C# 0 The Complete Reference

X

 and class 

Y

 are physically the same, it is not possible to assign a 

reference of type 

Y

 to a variable of type 

X

 because they have different types. Therefore, this 

line is incorrect because it causes a compile-time type mismatch:

x2 = y; // Error, not of same type

In general, an object reference variable can refer only to objects of its type.

There is, however, an important exception to C#’s strict type enforcement. A reference 

variable of a base class can be assigned a reference to an object of any class derived from 

that base class. This is legal because an instance of a derived type encapsulates an instance 

of the base type. Thus, a base class reference can refer to it. Here is an example:

// 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;

  }

}

class BaseRef {

  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; // OK because Y is derived from X

    Console.WriteLine("x2.a: " + x2.a);

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