一、继承的类型
1.1 实现继承和接口继承
C#中既有实现继承,也有接口继承。它们没有强弱之分,因为两种继承都完全内置于语言,因此很容易为不同的情形选择最好的体系结构。
实现继承:表示一个类型派生于一个基类型,拥有该类型的所有成员字段和函数。
接口继承:表示一个类型只继承了函数的签名,没有继承任何实现。在需要指定该类型具有某些可用的特征时,最好使用这种类型的继承。接口继承常常被看做提供了一种契约:通过类型派生于接口,从而保证为客户提供某个功能。
1.2 多重继承
C#不支持多重实现继承,但允许类型派生于多个接口。所以C#类可以派生于另一个类和任意多个接口。
1.3 结构和类
前面说过了结构是值类型,不支持继承,但是每个结构都自动派生于System.ValueType。实际上,不能建立结构的类型层次,但结构可以实现接口。
二、实现的继承
类派生于另一个类,语法如下:
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类派生于接口,则用都好分隔。
2.1 虚方法
在C#中可以把在方法或者属性的前面加上virtual,声明基类的方法为虚方法或者虚属性,虚方法和虚属性的规则相同。语法如下:
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注意:成员字段和静态函数不能被声明为virtual,因为这个概念只对类中的实例函数成员有意义。
2.2 隐藏方法
如果在基类和派生类中都声明了签名相同的方法,而基类中的方法又没有声明为virtual,派生类中的方法也没有声明成override,则派生类方法就会隐藏基类方法。结果是调用哪个类的方法取决于引用实例的变量类型,而不是实例本身的类型。
2.3 调用函数的基本版本
C#有一种特殊的语法用于从派生类中调用方法的基本版本:base.<MethodName>()。
2.4 抽象类和抽象函数
C#中的抽象类要声明为abstract,抽象类不能实例化,抽象函数没有执行代码,必须在非抽象的派生类中重写。显然抽象函数也是虚拟的,但不需要提供virtual关键字,而用abstract关键字。如果类包含了抽象函数,那么该类也是抽象的,类必须用abstract声明。
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C#允许把类和方法声明为sealed,对于类来说,这表示不能继承该类;对于方法来说,这表示不能重写该方法。
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在创建派生类的实例时,实际上会有多个构造函数在起作用。实例化类的构造函数本身不能初始化类,还必须调用基类中的构造函数。即派生类的实例构造是按层次结构进行的,顺序是先执行基类的构造函数,再执行派生类的构造函数。
基类的构造函数总是最先调用,那么派生类的构造函数可以在执行的过程中调用基类的方法、属性和其他成员,因为基类已经构造出来了,其他字段也初始化了。
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C#中完整的修饰符列表:
修饰符 | 应用于 | 说明 |
public | 所有类型或成员 | 任何代码均可以访问该方法 |
protected | 类型和内嵌类型的所有成员 | 只有派生的类型能访问该方法 |
internal | 类型和内嵌类型的所有成员 | 只能在包含它的程序集中访问该方法 |
private | 所有类型或成员 | 只能在它所属的类型中访问该方法 |
protected internal | 类型和内嵌类型的所有成员 | 只能在包含它的程序集中和派生类型的代码中访问该方法 |
四、接口
接口中只能包含方法、属性、索引器和事件的声明。接口不能实例化,不能有构造函数或字段,也不包含运算符重载。接口定义中还不允许声明成员的修饰符,成员接口总是public的,不能声明成virtual或者static
4.1 接口的定义和实现
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接口可以继承,其方式与类的继承相同。