C# 关键字Abstract

1 namespace FirstCode.EX1
2 {
3     abstract public class Shape
4     {
5         abstract public int area();
6     }
7 }

 1 namespace FirstCode.EX1
 2 {
 3     public class Rectangle : Shape
 4     {
 5         public int width;
 6         public int length;
 7 
 8         public Rectangle(int a,int b)
 9         {
10             width =a;
11             length = b;
12         }
13         public override int area()=>width*length;
14 
15     }
16 }

 1 namespace FirstCode
 2 {
 3     class Program
 4     {
 5         static void Main(string[] args)
 6         {
 7             Console.WriteLine("Hello World!");
 8 
 9             Rectangle rect1 = new Rectangle(2,3);
10             Console.WriteLine(rect1.area().ToString());
11         }
12     }
13 }