Csharp/CSharp Tutorial/Class/Derived Class

A base class reference can refer to a derived class object

<source lang="csharp">using System;

class BaseClass {

 public int a; 

 public BaseClass(int i) { 
   a = i; 
 } 

}

class DerivedClass : BaseClass {

 public int b; 

 public DerivedClass(int i, int j) : base(j) { 
   b = i; 
 } 

}

class MainClass {

 public static void Main() { 
   BaseClass x = new BaseClass(10); 
   BaseClass x2;  
   DerivedClass y = new DerivedClass(5, 6); 

   x2 = x; // OK, both of same type 
   Console.WriteLine("x2.a: " + x2.a); 

   x2 = y; // Ok because DerivedClass is derived from BaseClass 
   Console.WriteLine("x2.a: " + x2.a); 

   // BaseClass references know only about BaseClass members 
   x2.a = 19; // OK 

// x2.b = 27; // Error, BaseClass doesn"t have a b member

 } 

}</source>

x2.a: 10
x2.a: 6

Illustrates member accessibility in subclass

<source lang="csharp">using System; public class Car {

 private   string make;
 protected string model;
 public Car(string make, string model)
 {
   this.make = make;
   this.model = model;
 }
 public virtual void Start()
 {
   startCar();
   System.Console.WriteLine("Vehicle started");
 }
 private void startCar()
 {
   System.Console.WriteLine("Turning starter motor...");
 }

} public class MyFirstCar : Car {

 public MyFirstCar(string make, string model) : base(make, model)
 {
 }
 public override void Start()
 {
   Console.WriteLine("Starting " + model);  
   base.Start();  
 }

} class MainClass {

 public static void Main()
 {
   MyFirstCar myCar = new MyFirstCar("Toyota", "MR2");
   myCar.Start();
 }

}</source>

Starting MR2
Turning starter motor...
Vehicle started

Illustrates member hiding

<source lang="csharp">using System; public class BaseClass {

 public string lastName;
 public string firstName;
 public BaseClass(string lastName, string firstName)
 {
   Console.WriteLine("In BaseClass constructor");
   this.lastName = lastName;
   this.firstName = firstName;
   Console.WriteLine("this.lastName = " + this.lastName);
   Console.WriteLine("this.firstName = " + this.firstName);
 }
 public void DisplayfirstName()
 {
   Console.WriteLine("In BaseClass DisplayfirstName() method");
   Console.WriteLine("firstName = " + firstName);
 }

} public class DerivedClass : BaseClass {

 public new string firstName;
 public DerivedClass(string lastName, string firstName) : base(lastName, "Test")
 {
   Console.WriteLine("In DerivedClass constructor");
   this.firstName = firstName;
   Console.WriteLine("this.firstName = " + this.firstName);
 }
 public new void DisplayfirstName()
 {
   Console.WriteLine("In DerivedClass DisplayfirstName() method");
   Console.WriteLine("firstName = " + firstName);
   base.DisplayfirstName();
 }

}

class MainClass {

 public static void Main()
 {
   Console.WriteLine("Creating a DerivedClass object");
   DerivedClass myDerivedClass = new DerivedClass("A", "B");
   Console.WriteLine("Back in Main() method");
   Console.WriteLine("myDerivedClass.lastName = " + myDerivedClass.lastName);
   Console.WriteLine("myDerivedClass.firstName = " + myDerivedClass.firstName);
   Console.WriteLine("Calling myDerivedClass.DisplayfirstName()");
   myDerivedClass.DisplayfirstName();
 }

}</source>

Creating a DerivedClass object
In BaseClass constructor
this.lastName = A
this.firstName = Test
In DerivedClass constructor
this.firstName = B
Back in Main() method
myDerivedClass.lastName = A
myDerivedClass.firstName = B
Calling myDerivedClass.DisplayfirstName()
In DerivedClass DisplayfirstName() method
firstName = B
In BaseClass DisplayfirstName() method
firstName = Test