抽象与密封示例

     使用abstract申明抽象类，抽象类不能被实例化；

     使用sealed关键字申明密封类，不可从此类派生其他的类；

    下面以计算各类图形的面积为例：

这是计算圆的类（是一个密封的类）:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Shape
{
    public sealed class Circle:shape
    {
        private double radium;
        public Circle (double r)
        {
            radium = r;
        }
        public override double calculateArea()
        {
            return Math.PI * radium * radium;
        }
    }
}

矩形的类（密封）：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Shape
{
    public sealed class Rectangle:shape
    {
        private double length, width;
        public Rectangle (double l,double w)
        {
            length = l;
            width = 1;
        }
        public override double calculateArea()
        {
            return length * width;
        }
    }
}

三角形的类（密封）：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Shape
{
    public sealed class Triangle:shape
    {
        private double edge, high;
        public Triangle(double e, double h)
        {
            edge = e;
            high = h;
        }
        public override double calculateArea()
        {
            return edge * high / 2;
        }
    
    }
}

基类（抽象的）：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Shape
{
    public abstract class shape
    {
        public abstract double calculateArea();
    }
}

主函数（调用这些类）：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Shape
{
    class Program
    {
        static void Main(string[] args)
        {
            shape s;
            s = new Triangle(12.0,5.6);
            printArea(s);
            s = new Rectangle(10.0, 8.1);
            printArea(s);
            s = new Circle(3.9);
            printArea(s);
            Console.ReadLine();

        }
        static void printArea(shape s)
        {
            Console.WriteLine("这个图形是" + s.GetType().ToString());
            Console.WriteLine("图形面积是" + s.calculateArea().ToString());
        }
    }
}