How to create the right subclass based on value in SQL row

Question

I was feeling quite clever as I worked out how to create subclasses in C# for an ASP.NET project, then I found a problem - I didn't know how to create an object of the right subclass based on the results of a SQL query.

Say you have a class called Animal and two subclasses called Zebra and Elephant. You get the idea?

What I want to do is perform a SQL query and if the row returned has row["Type"]="Zebra" then load a Zebra object (or if it's an Elephant then..).

So, in principle, the Animal class would have a static method:

class Animal{
 public static Animal Load(DataRow row){
  if (row["Type"]=="Zebra"){
   return new Zebra();
  } 
}

class Zebra : Animal{
 //some code here
}

Is this at all possible or have I just plain got the idea of subclasses wrong. It should be obvious I'm no OO expert.

Thanks in advance, Jake

Answer 1

You can implement the method factory pattern. http://en.wikipedia.org/wiki/Factory_method_pattern

Answer 2

Try this:

    public interface IAnimal
{ }

public class Animal : IAnimal
{
    public static IAnimal Load(String type)
    {
        IAnimal animal = null;
        switch (type)
        {
            case "Zebra" :
                animal = new Zebra();
                break;
            case "Elephant" :
                animal = new Elephant();
                break;
            default:
                throw new Exception();

        }

        return animal;
    }
}

public class Zebra : Animal
{
    public int NrOfStripes { get; set; }

    public static Zebra ZebraFactory()
    {
        return new Zebra();
    }
}

public class Elephant : Animal
{
    public int LengthOfTrunk { get; set; }
}

And to try it:

    class Program
{
    static void Main(string[] args)
    {
        IAnimal zebra = Animal.Load("Zebra");
        IAnimal elephant = Animal.Load("Elephant");
    }
}

Answer 3

I think it's fine:

public class Animal
{
    public static Animal Load(string row)
    {
        if (row == "Zebra")
        {
            return new Zebra();
        }
        else if (row == "Elephant")
        {
            return new Elephant();
        }

        return null;
    }
}

public class Zebra : Animal
{
    public new string ToString()
    {
        return "Zebra";
    }
}

public class Elephant : Animal
{
    public new string ToString()
    {
        return "Elephant";
    }
}

static void Main(string[] args)
{
    Animal a1 = Animal.Load("Zebra");
    System.Console.WriteLine(((Zebra)a1).ToString());

    System.Console.WriteLine(((Elephant)a1).ToString()); // Exception

    Animal a2 = Animal.Load("Elephant");
    System.Console.WriteLine(a2.ToString());
}