public class Test 

{

    public static void Main(string[] args) 

    {

        Animal a = new Dog();  

        Animal x = a.Spawn();

        Dog b = new Dog();

        Dog y = (Dog) b.Spawn();

    }

​

}

​

​

class Animal 

{

    protected int _generation = 0;

​

    public Animal() : this(1) 

    {   

    }

    public Animal(int generation) 

    {

        _generation = 1;

    }

​

    public virtual Animal Spawn() 

    {

        System.Console.WriteLine("From Animal Spawn");

        return new Animal(_generation + 1);

    }

}

​

​

class Dog : Animal 

{   

    public Dog() : this(1)

    {

    }

    public Dog(int generation) 

    {

        _generation = generation;       

    }

​

    // Compilation error: 'Dog.Spawn()': return type must be 'Animal' to match overridden member 'Animal.Spawn()'

    public override Animal Spawn() 

    {

        System.Console.WriteLine("From Dog Spawn");

        return new Dog(_generation + 1);

    }

}