11.6 The Root of All Classes: Object

All C# classes, of any type, are treated as if they ultimately derive from a single class: Object. Object is the base class for all other classes.

A base class is the immediate "parent" of a derived class. A derived class can be the base to further derived classes, creating an inheritance "tree" or hierarchy. A root class is the topmost class in an inheritance hierarchy. In C#, the root class is Object. The nomenclature is a bit confusing until you imagine an upside-down tree, with the root on top and the derived classes below. Thus, the base class is considered to be "above" the derived class.

Object provides a number of methods that subclasses can and do override. These include Equals(), which determines if two objects are the same, GetType(), which returns the type of the object, and ToString(), which returns a string to represent the current object. Specifically, ToString() returns a string with the name of the class to which the object belongs. Table 11-1 summarizes the methods of Object.

Table 11-1. The Object class

Method	What it does
Equals()	Evaluates whether two objects are equivalent
GetHashCode()	Allows objects to provide their own hash function for use in collections (see Chapter 16)
GetType()	Provides access to the type object
ToString()	Provides a string representation of the object
Finalize()	Cleans up nonmemory resources; implemented by a destructor
MemberwiseClone()	Creates copies of the object; should never be implemented by your type
ReferenceEquals()	Evaluates whether two objects refer to the same instance

In Example 11-4, the Dog class overrides the ToString() method inherited from Object, to return the weight of the Dog. This example also takes advantage of the startling fact that intrinsic types (int, long, etc.) can also be treated as if they derive from Object, and thus you can call ToString() on an int variable! Calling ToString() on an intrinsic type returns a string representation of the variable's value.

Example 11-4. Overriding ToString

using System;

public class Dog
{
    private int weight;

    // constructor
    public Dog(int weight)
    {
        this.weight = weight;
    }

    // override Object.ToString
    public override string ToString()
    {
        return weight.ToString();
    }
}


public class Tester
{
    static void Main()
    {
        int i = 5;
        Console.WriteLine("The value of i is: {0}", i.ToString());

        Dog milo = new Dog(62);
        Console.WriteLine("My dog Milo weighs {0} pounds", milo.ToString());
    }
}
Output:
The value of i is: 5
My dog Milo weighs 62 pounds

The documentation for Object.ToString() reveals its signature:

public virtual string ToString();

It is a public virtual method that returns a string and takes no parameters. All the built-in types, such as int, derive from Object and so can invoke Object's methods.

The Console class' Write() and WriteLine() methods call ToString() for you on objects that you pass in for display.

If you comment out the overridden function, the base method will be invoked. The base class default behavior is to return a string with the name of the class itself. Thus, the output would be changed to the meaningless:

My dog Milo weighs Dog pounds

Classes do not need to declare explicitly that they derive from Object; the inheritance is implicit.