public abstract class Animal
{
     public abstract string AnimalType {get;}
     public override string ToString(){ return AnimalType; }
//...
//...
//...
}

public class Cat:Animal
{
   public override string AnimalType
  {
     get{ return "Cat";}
  }
}

public class Dog:Animal
{
   public override string AnimalType
  {
     get{return "Dog";}
  }
}


..
..
..

public class EnterClass
{
  public static void Main(string[] argv)
  {
      //      ...
      //start foreach animal in animals
      Console.WriteLine(animal.ToString());
      //end foreach
  }
}

------解决方案--------------------
用泛型：

C# code
class AnimalBuilder
{
    static public T Create<T>(T refobj) where T : new()
    { return new T(); }
}