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//Base class
public abstract class Animal
{
    public virtual string Name {get; set;}
    public virtual int Age {get; set;}
}

public class Dog : Animal
{
    public virtual bool WearsCollar {get; set;}
}

public class Cat: Animal
{
    public virtual bool IsLazy {get; set;}
    public virtual void Update(Pet pet)
    {
       //And now what?
    }
}

public class AnimalOwner
{
    public virtual int Id {get; set;}
    public virtual Animal Pet {get; set;}

    public virtual UpdatePet(Pet pet)
    {
        if(Pet==null)
        {
            Pet = pet;
            return;
        }
        Pet.Update(pet);
    }

}

//Now I want to do stuff like this@

[Test]
public void TestOwnersPet()
{
    var owner = Session.Get<Owner>(1);
    owner.Pet = new Dog();
    Assert.That(owner.Pet.WearsCollar, false); //Can't do this!!!!!
}