C # syntax exercise (12): class [4]-abstract class and abstract member, seal class and seal Member

Abstract classes cannot be directly instantiated:

 
Using system; abstract class myclass {} class program {static void main () {/* abstract class cannot be directly instantiated. the following error occurs */myclass OBJ = new myclass (); console. readkey ();}}

  
 

However, abstract classes can be instantiated by subclass:

 
Using system; abstract class parent {} class child: parent {} class program {static void main () {parent OBJ = new child (); console. writeline (obj. tostring (); // child console. readkey ();}}

  
 

Abstract METHODS can only be included in abstract classes:

Using system; abstract class parent {/* the abstract method is an implicit virtual method, but it cannot be modified using static or virtual */public abstract void Method1 (); /* abstract classes can contain non-Abstract METHODS */Public void method2 () {console. writeline ("method2");}/* can even contain static methods */public static void method3 () {console. writeline ("method3") ;}} class child: parent {/* use override */Public override void Method1 () {console. writeline ("Method1") ;}} class program {static void main () {parent. method3 (); // method3 child. method3 (); // method3 parent OBJ = new child (); obj. method1 (); // Method1 obj. method2 (); // method2 console. readkey ();}}

  
 

A derived class must implement the abstract method of the parent class, unless it is also an abstract class:

Using system; abstract class parent {public abstract void Method1 ();} abstract class Child1: parent {} class child2: Child1 {public override void Method1 () {console. writeline ("Method1") ;}} class program {static void main () {parent OBJ = new child2 (); obj. method1 (); // Method1 console. readkey ();}}

  
 

Abstract attributes:

 
Using system; abstract class shape {public abstract int area {Get ;}} class rectangle: Shape {private int fwidth, fheight; Public rectangle (int w, int H) {fwidth = W; fheight = H;} public override int area {get {return fwidth * fheight ;}} class program {static void main () {rectangle rect = new rectangle (20, 10); console. writeline (rect. area); // 200 console. readkey ();}}

  
 

Sealing class and sealing members:

Using system; Class Parent {Public Virtual void Method1 () {console. writeline ("Method1");} Public Virtual void method2 () {console. writeline ("method2");} Public Virtual void method3 () {console. writeline ("method3") ;}} class Child1: parent {/* the following two methods can continue to overwrite */Public override void Method1 () {console. writeline ("New Method1");} public override void method2 () {console. writeline ("New method2");}/* this method has been disabled by sealed, that is, the feature of the virtual function */Public sealed override void method3 () {console. writeline ("New method3");}/* This class uses sealed as a sealed class and cannot have a derived class. Regardless of its internal nature, */sealed class child2: child1 {public override void Method1 () {console. writeline ("New Method1");} public sealed override void method2 () {console. writeline ("New method2") ;}} class program {static void main () {child2 OBJ = new child2 (); obj. method1 (); // New Method1 obj. method2 (); // New method2 obj. method3 (); // new method3 console. readkey ();}}