Classes and Inheritance - Illustrated C# 2005 - page 142

All Classes are Derived From Class object
All classes, except special class object , are derived classes, even if they don't have a class-base
specification. Class object is the only one that is not derived, since it is the base of the inherit-
ance hierarchy.
Classes without a class-base specification are implicitly derived directly from class object .
Leaving off the class-base specification is just shorthand for specifying that object is the base
class. The two forms are completely equivalent.
Figure 7-2 shows both forms of declaration for the same class.

Figure 7-2. Direct inheritance from object

Other important facts about class derivation are the following:

￿ A class declaration can have only a single class listed in its class-base specification. This

is called single inheritance .

￿ Although a class can directly inherit from only a single base class, there is no limit to the

level of derivation. That is, the class listed as the base class might be derived from
another class, which is derived from another class, and so forth, until you eventually
reach object .

Base class and derived class are relative terms. All classes are derived classes, either from
object or from another class—so generally when we call a class a derived class, we mean that it
is immediately derived from some class other than object . Figure 7-3 shows a simple class
hierarchy. After this, I will not show object in the figures, since all classes are ultimately derived
from it.

Figure 7-3. A class hierarchy