All C# classes, of any type, 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 override. These include Equals( ), which determines whether two objects are the same, 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 |
|---|---|
| |
| Allows objects to provide their own hash function for use in collections (see Chapter 14) |
| Provides access to the type of the object |
| Provides a string representation of the object |
| Cleans up nonmemory resources; implemented by a finalizer |
In Example 11-4, the Dog class overrides the ToString( ) method inherited from Object, to return the weight of the Dog. You wouldn't expect to be able to convert a Dog object to a string, but if you override the ToString( ) method, that's essentially what you're doing.
Example 11-4. Overriding the ToString( ) method of Object allows a user-defined class to return a string
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);
}
}The output looks like this:
The value of i is: 5 My dog Milo weighs 62 pounds
Some classes (such as Console) have methods that expect a string (such as WriteLine( )). These methods will call the ToString( ) method on your class if you've overridden the inherited ToString( ) method from Object. This lets you pass a Dog to Console.WriteLine, and the correct information will display.
This example also takes advantage of the startling fact that intrinsic types (int, long, and so forth) can also be treated as though 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.
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's Write( ) and WriteLine( ) methods call ToString( ) for you on objects that you pass in for display. Thus, by overriding ToString( ) in the Dog class, you did not have to pass in milo.ToString( ), but rather could just pass in milo!
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