A base class reference can refer to a derived class object

/*
C#: The Complete Reference 
by Herbert Schildt 

Publisher: Osborne/McGraw-Hill (March 8, 2002)
ISBN: 0072134852
*/


// A base class reference can refer to a derived class object. 
 
using System; 
 
class X { 
  public int a; 
 
  public X(int i) { 
    a = i; 
  } 
} 
 
class Y : X { 
  public int b; 
 
  public Y(int i, int j) : base(j) { 
    b = i; 
  } 
} 
 
public class BaseRef { 
  public static void Main() { 
    X x = new X(10); 
    X x2;  
    Y y = new Y(5, 6); 
 
    x2 = x; // OK, both of same type 
    Console.WriteLine("x2.a: " + x2.a); 
 
    x2 = y; // still Ok because Y is derived from X 
    Console.WriteLine("x2.a: " + x2.a); 
 
    // X references know only about X members 
    x2.a = 19; // OK 
//    x2.b = 27; // Error, X doesn't have a b member 
  } 
}

1.	Inheritance 3
2.	Four layers of class hierarchy
3.	An example of inheritance-related name hiding
4.	Using base to overcome name hiding
5.	Call a hidden method
6.	A multilevel hierarchy 1
7.	Demonstrate when constructors are called
8.	illustrates inheritance
9.	Private field and public Property in inheritance
10.	Illustrates versioning
11.	Class Hierarchy test
12.	Class Hierarchy with two children class
13.	A simple class hierarchy
14.	Pass a derived class reference to a base class reference
15.	a multilevel hierarchy
16.	Build a derived class of Vehicle for trucks

A base class reference can refer to a derived class object : Class Inheritance « Class Interface « C# / C Sharp

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