C#Keyword(base,protected,override,....)

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(1)protected

The protected keyword is a member access modifier. A protected member is accessible from within the class in which it is declared, and from within any class derived from the class that declared this member.

//proteted修饰的方法（本类和派生类内部可访问）

class A 
{
   protected int x = 123;
}

class B : A 
{
   void F() 
   {
      A a = new A();  
      B b = new B();  
      a.x = 10;   // Error
      b.x = 10;   // OK
   }
}

(2)base

The base keyword is used to access members of the base class from within a derived class:

Call a method on the base class that has been overridden by another method.
Specify which base-class constructor should be called when creating instances of the derived class.

(用于访问基类的成员,仅限于在派生类内部. base.方法名())

A base class access is permitted only in a constructor, an instance method, or an instance property accessor.

(只可在派生类(derived class)的构造函数,实例方法,实例属性中用base访问基类)

// keywords_base.cs
// Accessing base class members
using System;
   public class Person 
   {
      protected string ssn = "444-55-6666";
      protected string name = "John L. Malgraine";

      public virtual void GetInfo() 
      {
         Console.WriteLine("Name: {0}", name);
         Console.WriteLine("SSN: {0}", ssn);
      }
   }
   class Employee: Person 
   {
      public string id = "ABC567EFG";

      public override void GetInfo() 
      {
         // Calling the base class GetInfo method:
         base.GetInfo();
         Console.WriteLine("Employee ID: {0}", id);
      }
   }

class TestClass {
   public static void Main() 
   {
      Employee E = new Employee();
      E.GetInfo();
   }
}

结果：

Name:John L. Malgraine

SSN:444-55-6666

Employee ID:ABC567EFG

(3) virtual -overrid （必配套使用）

基类中用virtual修饰的方法,在派生类中可用override重写

class TestClass
{
    public class Shape
    {
        public const double PI = Math.PI;
        protected double x, y;
        public Shape()
        {
        }
        public Shape(double x, double y)
        {
            this.x = x;
            this.y = y;
        }

        public virtual double Area()
        {
            return x * y;
        }
    }

    public class Circle : Shape
    {
        public Circle(double r) : base(r, 0)
        {
        }

        public override double Area()
        {
            return PI * x * x;
        }
    }

    class Sphere : Shape
    {
        public Sphere(double r) : base(r, 0)
        {
        }

        public override double Area()
        {
            return 4 * PI * x * x;
        }
    }

    class Cylinder : Shape
    {
        public Cylinder(double r, double h) : base(r, h)
        {
        }

        public override double Area()
        {
            return 2 * PI * x * x + 2 * PI * x * y;
        }
    }

    static void Main()
    {
        double r = 3.0, h = 5.0;
        Shape c = new Circle(r);
        Shape s = new Sphere(r);
        Shape l = new Cylinder(r, h);
        // Display results:
        Console.WriteLine("Area of Circle   = {0:F2}", c.Area());
        Console.WriteLine("Area of Sphere   = {0:F2}", s.Area());
        Console.WriteLine("Area of Cylinder = {0:F2}", l.Area());
        }
    }
    /*
        Output:
        Area of Circle   = 28.27
        Area of Sphere   = 113.10
        Area of Cylinder = 150.80
    */