public class Shape
{
    public virtual void Draw()
    {
      Console.WriteLine("base class drawing");
    }
}
public class Rectangle :Shape
{
    public override void Draw()
    {
      Console.WriteLine("Drawing a Rectangle");
    }
}
public class Square :Rectangle
{
    public override void Draw()
    {
      Console.WriteLine("Drawing a Square");
      base.Draw();
    }
}
class Program
{
    static void Main(string[]args)
    {
      System.Collections.Generic.List<Shape> shapes =new List<Shape>();
      shapes.Add(new Rectangle());
      shapes.Add(new Square());
      foreach(Shape s in shapes)
      {
        s.Draw();
      }
      Console.ReadKey();
      /*运行结果
       Drawing a Rectangle
       Drawing a Square
       Drawing a Rectangle
       */
    }
}

public new void Draw()
{
      Console.WriteLine("Drawing a Square");
      base.Draw();
}

/*运行结果
Drawing a Rectangle
Drawing a Rectangle
*/

public class Rectangle :Shape
{
    public sealed override voidDraw()
    {
      Console.WriteLine("Drawing a Rectangle");
    }
}

static void Main(string[] args)
{
      System.Collections.Generic.List<Shape>shapes =new List<Shape>();
      shapes.Add((Shape)new Rectangle());
      shapes.Add((Shape)new Square());
      foreach(Shape s inshapes)
      {
        s.Draw();
      }
      Console.ReadKey();
      /*运行结果
       Drawing a Rectangle
       Drawing a Square
       Drawing a Rectangle
       */
}

public abstract classShape
{
    public abstract void Draw();
}
public class Rectangle :Shape
{
    public override void Draw()
    {
      Console.WriteLine("Drawing a Rectangle");
    }
}
public class Square :Rectangle
{
    public override void Draw()
    {
      Console.WriteLine("Drawing a Square");
      base.Draw();
    }
}
class Program
{
    static void Main(string[]args)
    {
      System.Collections.Generic.List<Shape>shapes =new List<Shape>();
      shapes.Add(new Rectangle());
      shapes.Add(new Square());
      foreach(Shape s in shapes)
      {
        s.Draw();
      }
      Console.ReadKey();
     }
}

public interface IShape
{
    void Draw();
}
public class Rectangle :IShape
{
    public void Draw()
    {
      Console.WriteLine("Drawing a Rectangle");
    }
}
public class Square: IShape
{
    public void Draw()
    {
      Console.WriteLine("Drawing a Square");
    }
}
class Program
{
    static void Main(string[]args)
    {
      System.Collections.Generic.List<IShape>shapes =new List<IShape>();
      shapes.Add(new Rectangle());
      shapes.Add(new Square());
      foreach(IShape s inshapes)
      {
        s.Draw();
      }
      Console.ReadLine();
    }
}

public interface IShape
{
    void Draw();
}
public class Shape:IShape
{
    void IShape.Draw()
    {
      Console.WriteLine("Shape IShape.Draw()");
    }
    public virtual void Draw()
    {
      Console.WriteLine("Shape virtual Draw()");
    }
}
public class Rectangle :Shape,IShape
{
    void IShape.Draw()
    {
      Console.WriteLine("Rectangle IShape.Draw()");
    }
    public newvirtual void Draw()
    {
      Console.WriteLine("Rectangle virtual Draw()");
    }
}
public class Square :Rectangle
{
    public override void Draw()
    {
      Console.WriteLine("Square override Draw()");
    }
}
class Program
{
    static void Main(string[]args)
    {
      Square squre = new Square();
      Rectangle rect = squre;
      Shape shape = squre;
      IShape ishape = squre;
      squre.Draw();
      rect.Draw();
      shape.Draw();
      ishape.Draw();
      Console.ReadLine();
    }
}
/*运行结果：
Square override Draw()①
Square override Draw()②
Shape virtual Draw()③
Rectangle IShape.Draw()④
*/

public class Base
{
    public virtual void Write(int num)
    {
      Console.WriteLine("int:" + num.ToString());
    }
}
public class Derived :Base
{
    public override void Write(int num)
    {
      Console.WriteLine("derived:" + num.ToString());
    }
    public void Write(double num)
    {
      Console.WriteLine("derived double:" + num.ToString());
    }
}