C# DownCast vs UpCast

Polymorphism Polymorphism is one of the most powerful mechanisms of OOP

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);

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); } 

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?

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?

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(); }