【C#】学习笔记 abstract、virtual、interface使用的一些栗子

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abstract (C# Reference)

Abstract classes（抽象类） have the following features:

An abstract class cannot be instantiated. ①

An abstract class may contain abstract methods and accessors. ②

It is not possible to modify an abstract class with the sealed modifier because the two modifiers have opposite meanings. The sealed modifier prevents a class from being inherited and the abstract modifier requires a class to be inherited. ③

A non-abstract class derived from an abstract class must include actual implementations of all inherited abstract methods and accessors. ④

①抽象类无法被实例化

②抽象类可以包含抽象方法和访问器

③抽象类无法用sealed修饰，因为这俩是相反的。sealed是用来阻止被继承的，而abstract是需要一个类来继承的。

④一个非抽象派生类必须实现继承来的所以抽象方法和访问器。

 1 namespace AbstractDemo
 2 {
 3     abstract class Shape
 4     {
 5         public abstract int GetArea();
 6 
 7     }
 8     class Square : Shape
 9     {
10         int side;
11         public Square(int n)
12         {
13             side = n;
14         }
15         //简写： public Demo(int n) => side = n;
16 
17         public override int GetArea()
18         {
19             return side * side;
20         }
21         //简写： public override int GetArea() => side * side;
22     }
23 }

 1 namespace AbstractDemo
 2 {
 3     class Program
 4     {
 5         static void Main(string[] args)
 6         {
 7             Square square = new Square(4);
 8             System.Console.WriteLine(square.GetArea()); // 16
 9         }
10     }
11 }

Abstract methods（抽象方法） have the following features:

An abstract method is implicitly a virtual method.①

Abstract method declarations are only permitted in abstract classes.②

Because an abstract method declaration provides no actual implementation, there is no method body; the method declaration simply ends with a semicolon and there are no curly braces ({ }) following the signature.③

It is an error to use the static or virtual modifiers in an abstract method declaration.④

Abstract properties behave like abstract methods, except for the differences in declaration and invocation syntax.

It is an error to use the abstract modifier on a static property.⑤

An abstract inherited property can be overridden in a derived class by including a property declaration that uses the override modifier.⑥

①抽象方法是隐式的虚方法

②抽象方法只能在抽象类中进行声明。

③因为抽象方法声明却没有具体的实现，所以是没有方法体的“{ }”。以“；”结尾

④抽象方法声明时不能用static或virtual来修饰

⑤abstract不能用来修饰静态属性

⑥override用来修饰抽象属性派生类中的属性声明。

// virtual
 1 using System;
 2 namespace InterfaceDemo1
 3 {
 4     public class ConcreteRegularPolygon
 5     {
 6         public int NumberOfSides { get; set; }
 7         public int SideLength { get; set; }
 8 
 9         public ConcreteRegularPolygon(int sides, int length)
10         {
11             NumberOfSides = sides;
12             SideLength = length;
13         }
14         public double GetPerimeter()
15         {
16             return NumberOfSides * SideLength;
17         }
18         // 这里定义了一个virtual函数
19         public virtual double GetArea()
20         {
21             throw new NotImplementedException();
22         }
23     }
24 }

 1 namespace InterfaceDemo1
 2 {
 3     public class Square : ConcreteRegularPolygon
 4     {
 5         public Square(int length) : base(4, length)
 6         {
 7 
 8         }
 9         // 在这里进行覆盖重写
10         public override double GetArea()
11         {
12             return SideLength * SideLength;
13         }
14     }
15 }