1. C# DownCast vs UpCast

2. Polymorphism
Polymorphism is one of the most powerful mechanisms of OOP

3. Class Shape public class Shape { protected int m_xpos;
protected int m_ypos;
public Shape() }
public Shape(int x, int y)
m_xpos = x;
m_ypos = y;
public virtual void Draw()
Console.WriteLine("Drawing a SHAPE at {0},{1}", m_xpos, m_ypos);

4. Class Circle public class Circle : Shape { public Circle() }
public Circle(int x, int y) : base(x, y)
public override void Draw()
Console.WriteLine("Drawing a CIRCLE at {0},{1}", m_xpos, m_ypos); } 

5. Client Code: Upcast Consider the following statement:
Upcast: 1) only cast a class to its base class, 2) only consider the static type of an object
There are two type conversions:
Shape s = new Circle(100, 100);
Which type conversion does upcast belong to?

6. Downcast Downcast: only change the static type of an object. Example:
Shape s = new Circle(100, 100);
s.fillCircle();
Circle c;
c = (Circle)s;
s.fillCircle()
Which type conversion does downcast belong to?

7. More on Downcast
Since downcast is not safe, the runtime system checks whether the runtime type of an object is a derived class of the cast type or the cast type itself.
How to make your program without a runtime exception
foreach (Shape shape in shapes) {
shape.Draw();
  if (shape is Circle)
((Circle)shape).FillCircle();
  if (shape is Square)
((Square)shape).FillSquare(); }