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 
  } 
}