The C# Station Tutorial

by Joe Mayo
12/9/00, updated 12/8/01, 3/12/03, 2/21/08, 1/12/09

Lesson 8: Class Inheritance

This lesson teaches about C# Inheritance. Our objectives are as follows:

• Implement Base Classes.
• Implement Derived Classes.
• Initialize Base Classes from Derived Classes.
• Learn How to Call Base Class Members.
• Learn How to Hide Base Class Members.

Inheritance is one of the primary concepts of object-oriented programming. It allows you to reuse existing code. Through effective employment of reuse, you can save time in your programming.

Listing 8-1. Inheritance: BaseClass.cs

using System;

public class ParentClass
{
    public ParentClass()
    {
        Console.WriteLine("Parent Constructor.");
    }

    public void print()
    {
        Console.WriteLine("I'm a Parent Class.");
    }
}

public class ChildClass : ParentClass
{
    public ChildClass()
    {
        Console.WriteLine("Child Constructor.");
    }

    public static void Main()
    {
        ChildClass child = new ChildClass();

        child.print();
    }
}

Output:

Parent Constructor.
Child Constructor.
I'm a Parent Class.

Listing 8-1 shows two classes. The top class is named ParentClass and the main class is called ChildClass. What we want to do is create a child class, using existing code from ParentClass.

First we must declare our intention to use ParentClass as the base class of ChildClass. This is accomplished through the ChildClass declaration public class ChildClass : ParentClass. The base class is specified by adding a colon, ":", after the derived class identifier and then specifying the base class name.

Note: C# supports single class inheritance only. Therefore, you can specify only one base class to inherit from. However, it does allow multiple interface inheritance, a subject covered in a later lesson.

ChildClass has exactly the same capabilities as ParentClass. Because of this, you can also say ChildClass "is" a ParentClass. This is shown in the Main() method of ChildClass when the print() method is called. ChildClass does not have its own print() method, so it uses the ParentClass print() method. You can see the results in the 3rd line of output.

Base classes are automatically instantiated before derived classes. Notice the output from Listing 8-1. The ParentClass constructor executed before the ChildClass constructor.

Listing 8-2. Derived Class Communicating with Base Class: BaseTalk.cs

using System;

public class Parent

{

    string parentString;

    public Parent()

    {

        Console.WriteLine("Parent Constructor.");

    }

    public Parent(string myString)

    {

        parentString = myString;

        Console.WriteLine(parentString);

    }

    public void print()

    {

        Console.WriteLine("I'm a Parent Class.");

    }

}

public class Child : Parent

{

    public Child() : base("From Derived")

    {

        Console.WriteLine("Child Constructor.");

    }

    public new void print()

    {

        base.print();

        Console.WriteLine("I'm a Child Class.");

    }

    public static void Main()

    {

        Child child = new Child();

        child.print();

        ((Parent)child).print();

    }

}

Output:

From Derived
Child Constructor.
I'm a Parent Class.
I'm a Child Class.
I'm a Parent Class.

Derived classes can communicate with base classes during instantiation. Listing 8-2 shows how this is done at the child constructor declaration. The colon, ":", and keyword base call the base class constructor with the matching parameter list. If the code had not appended base("From Derived") to the Derived constructor, the code would have automatically called Parent(). The first line of output shows the base class constructor being called with the string "From Derived".

Sometimes you may want to create your own implementation of a method that exists in a base class. The Child class does this by declaring its own print() method. The Child print() method hides the Parent print() method. The effect is the Parent print() method will not be called, unless we do something special to make sure it is called.

Inside the Child print() method, we explicitly call the Parent print() method. This is done by prefixing the method name with "base.". Using the base keyword, you can access any of a base class public or protected class members. The output from the Child print() method is on output lines 3 and 4.

Another way to access base class members is through an explicit cast. This is done in the last statement of the Child class Main() method. Remember that a derived class is a specialization of its base class. This fact allows us to perform a cast on the derived class, making it an instance of its base class. The last line of output from Listing 8-2 shows the Parent print() method was indeed executed.

Notice the new modifier on the Child class print() method. This enables this method to hide the Parent class print() method and explicitly states your intention that you don't want polymorphism to occur. Without the new modifier, the compiler will produce a warning to draw your attention to this. See the next lesson for a detailed discussion of polymorphism.

In summary, you know how to create a derived/base class relationship. You can control instantiation of your base class and call its methods either implicitly or explicitly. You also understand that a derived class is a specialization of its base class.

I invite you to return for Lesson 9: Polymorphism.