol { get { return _symbol; } } public int AtomicNumber { get { return _atomicNumber; } } public Color color { get { return _color; } } // Known elements private static readonly Element _hydrogen = new Element("H", 1, Color.White); private static readonly Element _carbon = new Element("C", 6, Color.Black); private static readonly Element _oxygen = new Element("O", 8, Color.Red); public static Element Hydrogen { get { return _hydrogen; } } public static Element Carbon { get { return _carbon; } } public static Element Oxygen { get { return _oxygen; } } ... }

This way all information about an element can be in one place, rather than scattered in many different places

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I spend half my time in Java and their version of enum is more like this... I think that's why I was trying to force it into an enum to being with
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If you have a lot of enums you should reconsider if you should be using inheritance instead.

Instead of Element.Neon why not let Neon inherit from Element? 

class Element int Radius(); Color Color(); string Symbol(); class Neon : Element int Radius (return x); int Color (return Color.Red); int Symbol (return "NE");

If you only need one element, consider using a Singleton.

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Either an abstract class with some default logic or indeed, an interface.
I think this would be the best way as I recall from my chem classes there are groups of elements in the periodic table that are similar to each other. They should in this case share the same base class
I spend half my time in Java and their version of enum is more like this... This is the pattern I wanted to express
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Define your elements as classes, but use the Flyweight pattern.

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