Polymorphism and casting

Question

I want to understand polymorphism in c# so by trying out several constructs I came up with the following case:

class Shape
{
    public virtual void Draw()
    {
        Console.WriteLine("Shape.Draw()");
    }
}

class Circle : Shape
{
    public override void Draw()
    {
        Console.WriteLine("Circle.Draw()");
    }
}

I understand that in order to send the Draw() message to several related objects, so they can act according to its own implementation I must change the instance to which (in this case) shape is 'pointing' to:

Shape shape = new Circle();
shape.Draw(); //OK; This prints: Circle.Draw()

But why, when I do this:

Circle circle = new Circle();
circle.Draw(); //OK; This prints: Circle.Draw()

Shape shape = circle as Shape; // or Shape shape = (Shape)circle;
shape.Draw();

It prints: "Circle.Draw()"

Why it calls the Circle.Draw() instead Shape.Draw() after the cast? What is the reasoning for this?

Answer 1

Casting does not change run-time type of object and what implementation of particular virtual method each instance have.

Note that following 2 cases you have as sample are identical:

Shape shape = new Circle();
shape.Draw(); //OK; This prints: Circle.Draw()

and:

Circle circle = new Circle();
Shape shape = circle as Shape;
shape.Draw();

The first one is essentially shorter version of the second.