原理：基类可以定义并实现虚(virtual)方法，派生类可以重写(override)这些方法
实例代码：
class Shape
{
//
public int X{get;private set; }
public int Y{get;private set;}
public int Height{get;set;}
public int Width{get;set;}
//
public virtual void Draw()
{
Console.WriteLine("Performing base class drawing tasks");
}
}
class Circle : Shape
{
public override void Draw()
{
// Code to draw a circle...
Console.WriteLine("Drawing a circle");
base.Draw();
}
}
class Rectangle : Shape
{
public override void Draw()
{
// Code to draw a rectangle...
Console.WriteLine("Drawing a rectangle");
base.Draw();
}
}
class Triangle : Shape
{
public override void Draw()
{
// Code to draw a triangle...
Console.WriteLine("Drawing a triangle");
base.Draw();
}
}
class Program
{
static void Main(string[] args)
{
List<Shape> shapes = new List<Shape>();
shapes.Add(new Rectangle());
shapes.Add(new Triangle());
shapes.Add(new Circle());
foreach (Shape s in shapes)
{
s.Draw();
}
Console.WriteLine("press any key to exit.");
Console.ReadKey();
}
}