Unable to cast base class to derived class

Question

I have base class as Animal and child class as Dog

Here is the code:

class Animal
{
    public int Legs { get; set; }

    public string Name  { get; set; }
}

class Dog : Animal
{
    public int noOfTail { get; set; }
}

In My Main Method when I execute below I get the exception:

   static void Main(string[] args)
    {
        Animal a = new Animal();
        Dog d = new Dog();
        d = (Dog)a;
    }

but when I first cast my derived class to parent I don't get any exception, and that is why I am confused, can somebody explain reason behind it.

   static void Main(string[] args)
    {
        Animal a = new Animal();
        Dog d = new Dog();
        a = d;
        d = (Dog)a;
    }

Answer 1

Animal object cannot be cast to a Dog type. A Dog object, on the other hand, can be cast to Dog type even if you store a Dog object in a variable of Animal type.

In your first example you attempt to cast an Animal to Dog, which fails, because object is of a wrong class. In your second example you assign a = d, so now variable a has type Animal, but its runtime type is Dog. That is why the second cast works.

Note: This is only the technical part of your problem, but there is also a logical component to it: your class hierarchy lets you instantiate Animal class, which is not a leaf class.

A better approach would be to make Animal an interface or an abstract class. This way you would be able to instantiate only the "actual" animals (Dog and whatever else you wish to derive from Animal), while it would not be possible to create an instance of Animal without saying what kind of animal it represents.

Answer 2

In Case two you have an instance of dog and animal

Animal a = new Animal();
Dog d = new Dog();

Then you have reference copy of dog to animal

a = d;

So the a is points to d and d is a dog instance and instance properties like noOfTail is still exist but is hidden and not available in object a. and then you have this line :

d = (Dog)a;

In last line you have reference copy of d to a ,so d is point to where a is point to so everything is okay.

but in case one you want copy one Instance of child class Dog to an instance of Animal so the properties of child class will be lost. you need to confirm that with Compiler and say to him you know instance properties like noOfTail will not be available any more your code most be like this :

 Animal a = new Animal();
 Dog d = new Dog();
 d = (a as Dog); 

Answer 3

You cannot cast a base class to a derived class, but you can do the opposite: cast a derived class to a base class.

Your second attempt works for a very simple reason: a = d; => you are assigning a derived class instance to a base class instance.
that OOP fundamental is called polymorphism.

static void Main(string[] args)
    {
        Animal a = new Animal();
        Dog d = new Dog();
        a = d; // you can do that but you cant do d=a
        d = (Dog)a;
    }

To understand it better try to delve into that piece of code:

public void polymorphism()
{
    // thats wrong and the compiler will throw an exception (at design time) because animal is not a dog
    Dog d = new Animal();
    // thats correct becasue dog is an animal
    Animal a = new Dog();
}