All C# classes, of any type, are treated as if they ultimately derive from System.Object. Interestingly, this includes value types!

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( ) to determine if two objects are the same, GetType( ), which returns the type of the object (discussed in Chapter 18), and ToString( ), which returns a string to represent the current object (discussed in Chapter 10). Table 5-1 summarizes the methods of Object.

Example 5-4 illustrates the use of the ToString( ) method inherited from Object, as well as the fact that primitive datatypes such as int can be treated as if they inherit from Object.

using System;

public class SomeClass
{
   public SomeClass(int val)
   {
      value = val;
   }

   public virtual string ToString(  )
   {
      return value.ToString(  );
   }
   private int value;
}


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

      SomeClass s = new SomeClass(7);
      Console.WriteLine("The value of s is {0}", s.ToString(  ));
   }
}

Output:
The value of i is: 5
The value of s is 7

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

public virtual string ToString(  );

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

Example 5-4 overrides the virtual function for SomeClass, which is the usual case, so that the class’ ToString( ) method will return a meaningful value. If you comment out the overridden function, the base method will be invoked, which will change the output to:

The value of s is SomeClass

Thus, the default behavior is to return a string with the name of the class itself.

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