C# 0 The Complete Reference

x2

 can’t 

access

b

 even when it refers to a 

Y

 object. This makes sense because the base class has no 

knowledge of what a derived class adds to it. This is why the last line of code in the program 

is commented out.

Although the preceding discussion may seem a bit esoteric, it has some important 

practical applications. One is described here. The other is discussed later in this chapter, 

when virtual methods are covered.

An important place where derived class references are assigned to base class variables 

is when constructors are called in a class hierarchy. As you know, it is common for a class 

to define a constructor that takes an object of its class as a parameter. This allows the class to 

construct a copy of an object. Classes derived from such a class can take advantage of this 

feature. For example, consider the following versions of